New approach to rapid generation and screening of diverse catalytic materials on electrode surfaces

Article abstract of DOI:10.1021/ja002334u

This paper describes a general approach to rapid generation and screening of catalytic materials on electrode surfaces. The properties of the corresponding polymers, including catalytic performance, can be modulated by varying the monomer feed ratios, monomer concentrations, and applied polymerization potential. Thus, the generation of the polymeric TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxy) catalysts was performed by electrochemical copolymerization of 2,2'-bithiophene with the TEMPO catalyst precursors containing pyrrole side chains. A library of catalyst films was obtained over a wide range of bithiophene/pyrrole ratios upon repeated scanning of the applied potential from +0.5 to +1.4 V (vs Ag/AgCl). The resulting catalyst films were utilized in both chemical and electrochemical oxidation of primary alcohols to aldehydes.

Full text of DOI:10.1021/ja002334u

J. Am. Chem. Soc. 2000, 122, 11787-11790
11787
New Approach to Rapid Generation and Screening of Diverse
Catalytic Materials on Electrode Surfaces
Tung Siu, Shahla Yekta, and Andrei K. Yudin*
Contribution from the Department of Chemistry, UniVersity of Toronto, 80 St. George St.,
Toronto, Ