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MECHANISTIC AND SITE ISOLATION STUDIES 
OF TRANSITION METAL OXIDATION CATALYSTS 



By 
DAVID CHAPPEL PRIBICH 



A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL 
OF THE UNIVERSITY OF FLORIDA IN 
PARTIAL FULFILLMENT OF THE REQUIREMENTS 
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY 



UNIVERSITY OF FLORIDA 
1985 



To Susan, Jean, Mitch and Pat Pribich 



TABLE OF CONTENTS 

PAGE 
ABSTRACT ^ 

CHAPTER 

I GENERAL INTRODUCTION 1 



II THE SPECIFIC OXIDATION OF [Rh{CO),Cl], BY 

0, VIA THE COORDINATION OF IN SITO GENERATED 
HYDROGEN PEROXIDE 3 



Introduction 3 

Background 5 

Results and Discussion 3 

Characterization of the Catalyst 

as a Rhodium( III ) Chloride Complex .... 8 
Mechanism of the 0_ Oxidation of 
[Rh(CO)-Cl], to RhSdiumdII) 

Chloride . 19 

Conclusion 43 

Experimental Section 44 

Catalytic Oxidation of 1-Hexene 45 

Preparation of RhCl, (H^O) 2CH,- 

CH^OH (II) 45 

Determination of Acetone Production. ... 46 

Titration of [Rh(CO).Cl], with 

HOOH ^. 47 

III ENHANCED SITE ISOLATION ON SILICA GEL AND 

IMPROVED LIFETIMES OF SITE ISOLATED CATALYSTS . 49 

Introduction 49 

Results and Discussion 55 

Polymer Support of the Catalyst ^5 

Catalytic Oxidations of 1-Hexene by 

Supported Complexes ^3 

Alkyl Covering of the Silica 
Surface to Improve Site Isolation 

and Catalysis ^^ 

Catalytic Oxidations Using Alkylated 

Silica Gels as Solid Supports 70 

Possible Effects of a Different 

Rhodium Catalyst Characterization .... 84 

Conclusions 34 

ill 



Experimental 86 

General Procedures 86 

Preparation of Silica Gel Supports .... 86 

Determination of -SH on Silica Gel .... 87 

Catalytic Oxidations of 1-Hexene 89 

IV THE SYNTHESIS AND CATALYTIC APPLICATIONS OF A 
MULTIDENTATE LIGAND AND CORRESPONDING METAL 
COMPLEXES BOUND TO SILICA GEL 90 

Introduction 91 

Results and Discussion 94 

Synthesis of a Salen Ligand on 

Silica Gel 94 

Oxygen Transfer Using [ SG ]-Fe( III ) - 

SalDPT and [SG ]-Mn( II )SalDPT 101 

Synthesis of [ SG ]-Fe( II ) SalDPT 107 

Incorporation of an Active Metal 

into a Functionalized Support 110 

Synthesis of a Silica Gel Anion 

Exchange Resin 117 

Titanium Carbide as a Solid 

Support 118 

Fenton Chemistry 127 

Experimental 128 

Silica Gel 128 

Functionalization of Silica Gel 
with l-trimethoxysilyl-2-(p,m,- 

chloromethylphenylethane) 129 

Preparation of Silica Gel Bound 

3 , 3 ' -Immodipropionitrile 129 

Preparation of Silica Gel Bound 

bis-( 3-aminopropyl )amine [SG I-DPT .... 130 

Condensation of Salicylaldehyde with 

[SG]-DPT 131 

Incorporation of Metal Ions into 

LSG]-SalDPT 132 

Incorporation of Fe(II) into ^SG]- 

SalDPT 132 

Incorporation of Cu(II) into the 

Silica Gel Matrix 133 

Preparation of Silica Gel Anion 

Exchange Resins 1^'* 

Titani'um Carbide as a Solid 

Support 135 

Fenton Chemistry 137 

V GENERAL CONCLUSION 138 

REFERENCES 139 

BIOGRAPHICAL SKETCH 14 5 

iv 



Abstract of Dissertation Presented to the Graduate School 

of the University of Florida in Partial Fulfillment of the 

Requirements for the Degree of Doctor of Philosophy 



MECHANISTIC AND SITE ISOLATION STUDIES 
OF TRANSITION METAL OXIDATION CATALYSTS 



By 

David Chappel Pribich 
May, 1985 



Chairman: Russell S. Drago 

Major Department: Chemistry 



Mechanistic work reported here involves the investi- 
gation of the oxidation of 1-hexene to 2-hexanone catalyzed 
by Rh( III )/Cu( II ) mixtures. A number of results are obtained 
that indicate that [Rh(CO) _C1] -, as a catalyst precursor must 
be converted to a rhodium(III) complex before catalysis 
occurs. The oxidation of the rhodium(I) precursor to the 
active rhodium(III) catalyst in the absence of copper is 
reported in detail. An unusual mechanism results which 
involves the in situ production of hydrogen peroxide from the 
alcohol solvent reduction of 0^. The hydrogen peroxide then 
oxidizes [Rh(C0)2ClJ2 to an unstable [Rh (CO) (OOH) ( ? ) ] 



intermediate. This oxidation occurs only in solvents 
capable of reducing 0-, 

Cofunctionalization of silica gel with trialkylchloro- 
silanes and (CH,0) ,SiCH_CH2CH2SRh(CO) 2 produces a catalyst 
for the oxidation of 1-hexene that can be compared with a 
catalyst that does not have an R-Si- covering. Physical 
methods are used to determine the concentration of groups on 
the silica surface and also to determine the loading at 
which site separation occurs. The alkylated supports can be 
loaded with a greater concentration of the site-isolated 
rhodium complex producing catalysts which have greater 
activity per gram of catalyst and longer lifetimes than the 
corresponding catalysts produced with non-alkylated silica 
gels. Increasing the length of the alkyl group used to cover 
the surface decreases the effectiveness of the silica as a 
catalytic support and eventually leads to a catalyst surface 
that is not wetted by ethanol solvent. 

A three nitrogen, two oxygen salen-type ligand was 
successfully synthesized on silica gel. Cobalt, copper, 
manganese and iron metal ions were incorporated into the 
supported ligand system. The iron(III) and manganese( II ) 
systems achieved oxygen atom transfer from iodosylbenzene to 
a cyclohexene substrate producing different product ratios 
than homogeneous tetraphenylporphyrinato metal systems have 
yielded and are also the first supported systems to 
achieve this process. 



VI 



Several other results are reported, including the 
preparation of an air sensitive iron(II) silica gel bound 
complex, incorporation of copper into the silica gel matrix, 
preparation of a silica gel based anion exchange resin, and 
examination of titanium carbide as a catalytic support. 



Vll 



CHAPTER I 
GENERAL INTRODUCTION 



The efficient conversion of ubiquitous substances on 
this planet into other useful species is a desirable and 
perhaps even honorable goal which correspondingly attracts 
the efforts of large segments of the scientific community. 
Specifically, a subset of this broad area is the efficient 
catalytic use of naturally occurring dioxygen via interaction 
with transition metal centers. The following chapters will 
describe investigations which in some way involve this area 
of research. A rather detailed study on the formation and 
nature of a specific homogeneous rhodium oxidation catalyst 
is presented in the second chapter. 

The general interest in the Chapter II catalyst led to 
work which supported the catalytic system on an inorganic 
polymeric solid. Subsequent efforts to maximize the 
efficiency of this supported system while gaining detailed 
information on the support medium itself are represented in 
the third chapter. 



2 

A general broadening of study to include different 
metals, ligands, substrates and even solid supports led to 
further research; the various results of which are detailed 
in the fourth chapter. 

Specific background information and introduction is 
included with each topic as it is discussed. The general 
introduction provided by this brief overview should then 
allow comprehension of the pattern of research on 
interrelated topics presented in the following. 



CHAPTER II 

THE SPECIFIC OXIDATION OF [Rh(CO)-Cl]- 

BY Op VIA THE COORDINATION OF IN ^ITIT 

GENERATED HYDROGEN PEROXIDE 



Introduction 

Homogeneous catalysis has developed rapidly during the 
last twenty years as organometallic chemistry has been firmly 
established as a major discipline within inorganic chemistry. 
The search for new and more efficient catalysts was 
intensified as the finite nature of the crude oil supplies 
which provide the feedstocks of many important chemical 
processes was more closely realized. Oxidations have 
specifically received much attention; there have been 
estimates that an oxidation step is involved in the 
production of over 50% of the chemicals manufactured 
industrially. Water, alkylperoxides and dioxygen are 
frequently used as oxidation reagents. Dioxygen is the most 
desirable one, not only due to its obvious availability, but 
also because of its favorable energetics. 



4 
Homolytic and heterolytic are the two major categories 

into which metal catalyzed homogeneous oxidations have most 

2 3 

often been divided. ' Homolytic oxidations are those in 

which radical intermediates are produced during the oxidation 
process. However, the drawback of most radical processes is 
a lack of product selectivity, and that is also a major 
problem in these oxidations. One reason for this lack of 
specificity is that in many homogeneous metal catalyzed 

oxidations of hydrocarbon by 0- the metals are involved in 

4-9 

just the first step of reaction. These metals decompose 

peroxides and thus initiate free-radical autoxidations. 
These free radicals then react with substrates such as 
olefins leading to the formation of several products. 

Heterolytic oxidations, also known as nonf ree-radical 
oxidations, involve binding of a substrate to a metal center, 
rearrangement, and later release of products. The Pd/Cu 
catalyzed oxidation of ethylene to acetaldehyde in the 
presence of water (the Wacker process) is one well-studied 
example. The systems which undergo this type of oxidation 
have been the focus of much research aimed at adjusting the 
metal centers in order to improve catalytic activity. This 
has been especially true since the discovery of Vaska's 
complex, Ir ( P{CgHc ) ^) j^COCl, and the realization that it is 
able to reversibly bind dioxygen. This "oxygen-atom 
transfer process" does not involve free radicals and results 
in product specificity. Since Vaska's complex was discovered 
several 0- oxidations of non-organic substrates have been 



12-19 
judged to occur by the 0-atom transfer process. General 

knowledge of this type of reaction is largely dependent upon 

the ability to procure detailed mechanistic information. A 

mechanistic investigation of one reaction will be presented 

in the following sections. 



Background 

Considering specifically rhodium catalyzed oxidations 
of hydrocarbons by dioxygen, several examples have been 
reported which seemingly do not proceed by either free 
radical or simple Wacker processes. One of these is 

the rhodium/copper co-catalyzed oxidation of terminal olefins 

20 
to 2-ketones with > 98% specificity (reaction 1). 



2CH2=CHR +02" 2CH^C(0)R (1) 



The Wacker-Smidt process refers to the palladium( II )/- 

copper (II) co-catalyzed oxidation of olefins to ketones and 

? S — 2 fi 

aldehydes using water as the direct oxidant. 

Palladium(O) is produced as an intermediate in this process, 
and the copper(II) is necessary to oxidize it back to Pd(II), 
the active catalyst. Only a stoichiometric reaction occurs 
and Pd(0) metal is produced if no Cu(II) is used. The 
general steps of the mechanism and the net reaction are shown 



6 

below (reactions 2-5) for the catalytic oxidation of ethylene 
to acetaldehyde . 



Pd(II) + H^O + CH2=CH2 - Pd(0) + H^CCHO + 2h"^ (2) 
Pd(0) + 2Cu(II) - Pd(II) + 2Cu(I) (3) 

2Cu(I) + 1/2 0^ + 2H"^ -^ 2 Cu(II) + H2O (4) 



Net: CH2=CH2 + 1/2 0^ -* CH^CHO (5) 



Water is the direct oxidant here, even though 0^ is consumed. 
Dioxygen is reduced to H„0 by Cu(I), which reforms the Cu(II) 
necessary for oxidizing Pd(0) to PdCII). Other processes 
which are thought to proceed in a similar manner are referred 
to as having a "Wacker" mechanism. 

Mimoun and co-workers put forth great effort to rule 
out a Wacker-type mechanism for reaction 1. Several 

experimental observations could not be rationalized by a 

20 
Wacker mechanism. Thus, despite apparent similarities 

between Mimoun 's system and the earlier rhodium/copper 

catalyzed system which is reported to have a Wacker-type 

27 
mechanism, the differences are great enough to rule out the 

hydrolysis process found in Wacker chemistry for Mimoun 's 

system. 

Mimoun proposed that the catalyst in the Rh( III )/Cu( II ) 

oxidation system is a rhodium(I) complex produced from the 

ethanol reduction of RhCl, and Cu(II) in the initiation step 

shown in reaction 6. 



RhCl^ + 2CH2=CHR + Cu(C10^)2 + 1.5CH CH-OH ^ 

[Rh(CH_=CHR)-]C10. + CuCl. , + HClO, + 2HC1 + 
2. 2 4 ( S ) 4 

1.5 CH^CHO (6) 



The observations which lead to this proposal by Mimoun 
are the following: (1) active catalysts are formed by using 
[Rh(CgH, . )2C1]2 in the presence of 2 equivalents of HCl 
instead of RhCl,'3H_0; (2) an amount of acetaldehyde 
approximately equal to the moles of catalyst used is produced 
from ethanol oxidation at the beginning of the catalytic 
reaction; and (3) 85% of the Cu(II) precipitates as CuCl at 
the beginning of the catalytic 1-hexene oxidation. 
Initiation of the catalysts would then be possible by the 
binding of dioxygen to the reduced rhodium species. From 
reaction 6 it can be seen that there are at least two 
possible needs for an equivalent of Cu(II) to produce the 
best catalytic conditions. The copper(II) is either 
necessary for the reduction of RhCl,'3H~0 to Rh(I) with the 
stoichiometric production of CuCl, or for the removal of 
chloride ion from solution to yield a more unsaturated 
rhodium center. A combination of both roles for copper is 
possible, and Mimoun did not eliminate some other possible 
need for copper. 

The mechanism Mimoun proposed involves a metal centered 
oxygen atom transfer involving coordination of dioxygen and 



8 

olefin to the rhodium(I) cation from reaction 6, a 
rearrangement to a peroxometallocycle, and then a decom- 
position to the rhodium{I) complex and the oxidation 
products. Figure 1 shows this mechanism. It was based upon 
Mimoun's suggestion that the active catalyst was a rhodium! I) 
complex that led to the preparation of a silica gel organo- 
sulfide supported rhodium! I) complex. This heterogenized 

complex produced an active and stable catalyst for the 

2 8 
oxidation of 1-hexene to 2-hexanone (vide infra, Chapter 

III) . 

Results and Discussion 



Characterization of the Catalyst as a Rhodium! Ill) Chloride 
Complex 

The rhodium carbonyl dimer (A), [Rh(CO) _C1 ]2 » was 

selected as the catalyst precursor due to the facile loss of 

the CO ligands in the presence of 0-,, the presumed need for a 

rhodium(I) catalyst to coordinate 0_ and olefin, and the 

usefulness of the CO ligands for infrared analyses. 

Employing A as a homogeneous catalyst for reaction 1 at 

various chloride concentrations and in the presence of Cu!II) 

at 70 C, the data in Figure 2 were obtained. Note that the 

maximum initial 1-hexene oxidation rate ! indicated by the 

amount of 2-hexanone produced) is achieved using a 

chloride/rhodium mole ration of 3:1, as was originally 



Figure 1. The mechanism proposed by Mimoun to account 

for the Rh(III)/Cu(II) catalyzed O2 
oxidation of 1-hexene to 2-hexanone. 



10 



RhClj 



♦CHjCHjOH 

♦RCH«CH2 
♦CuX 



-CHjCHO 
-2HCI 
-Cvja 

R-CH.CHi 

*" \ *ljrRh'x(R-CH=CH2) 



I 



HY 



H;;C-R 



XRhVI 




R-CH«CH2 \ I 

XRh - OH 

Y 



m 

•XRh« 



x.cio;©! N05 
Y.cr or cio; 




0\0 R 
XRh ) ^C 



-t-HY 






d) 


0) 


c 


c 





0) 




s X '^-1 


3 Q) 


o 


•H 4J i= 




TJ M 1 


(U 


^rH 


o 


J2 r^ 


c 


v^ (0 M-i 


0) 


\-U 


tn 


(U (0 


0) 


TJ O C 


u 


•H 


a 


>-l 73 •'-• 




O C -U 


0) 


i-H (0 (t 


£ 


x; 13 


-u 


U W -H 




OJ X 


c 


OJ jj 


•H 


J= «5 




4J U CN " 


O 


(N • 


O^r-I 


o - 


C (0 0) 


M 


■H -r^ SZ 


•H M 


U) JJ -P 


05 -' 


(0 -H 


a 3 


(1) C M 


u 


i-l -H 


CN 


U 4-1 


in 4-1 


C (1) 





■H £ CO 


rs 


4J 0) 


c -u 


4-1 -H 


(0 c 


C 4J 


0) 


O H 


U r-i 


-(-> rH 


(0 


O O -H 


o > 


0) -H J3 


r^ -H 


4-1 -P (0 


3 


4-1 (0 -U 


-U CT 


W k^ CO 


nj 0) 



OI 



0) 
M 



12 




9uou^x9^-z »touuuj 



13 
reported for this system at 40 C. However, increasing the 
chloride/rhodium ratio to 5:1 at 70 C causes a considerable 
increase in catalyst stability with little change in the 
initial rates, in contrast to the result obtained at 40°C, 
where a large drop in activity was found for the 5:1 ratio. 
(For example, after 20 h at 70°C with a 3:1 Cl/Rh mole ratio, 
the catalytic activity is only 7.5% of the initial value, 
while after the same time period with a 5:1 Cl/Rh ratio the 
activity is 34% the initial rate.) Increasing the 
chloride/rhodium ratio to 10 causes an even further increase 
in catalyst stability at 70°C (the activity after 20 h is 51% 
of the initial rate). A similar beneficial effect of 
increasing the Cl:Rh ratio to 5:1 was observed in analogous 
experiments with Cu(II) absent. This suggests that a 
specific interaction of chloride with rhodium and not copper 
leads to catalyst improvement. Furthermore, the marked 
dependences of both initial rate and catalyst stability on 
such large chloride concentrations suggest a rhodium( III) - 
chloride interaction. These results, along with others to be 
presented, indicate that a rhodium(III) chloride complex is 
the catalyst or the immediate precursor. 

Experiments were conducted investigating the effect of 
dioxygen pressure on 1-hexene oxidation at 70 C. The use of 
[Rh(C0)2Cl l-f H2SO. and NaCl to catalyze the production of 
2-hexanone from 1-hexene can be accomplished in the absence 
of Cu(II). These results, along with the existence of an 
induction period, are shown in Figure 3. Also demonstrated 



Figure 3. Demonstration of the dependence of the length 
of the induction period on the pressure of 
Oj, in the absence of Cu(II) at 70 C, 
using [Rh(CO)2Cl]2 as the precursor. The 0- 
pressures are for A, 3.7 atm; B, 1.9 atm; C, 
1.4 atm; and D, 1.0 atm. 



15 




40 



80 



120 



hours 



16 

is the first-order dependence of the induction period on 
dioxygen pressure. During the induction period the solution 
color changes from light yellow (the color of [Rh( CO ) -,C1 ] -, ) 

to bright orange (normal for rhodium(III) chloride 

29 
complexes). No induction period is seen when RhCl,'3H_0 

is used rather than A, nor when 0.1 equivalent or more of 

Cu(II) is added to the reaction mixture. A proposal 

suggested by these experiments is that of the dioxygen 

oxidation of [Rh(CO)_Cl]^ to a rhodium(III) chloride complex 

(which is either the active catalyst or an immediate 

precursor) during the induction period. 

Further investigation of this system involved the 

reaction of A, dioxygen, and HCl in ethanol, at 40 C in the 

absence of 1-hexene substrate. A bright orange complex (C) 

was isolated as RhCl^ ( H2O) 2 •CH-CH-OH after 12 hours of 

reaction. This process is described by reaction 7. 



[Rh(CO)2Cl]2 + 4HC1 + 20^ + 4CH2CH2OH -^ 



2RhCl (H20)2*CH2CH20H + 2CH2CHO + 4C0 (7) 



The rhodium product (C) was characterized by elemental 
analysis, molecular weight determination, and its infrared 
and visible spectra. The stoichiometry for reaction 7 was 



verified in both [Rh ( CO ) ^Cl ] -, and HCl, but this was not 
possible for O2 , CH,CH20H, and CH,CHO due to the catalysis ol 
reaction 8 by C (vide infra). The addition of an equivalent 



17 
2CH CHROH +0- ^ 2CH2CRO + lE^O (8) 

R = CH^ or H 



of Cu(II) or the use of [Rh(CgH, . ) -CI] _ as the rhodium(I) 
precursor considerably speeds up reaction 7 (vide infra) but 
does not affect its outcome. Compound C catalyzes the 
oxidation of 1-hexene to 2-hexanone as efficiently as 
RhCl^'SH-O and without the occurrence of the induction period 
observed when using [RhCCO-Cl]^ as the catalyst precursor 
under identical conditions (see Figure 3). The observed 
solution color changes and other results described above 
indicate that under reaction conditions for the catalytic 
oxidation of 1-hexene (reaction 1), the rhodium(III) complex 
(C) is produced from the oxidation of [Rh(CO) -CI ]- • An 
explanation for the role of copper (II) and a more detailed 
study of reaction 7 will be discussed in the second part of 
this chapter. 

To rule out the possibility that RhCl, -SH-O (or C) may 
be subsequently reduced to a rhodium(I) complex by alcohol 
solvent to initiate catalysis (as proposed by Mimoun in 
reaction 6), we investigated the initiation step using 
RhCl^ 'SH-O and Cu( NO- ) 2 "2 . SH-O as precursors. For our study 
we chose isopropyl alcohol as solvent since upon oxidation 
this alcohol forms acetone which is much easier to 
quantitatively measure than the more volatile acetaldehyde 
(produced from ethanol). Using GLC, we found that in the 



18 

presence of 1 equivalent of Cu(II) only 0.5 equivalents of 
acetone is produced immediately on mixing all reagents 
necessary for the catalytic oxidation of 1-hexene and that 
its concentration remains constant for at least 0.5 h. From 
this result we can rule out the simultaneous reduction of 
Rh(III) and Cu(II) proposed in reaction 6 because this would 
require the oxidation of 1.5 equivalents of isopropyl 
alcohol. We propose that reaction 9 initiates the catalytic 
cycle for this system 



~, 1 atm d_ at 40°C 

Cl" + Cu + 0.5(CH-,)^CHOH ^ 



3 2 isopropyl alcohol 

1-hexene 

CuCl, , + 0.5(CH^)„CO + h"*" (9) 



when RhCl^>3H„0 is used as the catalyst precursor. Our 

proposal requires that only enough reducing equivalents are 

2 + 
provided by isopropyl alcohol for the reduction of Cu to 

form CuCl. Copper(I) chloride may be isolated from the 

reaction mixture in 85% yield without affecting the catalytic 

oxidation. 



19 

Mechanism of the ^ Oxidation of [Rh(CO) ^ Cl] ^ to RhodiumC III ) 
Chloride 



The oxidation of [RhCCO-Cl]. (A) to rhodium(III) 
trichloride (C) by 0„ was originally studied because of our 
interest in characterizing the active catalyst for the 
Rh/Cu-co-catalyzed 1-hexene oxidation (reaction 1). 
Copper(II) was excluded in these initial investigations to 
facilitate the interpretation of the electronic absorption 
spectra, elemental analysis and molecular weight data. The 
substrate 1-hexene was excluded from these solutions to avoid 
its catalytic oxidation to 2-hexanone after the formation of 
the rhodium(III) chloride product. As described in the 
previous section, the exclusion of Cu(II) and 1-hexene during 
these studies proved to be quite useful for characterizing 
the rhodium catalyst. The oxidation of [Rh(CO)2Cl]2 to 
rhodium(III) chloride was generally monitored by electronic 
absorption spectroscopy in order to determine the reaction's 
end point. In the course of these studies it was found that 
at elevated 0_ pressures (3-5 atm) an unusual intermediate 
exhibiting a visible absorption band at 385 nm could be 
detected. This observation led to a detailed investigation 
of the mechanism of the 0~ oxidation of [Rh(C0)2Cl]2 to 
rhodium(III) chloride in reaction 7, and the results are 
presented below. 

The electronic spectral changes accompanying the 
oxidation of the [Rh(CO)2Cl]2 (A) to rhodium(III) chloride 
(C) in the absence of both Cu(II) and 1-hexene is shown in 



20 
Figure 4 as a series of spectra recorded over the course of 
the reaction. The growth and decay of an intermediate (B) 
with an absorbance at 385 nm is noted. The charge-transfer 
band beginning at the shortest wavelength is a result of A, 
while the band at 480 nm is produced by C. As shown in 
Figure 5, intermediate B is more stable in the presence of 
excess Bronsted acid (8HC10./A). The presence of two 
isosbestic points (at 377 and 435 nm) in both Figures 4 and 5 
suggests the formation and subsequent reactivity of only one 
intermediate. The isosbestic point at 377 nm is a result of 
the reaction of A to form B at the beginning of reaction 7 
(before much final product C has been formed). Early in the 
reaction the decomposition of B to C becomes quite 
pronounced, and this point disappears. The second isosbestic 
point emerges at 435 nm when all A has been consumed and 
results from the exclusive reaction of B to form C. 

The initial rate of formation of B was found by visible 
spectroscopy to follow the rate expression shown in eq. 10, 



-d[A] -d[B ] = k[H ][0-] (10) 

-d^ °^ -dt 2 



which is independent of the concentration of A. The 
zero-order dependence on [A] rules out a mechanism involving 
initially the formation of a rhodium hydride followed by the 

insertion of dioxygen to form a rhodium hydroperoxide 

31e 
intermediate, which occurs with K2 [RhH (CN) . (H-O) ] , and is 



u 
a) > 

J= -H o 

(NO 

C -. 0) 

•-^ o -o 

u - o 

C O (X 0) 



0) 

jC 

E 
O 



4J 

(0 r-l 

(NH 



0) 

0) 
3 



CJ c 

o 



a-' 



(0 
M 

O 
0) 

• a 



<u 



£ o 
o 



O 
tn 

•H 

0) 
C/3 



c 
o 

JJ 

IT3 
X! 



CI 

(0 o 

O X 



0) 00 
O a> 
G • 
0) O 
W 

cNQ cn 
O (0 H 



o 



O 

•H 

0) 

a 

3 
O 

x: 

CN 



(0 



0) 

3 
C7> 



22 




8Dueqjosqe 



0) 




J= Q) 1 




A-> x: u 




4J 0) 




e j= 




c -u 




U -H 




IM (0 




U -U 4J 




D> 3 (0 




c o J3 -a 




•H ■«* 




4J * M 




r-{ 4J trO 




3 (0 O 'J-i 




W ^ 




<U (NJ 13 




u^ a- 0) 




^ w 




m u uj 3 




VJ CNU 




4-> ^ C M 




O O (U 10 


• 


0) U rH 


■<* 


Qjw (t3 01 




w x: > c 


0) 


05 H 


u 


(D^— ' 3 -H 


3 


r-l CT-iJ 


Oi 


ja >4-i 0) H 


•H 


.-to XJ 


Ct, 


U3 00 C 




•-• C 


c 


> y-i o 


•H 


•H 




U-l 4J 0) tj 


n3 0) e 


0) 


TD U (0 


Jj 


Wl -H C 0} 


c 


0) X OJ 


QJ 


— I W (1) 


w 


U <U U3 


<u 


0) CNftJ -H 


^ 


CO o a ? 


a 



m 



0) 

0» 



CM 



24 



o 

iD 
CD 



u 




o 
If) 



s 



o 

CO 



E 

c 



CO 



sDueqjosqe 



25 

32 
proposed in two other studies. The rate law in eq. 10 is 

surprising because it indicates that the initial step or 

steps in reaction 7 (those including and preceding the 

rate-determining step) involve a reaction between H , 0~, and 

possibly CH^CH-OH, forming an intermediate oxidant which 

subsequently reacts with [Rh(CO) -CI] „ . 

We investigated the possibility that peroxide could 

play the role of direct oxidant in reaction 7. This was 

confirmed by experiments involving the titration of A with 

HOOH to form C in the absence of dioxygen, monitored by 

visible spectroscopy (reaction 11, in which S' is solvent). 

[Rh(CO)2Cl]2 + 4HC1 + 2H00H + S' -»■ 

2RhCl2(H20)2*S' + 4C0 ' (11) 
Because HOOH slowly disproportionates or is reduced in 
ethanol, as well as being consumed in a competing side 
reaction (vide infra), the oxidation of A to C in this 
solvent goes to only 70% completion. Very significantly the 
oxidation of A by HOOH proceeds with the formation of the 
same intermediate [B] as produced with 0_ as oxidant, 
indicating that reaction 11 and reaction 7 occur through 
similar mechanisms. The kinetic rate law for reaction 11 was 
determined by using Fourier transform infrared spectroscopy 
(FT-IR) to monitor the consumption of A, and is shown in 
equation 12. Several reports describe the reaction between a 



26 

-d[A]/dt = k( iRh(C0)2Cl ;2) [HOOH] (12) 



metal complex and peroxide to form a coordination 
compound. The reaction in eq . 11 proceeding as 
described above would exhibit the rate law shown in eq . 12. 
On the basis of these arguments and those that follow, we 
propose that intermediate B results from the coordination of 
HOOH to [Rh(CO)2Cl]2- A proton NMR analysis at -70°C showed 
no evidence for B possessing a hydride ligand. Any hydride 
species generated from the oxidative addition of HOOH to A 
apparently are acidic, and fast exchange of all protons with 
the alcohol solvent is very likely occurring. 

The characterization of B was assisted by an FT-IR 
spectroscopic study of reaction 11. Immediately on adding 
HOOH to a solution of A and HCl in ethanol, the formation of 
a complex exhibiting a CO stretching band at 2102 cm is 
observed, and this band increases in intensity at the expense 
of the bands due to A (at 1995 and 2069 cm" ; Figure 6). The 
growth and subsequent decay of the CO band at 2102 cm was 
found to correlate directly with the band due to B at 385. nm 
observed in the electronic absorption spectra. Therefore, 
intermediate B retains one CO ligand. The assignment of the 
band at 2102 cm to coordinated CO was confirmed by use of 
CO in the experiment. 

That B is a hydroperoxo rather than a u-peroxo complex 
is evidenced by our ability to substitute tert -butyl 
hydroperoxide (t-BuOOH) for HOOH in reaction 11 to obtain a 



Figure 6. Series of infrared spectra obtained from the 
HOOH oxidation of [Rh(CO)2Cl]2 in ethanol. 
(A) Spectrum of [ Rh(CO) ^CI] ^ . Reaction was 
run using 0.84 x lO"^ M [ RhfcO) -CI] 2 • 
Spectra were recorded at (B) 0.25 hours; (C) 
0.55 hours; and (D) 1.83 hours. 



28 



rTl 



I'll'''' 



2900 2000 

«Mvt>4>«OQ 



29 

much slower reaction and the formation of an intermediate 

analogous to B with an electronic absorption band at about 

385 nm and an infrared CO stretch at 2099 cm"''", 3 cm""*" 

lower than that found for B with HOOH. Since t-BuOOH is not 

known to bridge two rhodium species in the y-peroxo 

configuration, we suggest that B is not a y-peroxo complex. 

Only a limited number of stable hydroperoxide and alkyl 

peroxide complexes of the platinum metals have been reported 

(not including those with Schiff base or bio-type ligands), 

and some of these are capable of oxidizing terminal olefins 

to 2-ketones. ' Intermediate B is only formed in the 

presence of HCl and is not formed on substituting either 

HCIO. or N(C^H_ ) .CI -H-O for HCl. Thus, both a proton and an 
4 2 5 4 2 ^ 

additional chloride are required in the formation of the 
intermediate. We propose that intermediate B is 
H„ [Rh(C0)Cl2- (OOH) ], produced as shown in eq. 13. Use of 
the oxidation 



[Rh(CO)2Cl]2 + 2H00H + 2HC1 



2H2[Rh(CO)Cl2(OOH) ] + 2C0 (13) 



state formalism to describe these species is potentially 
misleading. An oxidative addition of HOOH to a Rh(I) complex 
produces HRh OOH. Deprotonation generates Rh OOH . After 
a number of attempts, we have not been able to observe the 
0-0 stretch between 800 and 900 cm expected for a 
coordinated peroxo group in the infrared spectrum. This is 



30 

due to the low concentrations necessary to stabilize this 
intermediate, the poor window in this region of the infrared 
spectrum for ethanol solvent, and the hydrogen bonding in 
this system which would broaden this band. When the 
intermediate is generated with 0^ and excess 0^ is removed, it 
spontaneously decomposes to rhodium(III) chloride over 
several hours. This is consistent with the formulation of 
this intermediate as H- [Rh(C0)Cl2 (OOH) ] , for this species 
possesses two oxidizing equivalents in the peroxo (or 
hydroperoxo) ligand. 

Intermediate B is not formed in the absence of HCl in 
reaction 11, but the reaction in eq. 14 occurs under argon in 
ethanol solvent, 

Ar 
[Rh(CO)2Cl]2 + HOOH ^ 200^ + D (14) 

producing free CO- from the oxidation of a CO ligand and a 
very deep brown rhodium(I) complex (D) exhibiting a broad CO 
stretching band at 2048 cm" . Thus, [Rh(C0)_Cl]2 could be 
reformed from D by exposure to a CO atmosphere, and the 
production of CO^ from the CO oxidation by HOOH is catalytic 

in A in a CO atmosphere. This reaction has been previously 

34 
reported in benzene, where D precipitates. It is 

34 
reported that redissolving this solid in ethanol and 

exposing it to CO leads to the formation of a rhodium(I) 

carbonyl complex with a spectrum similar to A. Reforming 

[Rh(CO)2Cl]2 from D under CO is significant because it 



31 

indicates that D is a rhodium(I) complex and that the 
irreversible oxidation of rhodium(I) to rhodium( III) by HOOH 
in ethanol is slow relative to the oxidation of the CO 
ligands to CO2 (which takes several hours). These results 
further suggest that in the presence of HCl the reaction 
between A and HOOH produces the relatively stable rhodium(I) 
hydroperoxo coordination complex B as an intermediate. In 
the presence of HCl reaction 3 is faster than reaction 14, 
and the majority of HOOH is consumed to form B rather than 
CO- and D. That reaction 14 does occur to a small extent in 
the presence of HCl can be seen in Figure 6, in which the 
band at 2334 cm due to CO- is evident. This competing 
reaction accounts, at least in part, for the incompleteness 
noted for the titration of A with HOOH in reaction 11. 

The stabilization of B by excess Bronsted acid, 
evidenced by a comparison of the electronic absorption 
spectra in Figures 3 and 4, can be explained by considering 
the deprotonation of the RhOOH group as the initiation step 
for the reaction of B to form C. A further investigation of 
the decomposition characteristics of B also proved to be 
useful. Upon isolation of a mixture of B and C by quick 
evaporation of a dilute solution to dryness under vacuum, B 
decomposes within a minute to a wet solid. An infrared 
spectrum of this product revealed a large concentration of 
water in addition to a high-energy CO stretching band at 
2132 cm"-*- (shifted from 2102 cm"-"- for B). These are the 
results expected from the fast, autocatalytic decomposition 



32 
of a hydroperoxo complex. The reduction of the hydroperoxo 
ligand by Rh(I) will produce H-O and a rhodium(III) carbonyl. 
This oxidized metal complex will be much poorer at 
back-bonding into the CO than was the rhodium(I) in B, and 
therefore its stretching frequency would be shifted nearer to 
that for free CO (at 2143 cm ). Finally, in Figure 7 are 
presented data which illustrate the effect of increasing the 
rhodium concentration on the rate of decomposition of the 
intermediate. This rate was measured by monitoring the 
electronic absorbance for C at 480 nm. It is evident that at 
a threshold concentration in A between 0.98 x 10 M and 1.47 
x 10 M, the reaction of B to form C becomes autocatalytic 
in character. At 0.98 x lO""^ and 0.68 x 10~ M this reaction 
follows a much more regular course. These results indicate 
that a free radical decomposition of intermediate B is a 
sustained process only above the threshold rhodium 
concentration. Indeed, the electronic absorption spectra 
indicate that at much high concentrations, reaction 7 is less 
specific to the exclusive formation of C. In this case the 
band at 480 nm for C is present only as a plateau due to the 
absorbance of a secondary rhodium(III) product which absorbs 
at lower wavelengths. 

The characterization of intermediate B and the rate law 
in eq. 10 have an importance not yet discussed. They 
indicate that in reaction 7, hydroperoxide is initially 
formed from the Bronsted acid catalyzed reduction by dioxygen 
by ethanol or isopropyl alcohol solvents (eq. 15). The 



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35 



CH^CHROH + 02"^ CH^CRO + HOOH (15) 

R s CH^ or H 



oxidation of primary and secondary alcohols to aldehydes and 
ketones using dioxygen has been known for some time. ' 
These oxidations may be divided into two categories: (1) 
free radical initiated reactions, using asoisobutyronitrile 
(AIBN) for example, and (2) metal-catalyzed oxidations. The 

principal difference is that oxidations of the first type may 

9 
produce HOOH stoichiometrically (eq. 15) as long as 

precautions are taken to stabilize hydrogen peroxide while 

oxidations of the second type produce water (eq. 8), 

because the metals which catalyze HOOH formation also 

catalyze decomposition of hydroperoxides to HpO and O2 very 

efficiently. Thus, there is literature precedence supporting 

our proposal that HOOH is produced (albeit inefficiently) 

37 
with a Bronsted acid as a catalyst. 

Under our reaction conditions the HOOH concentration 

reaches a very low steady-state value, accounting for the 

slow initial rate of oxidation of A (eq. 7) and our inability 

to detect HOOH by iodometric techniques in acidic ethanol 

solutions under 0_ . However, we have obtained indirect 

evidence of HOOH production in two series of experiments in 

which H , Cu(II) and Rh(III) were checked for their ability 

to (1) speed up the oxidation of A (eq. 7) by producing HOOH 



36 
catalytically from the alcohol reduction of 0- and (2) 
catalyze the oxidation of isopropyl alcohol (eq. 8) as 
measured by acetone production. For the investigation of the 
effect of these three reagents on the rate of oxidation of A 
(eq. 7), we worked at 1 atm rather than at 80 psi of 0- 
(5.4 atm) because the reaction is much slower at this reduced 
pressure. As shown in experiment D in Table I, no final 
product is observed at 480 nm in the electronic absorption 
spectra in the absence of HCIO., Cu(II) or RhdII) after 1 
hour, and the reaction take 36 hours to come to completion. 
In contrast, the addition of 0.1 equiv of Cu(II) or Rh(III) 
at the beginning of the oxidation of A (eq. 7) results in a 
much faster reaction, with Cu(II) being most efficient 
(experiments A and B) . Doubling the acid concentration also 
speeds up the reaction but to a much lesser extent 
(experiment C). From these data it appears Cu(II) and 
Rh(III), and to a much lesser extent H , catalyze the 
production of HOOH from ethanol reduction of 0-, and this 
causes the increased rates observed for the oxidation of A 
(eq. 7) in their presence. 

Substantiation of this was found from our investigation 
of the effectiveness of Cu(II), Ru(III) and H in catalyzing 
the oxidation of isopropyl alcohol (eq. 8). Their 
effectiveness at the beginning of this reaction follows the 
order Cu(II) ^ RhdII) >>h"'', with rhodium resulting in 19 

turnovers (acetone/rhodium) in 25 h and H producing only a 

3 8 
trace amount of acetone. The consumption of dioxygen when 



37 
Rh(III) was used for reaction 8 was also followed on a gas 
burette for several hours and is linear over that time, 
indicating this catalyst is not slowly .rendered inactive. It 
is reasonable to suggest that HOOH, or peroxo metal 
complexes, are formed as intermediates in reaction 8 and that 
the efficient catalysis of this oxidation is evidence of 
the ability of a reagent to catalyze the production of 
peroxide from the alcohol reduction of 0_. Because both 
Cu(II) and Rh(III) are efficient hydroperoxide decomposition 
catalysts, the HOOH is produced at a very low steady-state 
concentration and may in fact never leave the coordination 
sphere of the catalysts. 

Experiment E in Table I demonstrates unequivocably that 
an intermediate oxidant (HOOH) is formed in the presence of 
only HCl, HCIO., ethanol, and 0-. This has been labled an 
"incubation" experiment because it involved stirring 3.2 x 

10~ M each of HCl and HCIO. in ethanol under 1 atm of 0- at 

4 2 

40°C for 48 hours, followed by the addition of the [Rh(C0)2- 
Cl]-j to initiate reaction 7. In this case upon adding A no 
induction period was observed and the reaction was complete 
in only 7 hours compared to the 36 hours necessary when 
mixing all reagents from the start. In agreement with the 
rate law in eq . 10, this further indicates that the first 
step in the oxidation of A involves the production of HOOH 
from H , 0- and CH^CH-OH and that hydroperoxide is the 
reagent directly responsible for the oxidation of [Rh(C0)2~ 



38 
Table I. Oxidation of [Rh(CO)2Cl]2 



Relative 

Absorbance Time for 

^ for Complex C Completion of 

Experiment Catalyst at 480 nm at 1.0 h Reaction 7 



A 0.15 mM Cu(II) 14 4 

B 0.17 mM Rh(III) 3 10 

C 4.2 mM HCIO4 0^ 14 

D None 36 



"Incubation"'^ 1 



These reactions were run at 40 C and 1 atm of 0_ , using 
0.80 X 10"^ M [Rh(CO) 2C1J2/ 3.2 x lO"^ M HCl , and 22.5 mL of 
ethanol as solvent, in addition to the catalyst listed. 

No final product II could be detected after 1.0 h. 

c -3 

This run involved stirring 3.2 x 10 M HCl and 

3.2 x lO""^ M HCIO^ in ethanol at 40°C and 1 atm of 0^ for 

48 h, followed by the addition of [Rh(CO)_Cl] to a 

-3 
0.90 X 10 M concentration. 



39 
Cl]~ to rhodium( III ) chloride in ethanol via a hydroperoxo- 
rhodium complex intermediate. Our proposed mechanism for 
this oxidation is shown in Scheme I and is substantiated by 
all of the preceding evidence and arguments. 



Scheme I 



CH^CHROH + O2 i::^ CH^CRO + HOOH (a) 



[Rh(CO)2Cl]2 + 2HC1 + 2H00H z::;!! 

2 [Rh(C0)Cl2(00H) ]^" + 4h"^ + 2C0 (b) 



[Rh(C0)Cl2(00H) ]^" + 2h'^ ::! [Rh(C0)Cl2 ( 00) ] ^" + SH"^ (c) 



[Rh(C0)Cl2(00) ]^~ + 4h'^ + CI + CH^CHROH -> 

RhCl2(H20)2-CH-jCHROH (d) 



An intermediate analogous to B is not produced when 
[Rh(CgH, . )2C1]2 is used instead of [Rh(CO)2Cl]2 as the 
rhodium(I) starting material in a reaction similar to that in 
eq. 7. In this case the reaction to form rhodium(III) 
chloride is complete in only 40 min (compared to 12 hours 
under identical conditions with A) , and no evidence for 
intermediates is found in the electronic absorption spectra. 
In contrast, [Rh( P( tolyl) ^ ) (CO)Cl ] 2 is not oxidized to 
rhodium(III) chloride even after 48 hours. 



40 



Since Scheme I requires solvent reducing equivalents to 
produce HOOH and subsequently [RhCCOCl- ( OOH) ] from , any 
solvent capable of this 0- reaction could lead to the 
oxidation of A by this mechanism. Indeed, we have found that 
methanol, ethanol, isopropyl alcohol, and to a lesser extent 
tetrahydrofuran (THF), all produce B as an intermediate in 
the oxidation of A by O2. In THF this could appear through 
the intermediacy of THF hydroperoxide, produced from the 
abstraction of an a -hydrogen atom by 0^ and a subsequent 
radical coupling reaction. The THF-hydroperoxide may react 
with [Rh(CO)2Cl]2 as do both HOOH and t-BuOOH. In contrast, 

use of the typically nonreducing solvents tert -butyl alcohol, 

39 
acetone, and N,N-diraethylf ormamide (DMF) does result in 

oxidation of [Rh(C0)2Cl]_ without forming B as a stable 

intermediate. In tert -butyl alcohol and acetone the reaction 

is very fast, finishing in 40 min under conditions for which 

it takes 10 hours for completion in ethanol. In DMF the 

reaction is much slower (also taking 10 hours) and was 

monitored by electronic absorption spectroscopy. In this 

solvent only one isosbestic point is observed (at 390 nm) due 

to the production of rhodium(III) trichloride from the 

oxidation of [Rh(C0)2Cl]2 without the formation of any stable 

intermediates (Figure 8). 

It is interesting that the oxidation of A (eq. 7) 

proceeds through the coordination of HOOH to [Rh(CO)2Cl]2 in 

primary and secondary alcohol solvents rather than proceeding 



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43 
by the alternative mechanism that seems to occur in tert - 
butyl alcohol, acetone and DMF. When THF is used as solvent 
the reaction is much faster and only poor resolution of the 
electronic absorption band due to B is observed. This is due 
to a lower concentration of B relative to that found in the 
alcohols, and possibly a low energy absorption band. In 
THF it may be that both mechanisms are functioning, which 
would explain the low concentration of B observed. It is 
important to note that we cannot rule out the alternate 
mechanism occurring in methanol, ethanol, and isopropyl 
alcohol solvents to a small degree. However, it is evident 
from the magnitude of the bands due to B in the electronic 
absorption and infrared spectra, as well as data from the 
kinetic rate law in eq. 10, that Scheme I involving the 
coordination of HOOH to [Rh(C0)2Cl]2 is dominant in primary 
and secondary alcohols. 

Conclusion 

It is usually necessary to include bio-type or 
Schiff-base ligands in order to prepare a hydroperoxide 
complex, which makes the characterization of B as H- [Rh- 
(CO)Cl- (OOH) ] noteworthy. The CO ligand seems to be 
responsible for at least some amount of the stability of this 
complex. It is this stability which does not lead to the 
characterization of the complex as an intermediate in the 
catalytic oxidation of 1-hexene to 2-hexanone. Complex B 



44 
appears to have only slight (if any) reactivity as a catalyst 
for 1-hexene oxidation. This result is not unexpected when a 
comparison is made between the rhodium(III) active catalyst 
and the rhodium(I) hydroperoxo complex, B. 

The fact that primary and secondary alcohols can form 
low concentrations of HOOH from the reduction of 0_ may also 
have several implications. Other metal complexes may have to 
be included with H , Cu(II) and Rh(III) as catalysts for the 
alcohol reduction of 0- to HOOH. Systems which require long 
term exposure to alcohol and 0_ may find the low 
concentrations of HOOH produced to be quite significant. 
Substrate oxidations of 0_ via HOOH formation may also be 
more prevalent than now realized. 

Further results have shown HOOH and t-BuOOH to both be 
effective reagents in the RhCl-'3H_0 catalyzed oxidation of 
1-hexene to exclusively 2-hexanone in the absence of 0.. The 
results are the same whether the solvent is ethanol or tert- 
butyl alcohol (which has no reducing equivalents). These 
facts provide still further justification for the character- 
ization of the active catalyst for reaction 1 as a 
rhodium(III) chloride complex. 

Experimental Section 

All solvents and reagents were of reagent grade and 
used without further purification. Literature methods were 
used to prepare [Rh(CO) Cl]_. Hydrogen peroxide and tert - 



45 
butyl peroxide were used as 30% and 70% aqueous solutions, 
respectively, and were standardized iodometrically. The 
acids HCl, H2S0^ , and HCIO^ were used as their concentrated 
aqueous solutions. 

Infrared spectra were recorded on a Nicolet 7000 Series 
Fourier transform infrared spectrometer. GLC spectra were 
obtained with a Varian Model 940 FID instrument using a 
3-m,l/16 in. i.d. copper column packed with Chromasorb P 
supported diethylene glycol adipate. For the detection of 
acetone a column temperature of 60 °C was employed, and the 
measurement of 2-hexanone was quantitated by using 
2-heptanone as an internal standard. The electronic 
absorption spectra were recorded on a Gary 14, and all 
samples were run in air at ambient temperature and pressures. 
Care was taken to verify that the intermediates monitored by 
this technique were stable over the course of the measure- 
ments. The molecular weight of C was determined in methanol 
by vapor pressure osmometry. 
Catalytic Oxidations of 1-Hexene 

All catalytic 1-hexene oxidations were run in 250-mL 
Parr pressure bottles equipped with bras Swagelok pressure 
heads. These were constructed to allow purging with 0~, as 
well as sampling of the solution under reaction conditions 
during the course of the reactions. 2-Hexanone production 
was measured by GLC. 

A typical catalytic reaction was run as follows: to a 

250-mL Parr bottle were added 0.074 mmol of [ Rh(CO) ^S ' ] BF . 

2 n 4 



46 

2 
(prepared as earlier reported and used immediately), 

O.OlVl.g of Cu(NO,)2'2.5H2 (0.074 nunol), 0.0219 g of NaCl 

(0.375 mmol for the case in which 5:1 mole ratio chloride/ 

rhodium was desired), 0.41 mL of 0.36 M H-SO (0.148 mmol as 

an ethanol solution prepared from aqueous concentrated 

H_SO. ), 0.568 mmol of 2-heptanone, 45 mL of absolute ethanol, 

and 15 mL of 1-hexene (purged through alumina to remove 

peroxides). This mixture was purged 5 times with 60 psi of 

0-, set to 40 psi of 0_ , and the reaction initiated by 

placing in a 70 C oil bath. 



Preparation of RhCl .^(H 0) CH CH OH (II) 

Compound C was prepared for characterization studies 
most easily under 40 psi of 0- at 70 C by mixing 0.0514 g of 
[Rh(CO)2Cl]2 (0.132 mmol), 1.10 mL of 0.48 M HCl (in ethanol, 
0.528 mmol), and 15 mL of ethanol. This produced a bright 
orange solution after a reaction overnight, from which C was 
isolated by rotovapping to dryness and drying in vacuo . The 
M^ for H^02Cl,Rh calcd. 245; found, 226. Anal. Calcd for 

^2"l0°3^-'-3^^* ^' ^-24; H, 3.46; CI, 36.50. Found: C, 8.41; 
H, 2.69; CI, 37.17. 



Determination of Acetone Production 

In Presence of 1-Hexene . The measurement of acetone 
produced in the initial stage of the Rh/Cu-catalyzed 
oxidation of 1-hexene with isopropyl alcohol as solvent was 
made by GLC as follows: To a 50-mL round-bottom flask were 



47 
added to 0.159 g of RhCl2"3H20 (0.605 mmol), 0.137 g of 
Cu(N0,)2 "2.51120 (0.589 mmol), and stir bar. This was purged 
20 min with O2 at 40°C and 30 mL of an 02-purged, 9/1 (v/v) 
solution of isopropyl alcohol/1-hexene added to initiate the 
reaction. GLC ' s were recorded after 4, 8, 17, 25 and 40 min. 
The amount of acetone produced was determined by comparison 
of peak heights with standards at the same time under 
identical conditions. When this 1:1 Cu/Rh ratio was used, 
0.5 mol of acetone was formed per mole or rhodium in the 
first 4-8 min. No further production was observed. When a 
2:1 Cu/Rh mole ratio was used, continuous, catalytic 
production of acetone was observed. 

In Absence of 1-Hexene . The catalytic production of 
acetone from the O2 oxidation of isopropyl alcohol was 
observed when 1-hexene was excluded from the solutions. Both 
RhCl,-3H_0 (0.140 mmol) and a stir bar were placed in a 15-mL 
round-bottom flask and purged 20 min with 0^ at 40 C. Into this 
was syringed 7 mL of isopropyl alcohol, purged itself with 
Op at 40 C, to initiate the reaction. Acetone production was 
measured as described above. 

Titration of [Rh(CO) 2 Cl] 2 with HOOH 

Because the reaction of HOOH with Rh(CO)2Cl2 (A) is 
quite slow in ethanol, it was run at 40 C. The visible 
spectrum of the intermediate, H2 [ Rh( C0)Cl2 ( OOH) ] (B), is not 
clearly observed by first adding 1.0 equiv of HOOH, followed 
an hour later by 0.5 equiv. The first addition causes the 



48 
reaction of much of the starting material A (which overlaps 

the band at 385 nm) , so that after the second addition the 

band due to C at 385 nm is easily observable in the 

electronic absorption spectrum. Intermediate B is easily 

detected in the oxidation of A by HOOH by using FT-IR. Since 

there is no overlap of the carbonyl bands of A and B, the 

intermediate is detected in the first addition of HOOH. 

Immediately after use HOOH was diluted in ethanol. Aqueous 

dilution causes the addition of too much H^O, which retards 

the reaction considerably. 



CHAPTER III 

ENHANCED SITE ISOLATION ON SILICA GEL 

AND IMPROVED LIFETIMES OF SITE ISOLATED CATALYSTS 



Introduction 

Functionalized polymers have been increasingly employed 
to support transition metal complexes as catalysts in recent 

years. There have been several reviews in the literature 

■1 • K *.u 4. 42-47 , . 

involving both organic supports and inorganic 

solids. The system of interest here involves 

functionalized supports which usually contain molecularly 

definable species. Such systems are largely in the minority, 

because they do not include either (1) the deposition of 

metals or metal oxides by the oxidation or reduction of metal 

complexes, or (2) the intercalation of metals in 

unfunctionalized inorganic oxides. 

The lack of physical methods which can routinely 

provide identification of surface supported metal complexes 

hinders the development of new catalytic systems. As a 

result, attempts to prepare supported catalysts are often 

based upon known homogeneous systems. The approaches for 



49 



50 

attaching metal complexes is often guided by the ligand 
system of the homogeneous catalyst. The methods which have 

been utilized include ligand exchange or substitution of the 

51 52 
metal complex to the support, ' addition to unsaturated 

metal complexes, ' and ionic attachment. ' 

Immobilization of metal complexes on solid supports can 

provide advantages over both homogeneous and heterogeneous 

catalysts. Included in these are (1) the ease of separating 

solid catalyst from the reaction mixture, which encourages 

the use of flow reactors, (2) the ability to greatly increase 

dispersion of the metal on the surface, allowing the use of 

less metal (and finances), and (3) reduced contact with the 

reaction vessel leading to reduced corrosion. This potential 

for gaining the advantages of both homogeneous and 

heterogeneous catalysts has many industrial applications 

which are being investigated. 

Following below is a brief discussion of the general 

properties of the inorganic solid, silica gel, which is used 

as a catalytic support in the research presented in this 

chapter. 

Silica gel has a surface which is very irregular 

consisting of hydroxyl and silicon bridging oxide groups. It 

is quite rigid and contains many channels throughout its 

structure and only the silica gel surface may be functional- 

ized. Silica gel samples which vary widely in surface area 

are available. Several characteristics of silica gel make it 

potentially preferable to organic polymers as a support for 



51 
metal complexes. The rigidity of its silicon oxide tetra- 
hedra is often an advantage for immobilizing and site 
isolating catalytic centers. The stability of silica gel at 
higher temperatures than organic polymers may also be 
important. Finally, the large number of organosilanes which 
are becoming commercially available and the mild conditions 
under which f unctionalization is achieved makes the number 
of possible silica supported moieties quite large. 

As mentioned above, organosilanes are employed to 
functionalize the surface of silica gel. It has been 
reported that the reaction proceeds by hydrolysis, hydrogen 
bonding and final bond formation as shown in Figure 9 as 
reactions a-c. The symbol [SG]-OH is used to represent 
unf unctionalied silica gel. 

An investigation of the nature of the binding of 

58 
organosilanes to the silica surface was recently reported. 

Before this study was reported, it was generally believed 

that three bonds from the silane to the silica suface were 

formed. However, in this report Waddell et al . state that most 

commonly only one bond is formed from silane to silica, while 

the formation of two bonds is possible, and tridentate 

bonding seems unlikely due to steric limitations. 

Another important characteristic of polymer bound 

metals is the ability to achieve site isolation of the metal 

complex on the polymer surface. Solid supports can function 

to avoid molecular aggregation and prevent the formation of 

multinuclear complexes or clusters. This usually requires 



Figure 9. General reaction scheme for the functional- 

ization of the surface of silica gel, 
[SG]-OH. 



53 



R^SiCl + H2O 



R^SKOH) + HCl 



(a) 



R^SKOH) + [SG]-OH ■* [SGJ-0 H OHCSDR^ (b) 



[SG ]-0— H OHCSDR^ ^ [SGI-OODR^ + H^O (c) 



54 
dilution of active sites on the surface in order to minimize 
contact of supported metal complexes, although another method 
has been successful at achieving site isolation ( vide infra ) . 
Most of the work reported in supporting metal catalysts has 
employed very large loadings of the polymeric surface with 
the desired functional group and metal complex. In these 
cases elemental analyses can often be utilized, which is 
advantageous since there are so few physical methods avail- 
able. However, this has led to a lack of research on systems 
with definite site isolation of active species, or possible 
improvement of existing catalysts by the establishment of 
site isolation. Rigid, non-flexible polymeric supports such 
as silica gel are usually required when attempts to obtain 
site isolation of a supported species are undertaken. 

There are several reports of functionalized silica gel 
being used to achieve site isolation, and perhaps other 
examples from industrial laboratories which have not reached 
any available literature. Among the reported examples are a 

bound imidazole iron tetraphenylporphyr in which is used to 

59 
reversibly bind dioxygen, and cyclopentadiene groups 

supported on silica gel which form stable mononuclear iron 

and cobalt carbonyl complexes. 



55 



Results and Discussion 

Polymer Support of the Catalyst 

The oxidation of terminal olefins to 2-ketones 
catalyzed by rhodium and copper was discussed in some detail 
in the previous chapter in relation to its homogeneous 
mechanisms. Here will be described an investigation to 
increase that catalyst's relatively short lifetime by 
attempting to provide an ideal supported environment for the 
catalytic species. Prevention of the suspected deactivating 
oxidative aggregation of the rhodium catalyst was the 
rationale for this approach. 

Required of a solid support is a solvent independent 
rigidity which could improve prospects for obtaining site 
isolation. Silica gel, unlike organic polymers such as poly- 
styrene, meets these requirements. Initial attempts at using 
an organosulf ide ligand on silica gel to bind the 
rhodium/copper cocatalysts were successful. Goals of 
improving the activity and lifetimes of supported catalysts 
while gaining a detailed knowledge of the silica surface 
precipitated the research described in this chapter. 

Silica gel supported organosulf ide samples (abbreviated 
[SG]-SH) were synthesized using ( 3-mercaptopropyl) trimethoxy- 
silane [ (CH-0) ,Si ( CH2CH2CH2SH )] reacted with plain silica 
gel. A simple model was used to help gain insight into the 
surface coverage on the silica gel. Approximating the 



56 
silicon-oxygen bond distance as two Angstroms and assuming 
tetrahedral coordination around silicon, then the distance 

o 

between oxygen atoms of the surface silanol groups is 6.5 A. 
In order to obtain an upper limit it is assumed that a flat 
surface exists and that all of the surface groups are 

silanols (-SiOH). Since the Davison Grade 62 silica gel 

2 
employed has a surface area of 340 m /g [SG], then there is 

-9 2 
approximately one surface hydroxyl group per 4.3 x 10 m of 

surface. Thus, it can be calculated that as an upper limit 

there are about one mmol of surface silanol groups per gram 

[SG]. With this value it can be calculated that the [SG]-SH 
samples prepared contain approximately 1/40, 1/20, 1/10 and 
1/5 mmol sulfide/mmol surface Si (S/Si) which correspond to 
0.025, 0.5, 0.10 and 0.20 mmol S/g[SG]. The number of mmol 
S/gLSG] was determined by electronic absorption for all 
samples as described in the experimental section and is 
independent of the model and calculations discussed above. 
The values for the number of mmol S/g[SG] varied only a small 
amount between preparations of functionalized silica gels. 
It was these experimental values which were employed to 
determine the amount of silica gel necessary for each 
catalytic oxidation. The gen-eral labels used to represent 

[SG]-SH samples of different sulfide loadings are shown in 
Table II. 

Also worthy of consideration is the relatively large 
potential surface area over which the supported rhodium 
complex may cover due to its attachment to a chain of six 



57 



Table II. Representations used to designate [SG]-SH samples 
of varying sulfide concentrations. 



SH Loadings on [ SG ] Surface 



0.025 mmole S/g [SG] 

0.05 mmole S/g [SG] 
0.10 mmole S/g [SG] 
0.20 mmole S/g [SG] 



1 surface -SH/ 40 surface 

-SiOH 

lS/20Si 

IS/lOSi 

IS/ 5Si 



58 
atoms. Despite not being linear, this chain still allows a 
380 square Angstrom area over which each rhodium may migrate, 
This number, along with the surface Si concentration, allows 
the calculation that the silica surface must have a 
concentration less than 1 S/g surface Si in order for site 
isolation of the rhodium to exist. This is a convenient 
number to which properties of actual [SG]-SH samples may be 
compared. 

Preparation of supported rhodium complexes on silica 

gel was achieved by utilizing freshly generated [Rh(CO)pS' ]■ 

6 2 
BF. (S' = solvent) in ethanol or tetrahydrof uran (THF) as 

shown in reaction 16. 



[Rh{CO)2Cl]2 + 2AgBF^ -^ 2 [Rh(CO) 2S ' ^ ]BF^ + 2AgCl (16 



This rhodium cation readily binds to lSG]-SH samples at room 
temperature producing silica gels varying from bright yellow 
to red-orange as the concentration of surface sulfide is 
increased. 

The carbonyl ligands in the rhodium complex bound to 
the silica gel allows the use of infrared spectroscopy to 
investigate the functionalized surface. In these studies 
freshly prepared [Rh(CO)-S' ]BF. (S' = solvent) was reacted 
with [SG]-SH sample under nitrogen and an infrared spectrum 
quickly taken. The supported rhodium complex produced on 
many dilute functionalized gels (e.g., lS/40Si) yields a 
two-band infrared spectrum (2055 and 2005 cm" ) characteristic 



59 

of [SGjSRhCCO-S' (S' = solvent). More concentrated 
[SGj-SH samples (e.g., lS/5Si) treated identically produce 

three-band spectra (2075(m), 2055(s), and 2005(s) cm"''-) 

5 2 
indicative of a supported dimer, ( [SG ] -S ) 2Rh2 (CO) . . These 

different surface species are represented in Figure 10. It 

seems very evident that concentration of surface sulfide 

groups past a certain point removes site isolation and 

permits supported rhodium dimerization. These trends and 

their implications will be discussed further in later 

sections . 

Catalytic Oxidations of 1-Hexene by Supported Complexes 

Silica gel organosulf ide supports ([SG]-SH) as 
described previously were used to immobilize rhodium and 
copper in order to investigate in detail the heterogeneous 
catalytic oxidation of 1-hexene to 2-hexanone (reaction 17). 



2CH2=CH(CH2)3CH2 + 02"^ 2CH2C ( CH2 ) 3CH2 (17) 



Electronic absorption (as described in the Experimental 

Section) was used to quantify the amount of sulfide in each 

silica gel sample. The amount of sulfide present determined 

the quantity of [SG ]-SH used as a support in each catalytic 

oxidation. Each oxidation to be discussed employs 0.074 mmol 

of both [Rh(CO)-S' ]bF, (S' = solvent) and Cu{ NO, ) ^ ' 3H-0. 
2 n 4 3 2 2 

The quantity of [SG]-SH which provides 0.16 mmol of sulfide 



T3 


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61 





62 
sites (a 10% excess) is used in each oxidation. Also added 
to each reaction were 2 equivalents of HCIO. (based on moles 
of rhodium), 15 equivalents of 2-haptanone internal standard, 
45 mL absolute ethanol solvent and 15 mL 1-hexene substrate. 
An apparatus of 250 mL Parr pressure bottles was used to 
conduct the experiments along with a pressure head which 
allowed the removing of aliquots of the reaction mixture 
under reaction conditions. Reactions were initiated by 
purging the pressure bottle containing all components of the 
reaction five times with 60 psi 0- , setting the pressure to 
40 psi 0^ and placing the apparatus in a 70 C oil bath. The 

HCIO. was used because Mimoun found a Bronsted acid to be 
4 

necessary when using rhodium(I) in his homogeneous 
investigation, and it is given a role in his proposed 
mechanism. Gas-liquid chromatography (GLC) was employed to 
follow the catalytic oxidations. A calibration curve of 
2-hexanone product versus 2-heptanone standard was used to 
calculate 2-hexanone production in millimoles. 

In preparing [SG]-SH supports, if it is assumed that 
all of the added ( CH^O) ^SiCH-CH-CH-SH reacts completely, one 
obtained values for [SG]-SH of 0.025, 0.050, 0.100 mmol 

S/g[SG] corresponding to lS/40Si, IS/ 20Si, and IS/lOSi, 
respectively. The analysis for sulfide (using a rhodium 
complex and electronic absorption as in the Experimental 
Section) for a series of [SG]-SH samples produces values of 
0.020, 0.041, and 0.076 mmol S/g[SG]. These values are 
reasonably close to those expected for complete reaction and 



63 
also have relative ratios to one another very similar to 
those which assume complete reaction. Again, it is the 
experimental values for sulfide content which are used in 
determining the quantity of [SG]-SH necessary for each 
catalytic oxidation. 

Results of the heterogeneous catalytic oxidation of 
1-hexene to 2-hexanone are most clearly displayed as graphs 
of mmol of product (2-hexanone) versus time (in hours). 
Figure 11 shows the oxidation results characteristic of three 
[SGJ-SH samples which vary in sulfide loading. The samples 
are referred to by a ratio of sulfide to surface silicon 
atoms in the silica gel. These values are obtained by 
assuming complete reaction when f unctionalizing the gels. 
However, the experimental sulfide concentration values were 
always used when determining the mass of silica gel necessary 
for reaction. Each curve in the oxidation graphs represents 
many experiments repeated with the same supply of 
f unctionalized silica gel. Also, other samples were prepared 
and compared to previous gels. The experimental sulfide 
concentrations were very similar between silica gel batches. 
The idealized S/Si ratios are then used for convenience in 
referring to several different supplies of similar silica 
gels . 

In Figure 11, if all of the rhodium and copper is 
effectively site isolated on these samples, then very similar 
activity should result from each of the catalysts. The 
lS/40Si (0.025 mmol S/g[SG]) sample, curve A, is less active 





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CO CO to 
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auouoxs^-^ loujuj 



66 

for 2-hexanone production than the lS/20Si and IS/lOSi 
samples (0.05 and 0.10 mmol S/g[SG]). This results from 
diffusion problems which will be discussed further later in 
this chapter. Curves B and C do show similar activity for 24 
hours as shown. Catalytic activity resulting from immobili- 
zation of the rhodium and copper complexes on silica gel 
certainly has been demonstrated, and several comparisons to 
these results will be made in following sections. 

After establishing supported activity, it is important 
to check for leaching of the metal complexes from the solid 
into solution. Catalytic oxidations were run for three hours 
and the solutions removed by syringe under reaction 
conditions. The filtrate, after measuring the volume, was 
then placed in a new Parr bottle with another equivalent of 
Cu(II) and the oxidation re-started in the original manner. 
The filtrate had activity ranging from 5 to 15 percent of the 
total normal supported activity depending on the silica gel 
sample being studied. Since the homogeneous rhodium/copper 
oxidation catalyst is much more active than the corresponding 
supported system, then it is apparent that the vast majority 
of the heterogeneous catalytic activity is due to [SG]-SH 
bound catalyst. 

It is relevant to stress that experiments to check for 
leaching must be done under reaction conditions. The 
chemical equilibria involved in leaching may be greatly 
affected by conditions milder than those during reaction and 



67 
thus leaching should be investigated only under reaction 
conditions. 

Alkyl Covering of the Silica Surface to Improve Site 
Isolation and Catalysis 

The silica gel supported catalytic system as described 
until this point offers several characteristics which make it 
ideally suited for further investigation of site separation 
in a quantitative manner. Already demonstrated has been a 
method for determining the surface loading of organosulf ide 
functional groups, a spectroscopic technique for establishing 
the identity of silica gel immobilized rhodium species, and 
an oxidation reaction which can accurately measure catalytic 
activities and catalyst lifetimes. Thus, any further 
attempts to alter the silica surface may be monitored rather 
precisely. 

The general supposition that increased site isolation 
of the silica gel supported rhodium species is directly 
related to increased catalytic activity is supported by the 
results obtained using [SG]-SH as the support. Very highly 
loaded [SG]-SH samples produce catalytic systems of very low 
activity. Further manipulation of the silica gel supports in 
order to produce better and/or longer lived site isolation 
was greatly desired. It was assumed that the general 
deactivation of the supported catalysts was dependent to some 
degree upon the aggregation, over time, of rhodium species on 
the silica surface. A mechanism could be logically 
postulated whereby sulfide bound rhodium species exchange 



68 

with surface silanol protons and thus migrate along the 
surface until aggregation occurs. As a result, a 
modification of the silica surface was sought which could 
remove this possible path of aggregation. One other goal was 
to obtain a method for increasing the loadings of site 
isolated species per gram of silica support. 

The functionalization of unused surface silanol (-SiOH) 
groups with alkylsilane moieties results in an effective 
method for achieving the previously stated goals. Chloro- 
alkylsilanes are utilized to produce alkyl groups bound to 
the silica surface. The reaction of the alkylchlorosilanes 
with the silica surface is identical to the surface reaction 
with ( 3-mercaptopropyl ) trimethoxysilane as described in 
reactions 1-3. The chloro group here reacts as does the 
methoxy group in those reactions, forming HCl instead of 
CH,OH. Chlorotrimethylsilane, chlorotriethylsilane and 
chlorotripropylsilane were each used to prepare silica gel 
samples, along with the same ( 3-mercaptopropyl ) trimethoxy- 
silane in each case. Samples prepared with alkyl groups and 
the sulfide functionality on the silica surface will be 
generally referred to as alkylated [SG]-SH, while those with 
only sulfide will be called either non-alkylated [SG]-SH or 
merely [SG]-SH. The f unctionalized silica gels will also be 
identified by a ratio of surface sulfur to surface silicon 
(S/Si) as described previously (e.g., IS/lOSi, lS/20Si). The 
alkylated gels will also be referred to by a percent 
alkylation which is the percent of all unused surface silicon 



69 
atoms reacted with chloroalkylsilane assuming complete 
reactions. The assumption of complete reaction is generally 
supported by elemental analyses for carbon which became 
possible at levels of substantial alkylation. 

Quantitative analyses for sulfide were also done for 
alkylated [SG]-SH samples. The values of mmol S/g [SG] for 
these were generally slightly lower than for their 
non-alkylated analogs. Again, the values obtained for 
sulfide content were used to determine the necessary quantity 
of silica gel for catalytic oxidations. 

Infrared spectroscopy provided a good deal of 

information concerning the properties of the alkylated [sg] 

-SH gels. As described earlier, reaction of Rh(CO)_S' BF. 
^ 2 n 4 

(S' = solvent) with [SG]-SH samples provides species whose 
infrared spectra are quite useful. Rhodium species bound to 
lS/40Si and lS/20Si [SGJ-SH gels produce the two band 
infrared spectrum indicative of the monomeric supported 
complex using both non-alkylated and alkylated [SG]-SH 
samples. The IR spectrum which results using IS/lOSi 
non-alkylated [SG]-SH indicates the presence of primarily 
dimeric rhodium complex on the silica surface. This 
generally supports the estimation discussed earlier in this 
chapter that the silica surface must be on the average about 
lS/9Si or more dilute in order to produce a system in which 
most of the rhodium is site isolated. The alkylated [SG]-SH 
gels (e.g., 80% methylated) produce the monomeric rhodium 
complex on the surface as indicated by their infrared 



70 

spectra. The alkyl surface covering allows the formation of 
a different surface species than its non-alkylated analog at 
identical loadings of metal per gram of silica gel. A 
mixture of monomeric and dimeric rhodium complexes is present 
on the surface of 80% methylated lS/5Si [SG]-SH, while 
primarily the dimer is produced on 80% methylated lS/2.5Si 
[SG]-SH. These infrared spectra are shown in Figure 12. It 
may then be concluded that the alkyl covering cannot prevent 
rhodium dimerization on the silica surface past a certain 
level of metal loading on the silica gel. 



Catalytic Oxidations Using Alkylated Silica Gels as Solid 
Supports 



The experimental procedures for conducting catalytic 
oxidations using alkylated [SG]-SH supports are identical to 
those for the non-alkylated systems described in Section B.2. 
Again, the most effective way to present data for these 
catalytic oxidations is in the form of graphs of millimoles 
of 2-hexanone product versus time as measured in hours. 

The oxidations obtained using [SGj-SH with 50% of its 
available surface covered with trimethylsilyl group (50% 
methylated) as supports for the rhodium and copper 
cocatalysts are shown in Figure 13. Not only do these 
catalysts demonstrate much greater initial activity in some 
cases than their unmodified (non-alkylated) counterparts in 
Figure 11, but they are still more active after 24 hours. 
The lS/20Si (0.05 mmole S/G[SG]) 50% methylated gel in curve 



Figure 12. Infrared spectra for inunobilized [Rh(CO)_- 

(C_H-OH) ]bF. using [SG]-SH supports witn 
(AT IS/l9si 50% Me; (B) lS/5Si 80% Me; and 
(C) lS/2.5Si 80% Me. 



72 






A 'tIos. 507o Me 




B '2/53, 80% Me 



C 72.55, 80°/° Me 



J \ L 



2300 2000 1800 

(cm-) 



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s 









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.— ( -pH 


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r-H (N 


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aU0UDX9L| 



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75 
B produces virtually linear 2-hexanone production over 24 
hours. Compared to the non-alkylated catalyst curve B of 
Figure 11 on almost two-fold increase in product has been 
obtained after 24 hours and a very active catalyst remains. 
At the end of four hours essentially the same amount of 
2-hexanone had been produced by curve B in Figures 11 and 13. 
Infrared spectra indicated that in both catalysts the 
essential species was monomeric rhodium complex before the 
start of catalysis. Thus, alkylation of silica gel in this 
instance has greatly enhanced catalyst lifetime. 

Comparison of IS/lOSi catalyst systems from curve C of 
Figures 11 and 13 reveals that the methylated catalyst is 
superior almost from the very start of the oxidation. This 
result correlates well with the fact that the non-alkylated 
catalyst is composed of rhodium dimer, while the alkylated 
one contains monomeric rhodium before reaction. This 
demonstrates that higher loadings of catalytically active 
complex can be obtained under site isolated conditions by 
diluting the f unctionalized reagent in the hydrocarbon matrix 
formed by -Si(CH,),. A very active catalyst is still present 
after 24 hours. 

A surprising result is represented by curve A in 
Figures 11 and 13. According to infrared spectroscopy site 
isolation (rhodium monomer) exists in both systems before 
reaction. The diffusion problem described in relation to 
Figure 12 has apparently been decreased by the greater 
affinity of alkene for the alkylated surface than for 



76 

solvent. Such a rationalization explains the greater initial 
activity of curve A in Figure 13 versus Figure 11 as well as 
the increased activity after 24 hours. 

Figure 14 shows the results of 1-hexene using catalysts 
supported on 50% and 80% ethylated silica gels. They are 
generally more active than the non-alky lated gel catalysts 
and are still very active after 24 hours. The IS/lOSi 80% 
ethylated catalyst (curve E) is less active and the lS/20Si 
80% ethylated gel (curve D) more active than might have been 
predicted. Samples which were 80% methylated yielded results 
similar to those of the 50% methylated gels. It is also seen 
in Figure 14 that increasing the percent alkylation does not 
have a drastic effect on catalytic activity. 

The 80% propylated [SG]-SH samples produce catalytic 
results very much like those of the non-alkylated silica gels 
of Figure 11. Figure 15 displays oxidations run for longer 
time periods, each done with lS/20Si [SGj-SH gels which have 
different surface modifications. The 80% methylated gel 
(curve B) is still active after 140 hours. It can be seen 
that the 80% ethylated gel is less active than the methylated 
one, while the propylated catalyst is nearly identical to the 
non-alkylated sample. The propylated silica gels, and to a 
lesser extent the ethylated ones, are markedly slow in 
"wetting" by ethanol. Perhaps it is this increased 
hydrocarbon-like nature of the surface which accounts for 
their decreased efficiency as catalytic supports relative to 
the methylated gels. Thus, even though enhanced solubility 





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81 

of the alkene in the surface layer may occur, exclusion of 
the solvent would inhibit the reaction. 

An investigation was undertaken to determine whether 
under conditions in which the large amounts of silica gel 
used in 1-hexene oxidations when the lS/40Si (0.025 mmol S/g 
[SG]) gels are employed as catalyst supports diffusion might 
become rate-controlling. A typical 1-hexene oxidation 
employing lS/40Si non-alkylated [SGJ-SH was run. A second 
oxidation using one-half the amount of rhodium and copper, 
and thus one-half of the silica gel support, was also run. 
All other initial conditions and amounts of reactants were 
identical. Assuming that diffusion is not rate-controlling 
the rate of this oxidation should have been one-half of the 
original rate since the oxidation is first order in rhodium 
concentration. Figure 16 clearly shows the reaction with 
one-half of the silica gel supported catalyst to have a rate 
greater than one-half of the normal oxidation. The dashed 
line C is one-half of the curve A. Thus, there seems to be 
some reaction inhibition due to the large amounts of silica 
gel support used in the lS/40Si oxidations (9-12 g), which is 
near the limit of the mass of gel which may be stirred in 
this experimental apparatus. This result may be ued to help 
explain why in general several lS/40Si (0.025 mmol S/g [SG ] ) 
supported catalysts are not as active as their lS/20Si (0.05 
mmol S/g [SG ] ) counterparts (e.g.. Figure 11). 



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84 

Possible Effects of a Different Rhodium Catalyst 
Characterization 

The design and implementation of the supported 

catalytic systems have been based on Mimoun's 

characterization of the active catalyst as a rhodium(I) 

complex. The possible characterization of the catalyst as a 

rhodium(III) species as discussed in Chapter II may have 

several implications. Chloride ion may also be quite 

important since 3-5 equivalents of chloride ion promote 

better homogeneous oxidations using [Rh(C0)2S' ]bf. (S' = 

solvent) and given that the most stable homogeneous catalyst 

64 
species may be rhodium chloride complexes of same type. 

Chloride ion cannot be added to the [SG]-SH systems without 

extensive leaching occurring. The possible use of a 

supported multi- dentate ligand for the immobilization of a 

rhodium(III) catalyst should be considered for future use. 

It would also be of interest to compare the effects of 

alkylation of a multidentate ligand silica gel support to 

those presented here for a monodentate ligand system. 



Conclusions 

The spectral properties and catalytic behavior of the 
described systems demonstrate that isolating f unctionalized 
[SG]-SH groups with [SG]-SiR- (where R is alkyl) leads to 
some very pronounced changes. The stability of the 
methylated catalysts is greatly enhanced over that of the 



85 



non-al)cyiated systems. On non-alJcylated [SG]-SH it is 
possible that the derivatized silanol groups may exchange 
with protons on nearby silanol groups and thus migrate along 
the surface, aggregate and become inactive. The alkylated 
gels remove -OH groups and would be able to effectively 
suppress such a process leading to a longer lifetime for 
site-isolated conditions. The mixing of reagents prior to 
functionalization also ensures a better distribution of 
functional groups over the surface under site isolated 
conditions and enables one to attain a more concentrated site- 
isolated catalyst. 

In the specific catalytic system studied, increasing 
the length of the alkyl chain covering the silica surface to 
ethyl and propyl decreases catalytic activity over the methyl 
covering. This may be due to solubility properties as 
discussed earlier, or perhaps due to increased efficiency at 
keeping the two cocatalysts apart. This approach should 
still find general application and utility in the area of 
hybrid catalysts where site isolation is a desired feature. 



86 

Experimental 

General Procedures 

All solvents and reagents were of reagent grade and 

used without further purification unless otherwise specified. 

The [Rh(CO)TCl].'*^ and [Rh(CO)^S' IbF.^^ (S' = solvent) were 
2. 2 2 n 4 

prepared as reported in the literature or purchased from 
Aldrich and recrystallized from n-hexane. The ^CH-0) ,Si (CH^CH^- 
CH2SH) and CISKCH.), were obtained from Aldrich and 
ClSiCCpHc)^ and ClSi(C,H-), from Petrarch Systems. Silica 
gel was grade 62 from Davison Chemical. 

Infrared spectra were recorded on a Perkin-Elmer model 
283B infrared spectrometer. The GLC were obtained with a Varian 
model 940 FID instrument using a 3m, 1/16 in. i.d. copper 
column packed with Chromasorb P supported diethylene 
glycoladipate or with a Varian model 3700 FID chromatograph 
using an 8 ft, 1/8 in. column of the same material. The 
2-nexanone production was quantified using 2-heptanone as an 
internal standard. Electronic absorption spectra were taken 
on a Gary 14, and all samples were run in air at ambient 
temperatures and pressures. 

Preparation of Silica Gel Supports 

All reactions of functional iz ing silanes with silica 
gel were done under argon with xylenes as solvent. Silica 
gel was stirred m xylenes under argon, followed by hearing, 
and then a solution of one or more silanes in xylenes was 



87 
added dropwise to the hot silica gel slurry. The non- 
alky lated [SG]-SH gel was made by adding (CH^O) ^Si(CH_CH_- 
CH2SH) to the gel. The [SG]-SH alkylated gels were made by 
adding a solution of the mercaptosilane mixed with R^SiCl (R= 
CH,, C-Hc or C-H^). This technique is preferable to adding 
each silane in separate steps. After refluxing 24 hours, the 
silica gels were thoroughly washed with xylenes, ethanol and 
then dried at '^'90 C. The silica gels remain white in 
appearance. 
Determination of -SH on Silica Gel 

Silica gel and its surface silanol groups were reacted 
with (CH20)2Si(CH2CH2CH2SH) to produce the [SGl-OSKOCH^ ) 2" 
(CH2CH2CH2-SH) (abbreviated as [SG]-SH). R SiCl (R=methyl, 
ethyl or propyl) was used to complete reaction of the silanol 
groups and cover the silica gel surface with alkyl groups. 
Evidence that trialkylsilyl groups are bound to the surface 
is obtained by a substantial increase in the percent carbon 
found in the elemental analyses and by an increase in the 
lifetime of the functionalized catalyst. 

Electronic absorption was used to determine the amount 
of sulfide on each silica gel sample ([SG]-SH). The [SG]-SH 
samples were stirred with freshly prepared [Rh(CO)-,S' ]bf. 
(S' = solvent), the solid filtered off and the electronic 
absorption spectrum of the filtrate taken. From a 
calibration curve of the rhodium complex concentration versus 
its absorbance at 390 nm the amount of rhodium in the 
filtrate is measured. The amount of rhodium on the gel is 



88 

determined by the difference of the known initial amount of 

rhodium and the amount of rhodium in the filtrate. Assuming 
complete reaction of one rhodium per sulfide, the amount of 
rhodium is equal to the amount of sulfide on the silica 
surface. An analysis of this type was done for all samples. 
Samples were prepared which varied in surface sulfide 
concentration and in percent of available surface silanol 
groups which were "alkylated" (reacted with trialkylchloro- 
silane). Assuming tetrahedral coordination around silicon 
and oxygen atoms with an approximate silicon-oxygen bond 
distance of two Angstroms, and assuming that as an upper 
limit a flat surface exists with all surface groups being 

o 

silanols, a separation of 6.5 A exists between the oxygens of 

surface hydroxyl groups. This corresponds to one surface 

-19 2 
hydroxyl per 4.3 x 10 m of surface so it can be 

calculated that as an upper limit there are about one mmol 

surface silanol group atoms per gram [SG] . With this value 

it can be determined that the [SG]-SH samples prepared 

contain approximately 1/40, 1/20, 1/10 and 1/5 millimoles 

sulf ide/millimole surface Si (S/Si) which corresponds to 

0.025, 0.05, 0.10, 0.20 and 0.40 mmol S/g[SG]. Silica gel 

samples used as catalytic supports will be referred to by a 

(S/Si) ratio and by a number of mmole S/g[SG]. In some 

instances the percent alkylation ( methylation, ethylation or 

propylation) of the surface silicon sites will also be used. 



89 
Catalytic Oxidations of 1-Hexene 

All catalytic 1-hexene oxidations were run in 250 mL 
Parr pressure bottles equipped with brass Swagelok pressure 
heads. These were constructed to allow purging with 0-, as 
well as sampling of the solution under reaction conditions 
during the course of the reactions. The 2-hexanone 
production was measured by GLC . 

All catalytic reactions were run using 0.074 mmol 
[RhCCO-S' ]BF. (prepared as reported earlier and used 
immediately) and 0.0179 g Cu( NO^ ) 2 • 3H2O (0.074 mmol). The 
amount of silica gel employed was varied to provide enough 
-SH sites to bind ail of the rhodium and copper with a 10% 
excess (e.g., 4.9 g of lS/20Si 80% Et [SG]-SH). Both metal 
ions are attached to the [SG]-SH samples through the -SH 
moiety producing a lemon-yellow to yellow-green supported 
catalyst. The catalyst was added to a 250 mL Parr bottle 
along with 0.32 mL 0.47 M HCIO. (0.150 mmol, as an ethanol 
solution prepared from aqueous cone. HCIO.), 1.136 mmol 
2-heptanone, 45 mL absolute ethanol, and 15 mL 1-hexene 
(purged through alumina to remove peroxides). This mixture 
was purged 5 times with 60 psi O^i set to 40 psi 0^ , and the 
reaction initiated by placing the system in a 70 C oil bath. 



CHAPTER IV 

THE SYNTHESIS AND CATALYTIC APPLICATIONS 

OF A MULTIDENTATE LIGAND AND CORRESPONDING METAL COMPLEXES 

BOUND TO SILICA GEL 



Introduction 

The use of a polymer as a support for catalysts was 
discussed in Chapter III for a specific system which employed 
a monodentate chelate on silica gel. The general use of 
polymers as supports for catalysts and chelates, and also as 
synthetic reagents, has developed enormously since their use 
in peptide synthesis was shown by Merrifield approximately 
twenty years ago. It was generally assumed for several 
years that there was little, if any, interaction between 
f unctionalized sites on resins such as polystyrene. This is 
now known not to be the case, and Chapter III demonstrated 
some of the rather stringent conditions which must be met in 
order to achieve site isolation on a support such as silica 
gel . 



90 



91 

Another general approach to preparing stable polymer 
bound metal complexes is that of first attaching a 
multidentate ligand system to a polymer backbone and then 
adding the metal ion to the previously formed ligand. There 
have been many reports discussing polymers which contain 
chelating groups of various sizes and donor atoms. ~ An 
excellent review of this area is cited here as reference 70. 
Most of the polymer bound chelates studied are mono- or 
bidentate, with only a few tri- and tetradentate systems 
having been investigated. ' Resins with bidentate 
chelate groups often must provide a metal ion with two or 
three chelating groups since many metal ions prefer forming 
four or six coordinate complexes. This led to the common use 
of uncrosslinked or lightly crosslinked polymers. The 
ambiguity involved in trying to characterize these polymer 
bound metal complexes makes it very difficult to interpret 
data from catalytic reactions or physical methods. 

In virtually all of the literature reports cited in 
this area, there is no attempt made to isolate functionalized 
sites from one another and in many cases two or more sites 
must interact in order to provide successful chelation. 

Chapter III discussed in detail the conditions required 
to achieve site isolation on silica gel and surface 
modifications which may enhance that isolation. The initial 
sections of this chapter will discuss the preparation of a 
stable multidentate ligand site isolated on silica gel which 
can complex a variety of metal ions into polymer bound 



92 

complexes of a known geometry. Investigations of several 
applications of these polymer bound complexes will then be 
presented, followed by investigations into the use of 
different solid matrices as chelate metal complex supports. 

The ligand prepared on silica gel is a pentadentate 
ligand which complexes several different metal ions. The 
silica gel supported complex of iron(III) was used to 
investigate some known homogeneous reactions. 

Biological oxidation processes are generally quite 

selective and sensitive and as a result attract the attention 

72 
of many chemists. Cytochrome P-450 is one enzymatic system 

which can catalyze several types of oxidative 

transformations, including selective alkane hydroxylations 

and has therefore been an object of much research effort. 

Several studies have proposed intermediate high-valent 

oxometalloporphyrin species in the catalytic cycles of 

7 3 74 
several heme-containing oxygenase enzymes. ' Simple 

chemical models for cytochrome P-450 were sought in order to 

provide insight into some of the basic processes involved in 

its chemical reactivity. A significant finding was the 

realization that iron, chromium, and manganese 

porphyrins could catalyze oxygen transfer from iodosylbenzene 

7 8 
to simple hydrocarbons. In all of these cases, a metal-oxo 

species is proposed as an intermediate, although complete 

mechanistic details have not been determined. However, it 

was the possibility of generating silica gel supported iron 

or manganese oxo species to achieve some type of oxidative 



93 

transformation which was the impetus behind the work reported 
here. The ability to form supported cobalt-dioxygen 
complexes (vida infra) helped to encourage the idea that it 
would be sterically possible to generate the supported iron 
or manganese oxo intermediates. Another goal was to 
determine the effect of limiting or eliminating interaction 
between metal centers via their support and site isolation on 
silica gel. 

Another topic briefly investigated is the long known 

"Fenton chemistry" of iron salts and hydrogen peroxide 

79 
affecting the hydroxylation of organic substrates. This 

area has recently been reinvestigated by Cheves Walling and 

ne gives an excellent overview of this area in reference 80. 

The original goal of achieving this type of peroxide induced 

oxidation with a supported iron complex was not realized due 

to the leaching of metal complex into solution in the 

presence of acid and peroxide. Thus, no mechanistic or 

catalytic information is provided, although some further 

knowledge of the silica surface is obtained. 

Catalytic processes which require two different metals 

have not achieved much success in being supported on 

polymers. A method which may successfully attain support of 

both metals is that of incorporating one active metal into 

the structure of the support while supporting the other metal 

on the surface of the solid. Incorporation of copper(II) 

ions into silica gel is described along with attempts at 

catalysis by resulting bimetallic systems. 



94 



The method of electron spectroscopy for chemical 

analyses (ESCA) holds promise for studying the surfaces of 

81 82 
solid catalysts. ' The technique basically involves the 

measuring of the energy spectrum of the electrons ejected 

from a sample after bombardment with monoenergetic x-rays. 

The orbitals of origin of the ejected electrons determine 

their energies, with each element having characteristic 

orbital ionization potentials. Thus, comparison of a 

sample's ejected electron energy spectrum to known spectra 

usually provides surface analysis and/or molecular structure 

information. Since x-rays can eject electrons from at most 

the top 100 Angstroms of a solid, ESCA is well suited for 

surface studies. The applications of this method to a few of 

the systems studied here will be discussed in the following 

sections, and data given which may justify its even wider use 

in future work. 

Results and Discussion 

Synthesis of a Salen Ligand on Silica Gel 

The synthesis of a multidentate ligand system on a 

rigid support such as silica gel was derived as a method for 

possibly achieving properties different than its homogeneous 

analogs. The ligand described here is a three nitrogen, two 

oxygen donor which had to be synthesized stepwise on the 

silica surface. The plain silica surface was first reacted 

with (H^CO) -,SiCH-CH„-(C,H. )-CHtC1 in order to produce the 
3322642 

chloromethyl functionality on the surface. In fact the 



95 

chloromethyl moiety attached to phenyl groups is similar to 
the surface of polystyrene. Earlier work in this research 

group led to the successful support of the desired ligand on 

8 3 
polystyrene. However, this polymer was not rigid enough to 

produce all of the desired characteristics (vida infra). The 
next step in the ligand synthesis is to react the chloro- 
methyl groups with HNCCH-CH-CN) _ ( 3 , 3 ' -iminodipropio- 
nitrile) which will be referred to by its common name 
dicyanoethylamine, DCEA. This reaction does not proceed 
under routine conditions of mixing the DCEA with silica gel 
in a solvent under nitrogen. Attempts to react DCEA with 
chloromethylated silica gel under a normal inert atmosphere 
leads to the polymerization of the DCEA into a viscous mass 
accompanied by little DCEA incorporation. Experiments were 
performed under high argon pressures after concluding that 
rigorous exclusion of oxygen might be necessary. It was also 
discovered that Alum and coworkers had reacted the chloro- 
propyl group bound to silica gel with diethylamine at 
elevated argon pressures to produce the bound tertiary amine. 
Reaction of the silica gel with pure DCEA (without solvent) 
in a pressure bottle of the type used for catalysis under 100 
psi of argon pressure produced the DCEA bound to silica gel 
as shown below. 

[SG]-CH2C1 + HN(CH2CH2CN)2 ->■ 

[SG]-CH2-N(CH2CH2CN)2 + HCl (18) 



96 

Next, the cyano groups are reduced to amine 
functionalities by reaction with B-Hg/THF solutions under 
nitrogen. This supported ligand is referred to as [SG ]-DPT 
from the common name dipropyltriamine (DPT). The general 
scheme for the preparation of [SG]-DPT is shown in Figure 17. 

Finally, the Schiff base condensation of Lscl-DPT with 
salicylaldehyde is conducted in order to produce the desired 
[SG]-SalDPT as shown in Figure 18. This supported ligand is 
stable in air and the entire silica gel support is pale 
yellow in color. Thorough washing, Soxhlet extraction and 
complete drying of the silica gel after each step in this 
procedure is very important. The [SG]-SalDPT was also 
prepared with alkylation of the non-reacted surface silanol 
groups, as were samples which varied in surface 
concentrations of the -SalDPT ligand. A slight variation of 
the ligand was synthesized using 3 , 5-dibromosalicylaldehyde 
to produce [SG ]-BrSalDPT. 

Characterization of these supported ligands is not 
easily attained. The low loadings on the surface often rule 
out the use of elemental analyses. The best method available 
routinely is that of incorporating certain metals into the 
silica bound ligand and comparing their electron spin 
resonance (ESR) spectra to the spectra of their homogeneous 
and polystyrene bound analogs. The [SG ]-DPT ligand when 
stirred with copper(II) acetate in DMF binds the metal ion to 
form [SG J-Cu( II )DPT which is green in color. Similarly, 
[SG ]-Co(II)SalDPT is easily formed by mixing of the silica 



Figure 17. General scheme for the preparation of 

[SG ]-DPT. 



98 



n 




HN(CH,CHXN), BH,/THF 



CH2CI 



•> SG 



CH. 



CH, 



NH, 



@^^ 



CH, 



CHjN 



CH 



DPT 



2 

CK 



CH 



^H, 



Figure 18. The preparation of [ SG ]-SalDPT from [SG]-DPT, 



100 




CH 



'^O^CH^N 



r 



NH. 



NK 




OH 




benzene 



'^ 



^- ^ 



CH, — N 






•N 




(SG)- Sal DPT 



101 

gel supported SalDPT ligand with cobalt(II) acetate in DMF 
solution. The ESR spectra of these complexes can then be 
used for characterization to verify that the silica gel 
supported species have been prepared. The oxygen adduct, 
[SG ]-Co(II)SalDPT -O-f must be used in order to obtain a 
spectrum for the cobalt system. The ESR spectra of these 

silica gel bound complexed are then almost identical to the 

8 3 
spectra of the polystyrene bound complexes. The attainment 

of a stable multidentate ligand which is covalently bound to 

a rigid silica gel support with site separation achieved by 

surface alkylation is a development which has many possible 

applications, several of which will be discussed in the 

following sections. 



Oxygen Transfer Using [SG ]-Fe( III ) SalDPT and [SG ]- 
MndDSalDPT 



Among the general interest in cytochrome P450 
oxidations has been specific interest in aromatic and 
aliphatic hydroxylations , alkene epoxidation, and interest in 
the nature of the active oxidant. As mentioned previously, 
most of the systems used to investigate these processes 

involve iron or manganese tetraphenylporphyr in 

7 2 — 7 8 
complexes. Since silica gel supported iron(III) and 

manganese(II) complexes can be easily prepared from the [SG ] 

-SalDPT ligand, these complexes were employed to determine 



102 
whether they could provide a polymer bound model for any of 
the above reactivity. 

Pressurizing the silica gel supported iron or manganese 
complexes with dioxygen at 60 psi 0- and 40°C in methanol 
produced no oxidation of a cyclohexene substrate. This was 
not surprising, however, since other oxygen sources are 
usually required when examining the cytochrome P450 systems, 
lodosylbenzene, commonly used, is the oxygen source for all 
of the results which follow. 

Cyclohexene was employed as a substrate in order to 
investigate epoxidation and hydroxy lation of alkenes. 
Cyclohexene, lodosylbenzene, and a cyclopentanone internal 
standard were used in methylene chloride solvent along with a 
polymer supported metal complex. Product formation was 
determined by use of gas-liquid chromatography, using a 
carbowax column at 130 C. The retention times varied with 
fluctuations in gas flow, but the approximate average 
retention times (in minutes) were the following: 0.8 for 
CH^Clp, 2.6 for cyclohexene oxide, and 8.7 for cyclohexenoi . 

The iron(III) complex, [SG ] -Fe( III ) SalDPT, converted 
cyclohexene to cyclohex-2-enol and cyclohexene oxide, along 
with producing iodobenzene from the lodosylbenzene in CH^Cl^ 
at room temperature. The general reaction using 
[SG ]-FeSalDPT or [SG ]-MnSalDPT is represented in Figure 19. 
This supported iron complex achieved oxygen transfer, but 
also produced an interesting product distribution. Using a 
homogeneous solution of tetraphenylporphinatoiron( III ) 



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105 
chloride (FelllTPPCl) Groves et al. reported production of 
cyclohexene oxide and cyclohexenol in a ratio of 3.7:1 in 
CH2CI2 at room temperature. The silica gel supported 
iron(III) produces cyclohexenol almost exclusively, with only 
trace amounts of cyclohexene oxide formed. The homogeneous 
system produces 4.2 total turnovers (based on mmol products 
and mmol metal complex) in the relatively short time period 
of 1-2 hours. The supported system reacts more slowly, 
requiring 47 hours to produce 2.4 turnovers of the 
cyclohexenol product. Also, commonly cited is the 
destruction of catalyst by iodosylbenzene. This process does 
not appear to occur to the silica gel bound complexes, in 
that there is no color loss from the support into solution 
and the [SG ]-FeIIISalDPT resin when re-used several times 
produces identical results each time in terms of the amount 
of product formed and the rate of product formation. 

The silica supported manganese complex, [SG ]-MnSalDPT, 
also converts cyclohexene to cyclohexenol and cyclohexene 
oxide. In this system, however, only cyclohexene oxide is 
produced for the first 30-45 hours after iodosylbenzene 
addition. Cyclohexenol is then produced relatively quickly 
until the cyclohexenol to cyclohexene oxide ratio is 1.3:1 
after approximately 53 hours of reaction. The supported 
system yields 1.4 turnovers of exclusively cyclohexene oxide 
by the end of 45 hours, but after 53 hours has produced 2.1 
turnovers of the cyclohexene oxide product and 2.7 turnovers 
of the cyclohexenol product. The supported Mn(II) system's 



106 

reactions were conducted under a nitrogen atmosphere. The 
re-use of [SG]-MnSalDPT samples indicated the activity of the 
used silica gel and showed no evidence of color loss into 
solution. The yield of products was only monitored 
approximately every 12 hours during re-use, but indicated the 
same 1.3:1 product ratio of cyclohexenol to cyclohexene oxide 
produced at approximately the same rate as the original 
reaction had during its last hours. Cyclohexane is not 
oxidized by either the supported iron or manganese systems 
under the conditions used for cyclohexene oxidation. 

A product of the reaction of ClMn(III)TPP derivatives 
with iodosylbenzene is [ClMn(IV)TPP(OIPh) ] -0, which is 

capable of oxidizing alkane and alkene substrates at room 

84 
temperature under an inert atmosphere. Reaction of the 

manganese dimer above with cyclohexene produces the following 

oxidation products in one hour: 3-chlorocyclohexene, 32%; 

cyclohexenol, 7%; cyclohexene oxide, 31%; 2-cyclohexen-l-one, 

1% (percents based on three oxidizing equivalents for the 

dimer). The incorporation of chloride into a product, along 

with the relatively small production of cyclohexenol, are 

results which differ from those of the silica gel supported 

system. Both the homogeneous iron and manganese TPP 

complexes yield products which contain anions which have been 

incorporated from the TPP complexes. Obviously, the lack of 

anions from the supported metal complexes excludes the 

possibility of formation of any analogous products by the 

supported systems. 



107 

In conclusion, the product distributions and lack of 
catalyst degradation make the silica gel supported SalDPT 
iron and manganese complexes, despite their slower rate of 
activity, both interesting and worthy of further 
investigations. Future adjustment of metal complex 
concentrations or reaction conditions might provide progress 
toward activation of alkanes by these heterogeneous 
catalysts. 

Synthesis of [SG] -Fe( II ) SalDPT 

Complexes of iron(II) are of great interest, although 
they are generally difficult to prepare as air stable 
compounds. Previous attempts to prepare crystalline Fe(II)- 
SalDPT resulted in irreversible oxidation presumably to the 
oxo-bridged dimer. Preparation of Fe(II)SalDPT immobilized 
on a rigid silica gel support was desired in order to compare 
its properties to those of the attempted crystalline analogs. 

Great care must be used to eliminate oxygen when 
preparing the supported iron(II) complex. Performing cycles 
of a f reeze-pump-thawing procedure on [SG] -SalDPT in DMF 
solvent is the best way to exclude oxygen initially. The 
compound Fe(CO)- is employed as the source of iron and is 
reacted with the silica gel supported ligand in Schlenk-type 
apparatus. After reaction, filtering and thorough washing 
(see Experimental Section) a purple/pink silica gel results. 
This silica gel is very different in appearance from the [SG] 
-Fe( IIDSalDPT complex which is a tan/brown color. Samples 



108 

of [SG]-SalDPT with 80% of its surface methylated (covered 
with -Si(CH^)- groups) and 0.05 mmol ligand per gram [SG] (1 
ligand/20 surface Si) were used to prepare the iron(II) 
complex in order to provide a surface which should have site 
isolation of the iron centers (see Chapter III for details of 
the alkylation process). The SalDPT ligand is attached to a 
longer chain of atoms, and occupies a larger area, than the 
sulfide ligand ( [SG]-SH) discussed in Chapter III. As a 

result, it is theoretically able to cover a greater area (780 

2 

A ) than the sulfide ligand and therefore requires a loading 

of 1 ligand/18 surface SiOH or more dilute in order to 
achieve site isolation. This is the loading necessary to 
avoid any contact between ligands; however, alkylation of the 
silica surface very likely allows site isolation at higher 
ligand loadings. The larger steric bulk of the multidentate 
ligand may also allow some amount of ligand-ligand contact 
without the corresponding metal center interaction. 

Mossbauer spectroscopy was used to help determine the 

85 
oxidation states of iron in silica gel supported species. 

Spectra of supported complexes prepared with Fe(III) give 

data indicative of an Fe(III) complex. The spectra are not 

very intense or sharp due to the relatively low loading of 

iron used to obtain site isolation coupled with the fact that 

only the 2% abundant Fe isotope of iron is Mossbauer 

active. The silica gel supported Fe(III) complex yields 

Mossbauer spectra very close to those reported for similar 

8 3 
polystyrene bound Fe(III) complexes. 



109 

The Mossbauer spectrum of a small [SG] -Fe( IDSalDPT 
sample sealed under nitrogen was not intense enough to 
provide a meaningful characterization. 

The goal of preparing the [SG]-Fe(II )SalDPT complex was 
to obtain site isolation of the iron centers and thus remove 
the possibility of forming any type of oxo-bridged dimer 
when exposed to oxygen. However, the supported iron (II) 
silica gel turns from its original pink color to the tan 
color of supported iron(IH) within minutes upon exposure to 
air. This change is non-reversible by treatment with vacuum 
and the change also occurs at liquid nitrogen temperatures, 
although more slowly. Due to the rigid nature of the silica 
gel structure and the presence of surface methyl groups, it 
seems unlikely that neighboring iron centers are able to 
contact one another or to form oxo-bridged species. The 
preparation of supported iron(II) complexes with even lower 
surface concentrations of iron would be of interest to help 
confirm that neighboring iron centers are not interacting. 
Silica gel loadings of 1 ligand/50 surface Si (0.05 mmol 
ligand/g [SG] ) or even 1 ligand/100 surface Si (0.01 mmol 
ligand/g [ SG] ) would be worthy of investigation. At such low 
concentrations it would be difficult to obtain Mossbauer 
spectra, so the distinct color change observed upon oxidation 
from iron(II) to iron(III) would be the most convenient 
method of determining whether that oxidation is occurring 
upon exposure of the [ SG] -Fe(II) SalDPT to air. Even at low 
metal complex surface concentrations, the purple/pink 



110 

iron(II) color and the tan/brown iron(III) color should be 
readily distinguishable on the silica gel. 

The preparation and characterization of an iron(II)- 
multidentate complex supported on a rigid inorganic polymer, 
silica gel, has been accomplished. The possibility of 
achieving reversible 0- binding may be reached by this system 
with further "fine-tuning" of the surface. Application of 
the rather detailed information of the silica surface and of 
effects of surface modification described in Chapter III 
should be of use in future studies of this iron(II) system. 



Incorporation of an Active Metal into a Functionalized 
Support 



The design of polymeric supports and of ligands to 
immobilize catalytic species has been discussed and 
demonstrated in earlier sections. A logical extension of 
this concept is to incorporate one metal of a bimetallic 
catalytic system into the support material while retaining 
the ability to f unctionalize the support leading to the 
immobilization of a second metal species on the support 
surface. 

2 S 2 6 
The Wacker process ' referred to in Chapter II is a 

well-known bimetallic system utilizing copper and palladium 

complexes. The general mechanism for this process is shown 

in Figure 20. The Mimoun system discussed at length in 

Chapters I and II is a bimetallic system using copper and 

rhodium complexes. Interest in these two systems was the 



Figure 20. General catalytic cycle for the Wacker 
process. 



112 



CHjCHO 




CH2OH 



113 

primary justification to attempt to incorporate Cu(II) ions 
into silica gel. 

Copper(II) ions when mixed with plain silica gel in 
water have no reaction with the silica surface. However, 
silica gel treated with dilute aqueous hydroxide solution, 
which is then thoroughly rinsed with water, reacts with 
aqueous Cu(N0^)2 to produce enough copper (II) on the surface 
to give the entire silica gel a pale blue color. The 
hydroxide solution presumably ensures that the silica surface 
reaches its maximum concentration of surface -SiO Na groups. 
When the hydroxide treated silica gel is mixed with excess 
Cu(NO,)_, most of the copper ions do not stay on the silica 
gel, but enough react to give the gel a very distinct blue 
color. Techniques were later used to estimate the amount of 
copper on the surface (vide infra). However, extensive 
Soxhlet extraction of the silica gel with boiling water does 
not remove copper ions, confirming that the copper is well 
bound to the surface. 

Silica gel was prepared with an anion exchange 
capability with general formula [ SG] -OSi ( CH2CH2CH2 )N(CH2 ) ^I . 
Details of these types of silica gels and their preparation 
will be discussed in a later section. These samples can then 
be rinsed with hydroxide and treated with copper(II) ions. 
The typical blue copper color is then evident in the 
resulting gel. The fact that some of the iodide ions on the 
silica gel surface may be replaced by hydroxide ions during 
this procedure does not matter in this application. The 



114 

anion exchange silica gel with copper ions incorporated is 
then able to react with K^PdCl. producing a green silica gel 
with tetrachloropalladate ions exchanged onto the surface. 

Homogeneous palladium( II )/copper ( II ) Wacker oxidations 
were conducted using 1-hexene substrate yielding 2-hexanone 
product. The homogeneous reaction was conducted at 70°C, 40 
psi 0- in an aqueous DMF solvent with 0.06 mmol of both 
metals producing approximately 30 turnovers in 24 hours. The 

heterogeneous oxidation was then conducted using the silica 

2+ 
gel with Cu on its surface and palladium bound to it. The 

conditions used were equivalent to those for the homogeneous 

reaction, with approximately 0.07 mxnol of palladium supported 

on the silica gel. It is difficult to estimate the amount of 

copper on the surface, it being the amount which is bound 

after the surface is treated with NaOH. This heterogeneous 

catalyst produced roughly 0.5 turnovers initially and then 

did not yield any further product over a long time period. 

There was no visible change to the silica gel catalyst. 

The initial activity of the silica gel supported 

catalysts is a positive result. Leaching experiments 

indicate no activity in solution, so that any activity 

observed is due to metals supported on the surface. One 

possible factor limiting activity is that the palladium 

complex is at the end of a relatively long chain of atoms 

which bind it to the silica surface, while the copper ions 

are located on the silica surface. The physical separation 

between palladium and copper may well limit long term 



115 

catalytic activity. Normally one would predict that lack of 
Pd/Cu contact would result in the production of reduced 
metallic palladium and an accompanying grey/black color 
change. In this case it is uncertain whether such a color 
change would be visible due to the loading of the surface 
with blue copper ions, and the low concentration (one 
palladium ion per every 75 surface silanol groups) of surface 
palladium. In this case, rather than site isolation, higher 
surface loadings may improve activity. 

Electron spectroscopy for chemical analysis (ESCA) was 
employed to provide some information about the f unctionalized 
silica surface. This work was done at the Major Analytical 
Instrumentation Center (MAIC) of the University of Florida 
through the assistance of Susan Hofmeister. The x-ray 
photoelectron spectra resulting from a study of the Pd/Cu 
silica supported system described above indicate the presence 
of very small amounts of copper. Although ESCA is not a 
precisely quantitative method, it does indicate the relative 
amounts of species on a solid's surface. Since ESCA can 
examine at the most a depth of 100 Angstroms on a surface, 
this result would tend to indicate, despite the definite blue 
color of the silica gel, a relatively low loading of copper 
on the silica surface. This led to attempts to incorporate 
copper ions into silica gel (vide infra) and suggested that 
increased loadings of palladium may also be needed. 



116 

Apparently low loadings of copper ions on the silica 
gel surface led to attempts to produce silica gel having 
copper incorporated throughout the silica structure. The 
approach taken was that of dissolving the silica gel and then 
reforming the solid with copper ions interspersed in the 
silica gel. The technique of dissolving silica gel in acid 
(HCl), adding Cu(N0^)2f and neutralizing with base (NaOH) 
produces a very blue silica gel which does not change after 
extensive aqueous washings and extractions. The drawback to 
the copper incorporated silica gel produced is that it 
surface area is very much lower than that of the initial 
reactant silica gel. All of the experiments conducted 
support the validity of the chemical principles utilized in 
this approach. However, in discussions with chemists from 
W. R. Grace (manufacturers of Davison brand silica gel) the 

rather sophisticated technology involved in production of 

86 
high surface area silica gel was revealed. It is uncertain 

whether conditions sufficient to produce high surface area 

silica gel can be obtained with materials available at a 

typical academic laboratory. Industrial collaboration may be 

worthwhile in order to produce a high surface area silica gel 

with a metal ion incorporated into the silica gel matrix. 

Surface f unctionalization in the usual manner should then be 

possible ultimately yielding a bimetallic system with both 

metals in and on a solid heterogeneous catalyst. 



117 

Synthesis of a Silica Gel Anion Exchange Resin 

Anion exchange resins are conunercially available 
through several companies. However, they are virtually all 
composed of polystyrene or other similar organic polymers. 
The drawbacks to these substances are their lack of rigidity 
and their inability to tolerate increased temperatures. 
Polystyrene itself begins to soften at 85°C, and also has 
solvent dependent swelling properties. 

An anion exchange capability on silica gel was prepared 
by first putting an amine moiety on the surface. The surface 
is also covered with -Si(CH-)^ groups in order to achieve 
site isolation of the other groups and in order to help 
protect the surface 0-Si bonds of the functional groups from 
hydrolysis. The amine groups are then reacted with 
chloromethane in order to produce quaternary ammonium 
chloride functionality on the surface. The analogous 
quaternary ammonium iodide silica gel can be prepared by 
reacting iodomethane with the supported amine. 

The silica gel anion exchange resin reacted as would be 

predicted in all of the applications in which it was used. 

2- 
The gel picked up PdCl. ions as described previously. The 

f unctionalized silica gel was shipped to the Sybron 

Corporation where testing is being conducted to determine how 

well this substance meets industrial criteria for a good 

anion exchange resin. Initial results appeared encouraging, 

but final results have not been reported. 



118 

The surface loading on the silica may be easily varied 
to accommodate different applications. Lower loadings would 
allow site isolation of species such as metal complex, while 
higher loadings would of course provide the capacity to bind 
larger quantities of an anion from solution. Silica gels of 
several loadings were prepared. Due to the nature of silica 
gel, each sample is independent of swelling problems and is 
stable at much high temperatures than polystyrene resins. 

Titanium Carbide as a Solid Support 

The stability of any substance bound to a solid support 
is ultimately related to the strength of the bond from the 
surface of the support to the species attached to that 
surface. The presumption that bonds of carbon or carbanions 
to metal centers might be significantly stable led to an 
investigation of titanium carbide (TiC) as a support 
material . 

Titanium carbide is very unreactive and so initial 
attempts were made to alter the surface in order to generate 
active surface species. Halogenation of the surface was 
attempted by the passing of chlorine gas over TiC contained 
in a tube furnace. Temperature of 150 C - 350 C during 
exposure produced liquid TiCl. along with black TiC of 
unknown surface composition. Gradual heating up to 150 C 
yielded no TiCl. liquid. 

Based on an estimation of the very small particle size 
of titanium carbide, it is calculated that even complete 



119 
chlorination of the surface would yield a very small percent 
of chlorine by weight which could not be measured by any 
normal elemental analysis technique. Thus, it is difficult 
to determine whether any chlorine atoms have been bound to 
the carbide surface after treatment with chlorine. Several 
unsuccessful attempts were made to react other species with 
the alleged surface chlorides. The stirring of n-butyl 
lithium in hexanes with chlorinated TiC, followed by addition 
of the RhCCO)- cation in THF was an attempt to place an 
active metal center on the carbide surface. However, the 
resultant solid was inactive for the oxidation of 1-hexene 
with copper(II) added into solution under 0- pressure (40 
psi) at 70°C in ethanol solvent. As discussed earlier 
(Chapter III), rhodium centers supported on silica gel under 
similar conditions successfully oxidize 1-hexene to 
exclusively 2-hexanone. Attempts to hydrogenate 1-hexene (as 
solvent) under 50 psi H_ pressure at 45 C by the same 
"TiC-Rh(CO) 2" solid were also unsuccessful. The chlorinated 
titanium carbide was stirred in a nitrobenzene solution 
saturated with ammonia in an attempt to attach an -NH_ 
functionality to the carbide surface. No adequate technique 
was found to quantitate the small amount, if any, of surface 
amine produced. Attempts to titrate the surface amine with 
acid were also unsucccesf ul . Liquid bromine was stirred with 
plain titanium carbide in an effort to achieve bromination, 
rather than chlorination, of the carbide surface. Here 
again, the problem faced was that of determining the exent of 



120 

reaction that had occurred. Each of the approaches tried 
with the chlorinated carbide was also used with the 
brominated samples, producing equally unsuccessful results. 

It was decided that more detailed knowledge of the 
carbide surface is required to aid future attempts at 
functionalization. As a result, ESCA studies of the plain 
titanium carbide surface were conducted indicating at least 
two different types of surface titanium atoms and more than 
one type of surface carbon species. The overall ESCA 
spectrum of titanium carbide is shown in Figure 21. An 
expanded version of the carbon region of the spectrum reveals 
two different types of carbon present and is represented in 
Figure 22. The peak on the left represents carbon atoms 
which are bound to oxygen or other carbon atoms, while the 
peak on the right represents carbon atoms bound to titanium. 
The titanium region of the ESCA spectrum shown in Figure 23 
reveals at least two different types of titanium species 
present. The ESCA technique provides a general indication of 
relative amounts of surface species measured. Thus, all 
further surface reactions on the carbide support should be 
monitored by ESCA methods to determine the type and quantity 
of resultant surface species. Other methods which yield more 
accurate quantitative data are also available at the Major 
Analytical Instrumentation Center, and these may also be 
employed as needed. 



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Fenton Chemistry 

The conversion of isopropanol to acetone and 1,2- 

propanediol is one example of the so-called Fenton 

8 
chemistry. This conversion was accomplished by aqueous 

solutions of either FeCl2'4H20 or FeCl, (one mmol) at room 

temperature in the presence of aqueous 0.1 N HCIO. and 

hydrogen peroxide. Attempts were made to use [SG] 

-Fe( IIDSalDPT as a heterogenous model to achieve the same 

reactivity. However, leaching of the iron complex off of the 

silica gel support occurs under reaction conditions identical 

to those described above. The silica gel completely loses 

its brown color and a white gel is produced. Addition of 

hydroxide ion to the colorless filtrate after reaction 

produces a rust colored precipitate, presumably an iron oxide 

compound. The silica gel supported iron(III) complex is the 

presence of all starting materials except H_0~ does not 

completely leach into solution, as a pale brown color remains 

on the gel. Hydroxide ion again confirms the presence of 

iron in solution. It seems that leaching by acid alone is 

not as complete as the metal loss in the presence of both 

acid and hydrogen peroxide. The estimation of leaching was 

based simply on the color changes involved, primarily loss of 

color from the silica gel. If the amounts of leaching had 

been small, some attempt would have been made to quantitate 

the amount of iron leached into solution. However, in all of 

the cases studied the amounts of leaching were quite large. 



128 

The use of pure isopropanol and of several different 
isopropanol/water mixtures as solvents, as well as 
acetonitrile solvent, produced no observable reduction in loss 
of iron complex from the silica surface. The silica gel 
samples tested had all of their available surface covered 
with trimethylsilyl groups in an unsuccessful attempt to 
protect the surface functional groups from hydrolysis under 
these conditions. Systems which require the use of acid and 
hydrogen peroxide are seemingly going to be very difficult to 
catalyze by silica gel supported species. It is not known 
whether any other surface groups would be able to better 
protect silica supported functional groups under similar 
conditions . 



Experimental 
Silica Gel 

Silica gel employed in all of the following experiments 
is Davison Grade 62 silica gel from W. R. Grace. This silica 

gel has a wide pore diameter of 14 nm, a specific area of 

2 3 

340 m , and a pore volume of 1.1 cm /g as stated on the label 

of the container. Maximum surface reactivity of the silica 

gel is achieved by washing the silica gel with dilute acid 

(0.1 M HCl) followed by thorough drying at 100°C under 

vacuum. The silica gel retains its very white color 

throughout the above washing procedure. 



129 

Functionalization of Silica Gel with 1-trimethoxysilyl- 
2-(p/m, chloromethylphenyle thane) 

The silane, (H^CO) ^Si (CH2CH2CgH^CH2Cl ) , was purchasd 
from Petrarch Systems, Inc. The acid washed silica gel, 
30 g, was placed in a 500 mL round bottom flask fitted with a 
condenser and argon inlet. Xylenes, 225 mL, were stirred 
with the silica gel with argon bubbling in from a plastic 
needle for 45 minutes to an hour. An addition funnel with 
another 75 mL of xylenes solvent was degassed with argon from 
a separate argon inlet simultaneously. The appropriate 
amount of silane was syringed into the addition funnel, along 
with chlorotrimethylsilane if alJcylation of the silica 
surface is also desired. The silanes are then added dropwise 
to the silica ''^l slurry which has been heated to near reflux 
under an argon atmosphere. After addition (1-2 hours) the 
slurry is stirred at gentle reflux overnight. 

The following day the silica gel is cooled, then 
filtered in air and washed with xylenes, benzene and 
methanol. The silica gel is then Soxhlet extracted for 24 
hours with benzene. The gel is finally dried at 70-80 C 
under vacuum. This f unctionalized silica gel will be 
referred to as [SG J-PhCH^Cl . 

Preparation of Silica Gel Bound 3 , 3 ' -Iminodi- 
propionitrile 

The nitrile, HNCCH-CH-CN) ^ , was purchasd from Kodak. A 
sample of [SGjPhCH-Cl (9-10 g) as described in the previous 



130 

section was placed in a Parr pressure bottle with 30-35 mL of 
3 , 3 ' -iminodipropionitrile and a magnetic stirring bar. This 
was attached to a pressure head identical to that used for 
catalysis (see Chapter II) except for its attachment to an 
argon cylinder. The silica gel slurry was purged 8 times 
with 80 psi Ar, then set to 90 psi Ar and placed in a 90°C 
oil bath for 24 hours. Vapor pressure of the nitrile causes 
the pressure to increase, but is vented if necessary to keep 
the total pressure at 100 psi. 

After one day, the slurry is cooled and the argon 
pressure removed. Placing 20-30 mL methanol into the 
pressure bottle and letting stand for a short while 
facilitates filtering, which is best done on a Buchner funnel 
with filter paper rather than a fritted filter. The silica 
gel is washed with methanol, benzene and then more methanol 
followed by Soxhlet extraction for 24 hours with methanol. 
The silica gel is then dried at 70-80°C under vacuum. 



Preparation of Silica Gel Bound bis- ( 3-aminopropyl ) - 

amine |SGl-DPT 



A sample of silica gel containing cyano groups on the 
surface as described in the previous section was placed into 
a 500 mL 3 neck round bottom flask with a stir bar. Cycles 
of evacuation and N„ filling were alternated, followed by 
syringed addition of 1 M BH^/THF solution (200 mL for a 25 g 
sample). If the silica gel sample used has not been 
thoroughly dried of methanol, a bubbling borane-methanol 



131 
reaction will be observed upon BH-/THF addition. The silica 
gel is stirred in the borane solution for 24 hours under a 
nitrogen atmosphere. The slurry is then heated to near 
reflux for 2 hours, allowed to cool followed by the addition 
of enough methanol to destroy all of the excess borane. The 
mixture is again heated to near reflux for 1-2 hours, cooled 
and filtered in air. 

Washing procedure on a polymer such as polystyrene 
would entail treatment with acid while heating, washing and 
then treatment with base. However, there is a fear of 
hydrolyzing the functional groups off of a silica surface with 
acid. As a result, THF and toluene were used to wash the 
filtered silica gel. Dilute (0.1 M) HCl in dioxane was then 
used to briefly treat the silica gel, then washed, and 
finally pyridine in dioxane stirred with the silica gel. 
Finally, thorough washing with methanol was done followed by 
drying at 50-60 C under vacuum. Later experiments indicated 
that heating with 3 M HCl/dioxane probably does remove the 
functional groups from the silica surface as indicated by its 

inability to react with salicylaldehyde (the next step) or to 

87 
incorporate copper (II) ions. 



Condensation of Salicylaldehyde with [SG]-DPT 

Benzene is added to [SG] -DPT to make a slurry, followed 
by addition of a large excess of salicylaldehyde and several 
hours of stirring, although reaction occurs very quickly. 



132 
The slurry is also heated for an hour to insure complete 

reaction. The product with salicylaldehyde incorporated, 

[SG]-SalDPT, is washed with benzene and ethanol thoroughly, 

and then dried under vacuum at 60 C. The product silica gel 

is a bright yellow in color due to the yellow Schiff base. 

Incorporation of Metal Ions into [SG]-SalDPT 

Several metal ions are incorporated into the bound 
[SG]-SalDPT ligand by stirring the f unctionalized silica gel 
with a DMF solution of the metal ion. Iron(III) chloride in 
1:1 (v:v) DMF/pyridine and cobalt(II) acetate in DMF are used 
to prepare [SG] -Fe ( III ) SalDPT and [SG] -Co( II ) SalDPT, 
respectively. The cobalt (II) analog is prepared under argon, 
but both Fe(III) and Co(II) analogs are air stable. 
Copper(II) acetate in DMF is used with the [SG]-DPT ligand to 
prepare the [SG] -Cu( II )DPT complex. In each case the silica 
gel ligand and metal ion solution are stirred overnight, 
filtered and thoroughly washed or Soxhlet extracted. 

The [SG]-Cu(II)DPT and [SG] -Co( II ) SalDPT -02 (oxygen 
adduct) both give characteristic ESR spectra similar to those 
of their homogeneous analogs. 

Incorporation of Fe(II) into [SG]-SalDPT 

The Fe(II) supported complex preparation procedure is 
quite air sensitive, and much care must be employed to 
exclude oxygen. Distilled DMF (30 mL) is added to 2.0 g [SG] 
-SalDPT or [SG] -BrSalDPT and the slurry f reeze-pump-thawed 



133 
many (at least five) times. This is seemingly the best way 

to exclude oxygen from the initial silica gel ligand. The 

source of iron is FeCCO)-, which is poured through glass 

wooland then placed under a nitrogen atmosphere. The Fe(CO)_ 

(2 mL) is syringed into the silica gel slurry and stirred 

overnight in the dark, under inert atmosphere. The next day 

the slurry (deep red in color) is heated for 3-4 hours (not 

to reflux). The slurry turns a deep purple color during 

heating. The slurry is cooled, then filtered in a Schlenk 

apparatus which has been attached to the round bottom flask 

throughout reaction. The silica gel was washed with DMF, 

then the entire apparatus was transferred into an inert 

atmosphere glove box. Here the silica gel is washed with 

DMF, THF and diethyl ether until clear filtrates are 

obtained. The pink silica gel is then dried under vacuum for 

24 hours, and stored in the inert atmosphere box. The colors 

during preparation and of the final solid correspond to those 

8 3 
found from similiar work conducted on polystyrene. 

8 8 
Mossbauer simulations were done by program DMOSFIT with 

nuclear energy levels calculated using spin Hamiltonian 

calculated data points fitted to experimental data using the 

89 
non-linear least squares program DSTEPIT. 

Incorporation of Cu(II) into the Silica Gel Matrix 

Davison Grade 62 silica gel, washed with 1 M NaOH, and 
then copious amounts of water, is stirred with a copper (II) 
nitrate solution for 24 hours. The resulting blue silica gel 



134 
can then be Soxhlet extracted with boiling water for several 
days without the appearance of any blue copper ions in the 
extraction water. This procedure places copper ions on the 
silica surface. 

Silica gel is dissolved in concentrated hydrochloric 
acid in the first step to incorporating copper ions into the 
silica gel. Excess Cu(NO^ ) 2' 3H2O is added to the solution 
and stirred for several hours. The solution is then 
neutralized to pH 7 with large amounts of NaOH solution. 
Upon reaching a pH of 7, the green solution turns to cloudy 
blue as solid is formed. After 2 hours the solid is filtered 
and washed with copius amounts of water. This very bluesolid 
is dried under vacuum to yield a rather brittle, 
non-unif ormly sized silica gel. This resulting silica gel 
appears to have copper ions as part of its structure, but its 
surface area has been greatly reduced as evidenced by the 
large chunk-like nature of the resulting solid. Extensive 
physical crushing of the solid produces irregularly shaped, 
brittle pieces which do not approach the small homogeneous 
grain-like nature of the original silica gel. 

Preparation of Silica Gel Anion Exchange Resins 

Plain silica gel is reacted with (CH2CH2O) ^Si (CH2CH2- 
CH-NH-) and CISKCH..) (if desired) in xylenes in the same 
manner as was described earlier in preparing chloromethyl- 
phenyl silica gel. The f unctionaliz ing silanes are added 
from an addition funnel dropwise into a slurry of the silica 



135 
gel in xylenes which has been heated nearly to reflux. An 
amount of 2.8 mL (12 mmol) or 28.0 mL (120 mmol) of the 
aminosilane may be reacted with 60 grams of plain silica gel 
to yield products having 0.2 mmol -NH_/g [ SG] or 2.0 mmol 
HN-Zg [SG], respectively. Some amount of ClSi(CH^)^ may also 
be added to react with a percentage of the remaining surface 
silanol groups (11.7 mL to react with all of the remaining 
-SiOH groups). After 24 hours of reaction, the silica gel is 
cooled, filtered and washed with xylenes, ethanol and 
benzene. Soxhlet extraction in benzene and the usual vacuum 
drying follow. 

The resultant silica gel is then stirred overnight at 
room temperature with a DMF solution which has been saturated 
with CH,C1 gas. Also added to the slurry is a small amount 
of 2 , 6-dimethylpyridine ( 2 , 6-lutidine) to act as a proton 
sponge. Filtering, washing with acetone and benzene, and 
drying under vacuum produce a silica gel with a quaternary 
ammonium chloride functionality bound to the surface. A 
quaternary ammonium iodide functionality is produced in an 
identical manner by substituting iodomethane for 
chloromethane above. These silica gels are then able to 
function as anion exchange resins. 

Titanium Carbide as a Solid Support 

Titanium carbide was purchased from Aldrich and used 
without further purification. A tube furnace was used with a 
quartz tube with a three-way stopcock at one end and a two- 



136 
way stopcock at the opposite site. Titanium carbide was 
placed in the center of the tube and purged with nitrogen, 
the outlet end being attached to a bubbler. Chlorine gas 
from a lecture bottle was slowly passed over the carbide 
solid as the tube was slowly heated to 125 C for two hours, 
then heated to 150°C for another hour. Other experiments 
where the solid is heated to 250°C (or 350°C) produces a 
clear liquid which fumes vigorously upon exposure to air 
characteristic of TiCl.. The black solid is filtered and 
washed with toluene. Both of the products described above 
were used in further reactions as chlorinated titanium 
carbide. These samples were stirred in a nitrobenzene 
solution saturated with NH, in an attempt to produce amine 
groups on the surface. The chlorinated samples were also 
treated with n-butyl lithium in hexanes under argon, followed 
by reaction with [ Rh(CO) 2( solvent ) ] BF . in THF in an attempt 
to bind the rhodium complex to the carbide surface. As 
described in the Results and Discussion Section, no adequate 
technique was employed to determine the success of these 
reactions. 

Bromine was also reacted with titanium carbide in 
attempts to produce bromine atoms bound to the carbide 
surface. Liquid bromine was stirred with titanium carbide 
for 24 hours and then thoroughly washed with toluene. 
Soxhlet extraction is strongly recommended in these systems 
to remove reactants from the solid. Reaction schemes similar 



137 
to those carried out on the chlorinated carbide samples were 
also conducted on the brominated carbides. 
Fenton Chemistry 

The substances FeCl2'4H20 (0.240 g) and .50 mL aqueous 
O.IN NCIO. are mixed together and degassed with N„ into which 
1.1 mL isopropanol (2-propanol) is syringed followed by 0.1 
mL 30% H-O-. This mixture is stirred under N„, aliquots 
withdrawn and examined by GLC ( DEGA column, 100°C). The same 
procedure is employed using FeCl^ as the iron source. 

The materials 0.18 g [SG J-Fe(III ) SalDPT (0.5 mmol 
ligand/g [SG]), 50 mL . IN HCIO^ , 1 . 1 mL isopropanol and 0.5 
mL 30% H_0- were used for supported reactions. After 24 
hours of reaction, both the silica gel and filtrate were 
colorless. Addition of hydroxide ion to the filtrate 
produced a rust colored precipitate. 

Into large screw top vials were placed 0.1 g samples of 
[SG]-Fe(III)SalDPT, 5 mL of O.IN HCIO^ prepared in the 
solvent of choice, and 0.0 to 0.5 mL of aqueous 30% H„0-. 
The choice of solvent (water, isopropanol, several water/- 
isopropanol mixtures, acetonitrile) had very little effect on 
the extent of leaching, at least within the limits of 
comparing color change by eye. The acid-only systems showed 
markedly less leaching than did the acid/peroxide samples. 
The silica gel samples treated with both acid and peroxide 
were either completely white or very slightly brown after 24 
hours, showing complete or nearly complete loss of metal from 
the silica support. 



CHAPTER V 
GENERAL CONCLUSION 



Throughout the previous chapters, research which 
involved the use of transition metal centers as oxidation 
catalysts has been described. The importance of careful 
characterization of the active catalytic species in a system 
has been presented. Also demonstrated has been the need to 
obtain detailed knowledge of the surface in preparing 
immobilized, catalytically active species on an inorganic 
polymer such as silica gel. Applications to several ligand 
systems and transition metals were presented, and the 
potential for further expansion of this research pointed out. 

Specific, detailed chemical conclusions were presented 
with each topic as it was discussed. Overall, the goal of 
this work has been to provide some insight into the nature, 
design, support, application and refinement of several 
transition metal oxidation catalysts. 



138 



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38. The measurement of small amounts of acetone production 
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40. This latter explanation is somewhat speculative due to 
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adduct (see ref. 7). 



142 

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144 

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la 



BIOGRAPHICAL SKETCH 



The author, David Chappel Pribich, was born in Detroit, 
Michiaan, on February 15, 1957. He was raised in Royal Oak 
with his sister, Jean, by Mr. and Mrs. Milan Pribich and 
graduated from Kimball High School in June, 1975. He 
attended the University of Michigan in Ann Arbor and 
graduated with honors in April, 1979, with a Bachelor of 
Science degree. He began graduate studies at the University 
of Illinois in Urbana and moved to the University of Florida 
in Gainesville with the chairman of his doctoral committee in 
order to complete the requirements for his Ph.D. degree. On 
July 30, 1983, the author was married to Susan Elizabeth 
Alberti, daughter of Mr and Mrs. Guido A. Alberti. The 
author began employment by acquiring a postdoctoral and 
teaching position at the California State University in 
Northridge, California. 



145 



I certify that I have read this study and that in my 
opinion it conforms to acceptable standards of scholarly 
presentation and is fully adequate, in scope and quality, as 
a dissertation for the degree of Doctor of Philosophy. 




Russell S. urago, cna-irman 
Professor of Chemistry 



I certify that I have read this study and that in my 
opinion it conforms to acceptable standards of scholarly 
presentation and is fully adequate, in scope and quality, as 
a dissertation for the degree of Doctor of Philosophy. 




'^<^^' 




I'^iinici^H'^ 



David E. Richardson 
Assistant Professor 
of Chemistry 



I certify that I have read this study and that in my 
opinion it conforms to acceptable standards of scholarly 
presentation and is fully adequate, in scope and quality, as 
a dissertation for the degree of Doctor of Philosophy. 




R. Carl Stoufer 
Associate Professor 
of Chemistry 



I certify that I have read this study and that in my 
opinion it conforms to acceptable standards of scholarly 
presentation and is fully adequate, in scope and quality, as 
a dissertation for the degree of Doctor of Philosophy. 





j;:::^ 



Jdhn Q. Dorsey 
Assistant Professor 
of^Chemistry 




I certify that I have read this study and that in my 
opinion it conforms to acceptable standards of scholarly 
presentation and is fully adequate, in scope and quality, as 
a dissertation for the degree of Doctor of Philosophy. 



I-Ij- 1-1 ■ n^ 



Hal H. Rennert 
Associate Professor of 

Germanic and Slavic 
Languages and 
Literatures 



This dissertation was submitted to the Graduate Faculty 
of the Department of Chemistry in the College of Liberal Arts 
and Sciences and to the Graduate School, and was accepted as 
partial fulfillment of the requirements for the degree of 
Doctor of Philosophy. 



May, 1985 



Dean, Graduate School 



UNIVERSITY OF FLORIDA 



3 1262 08554 1372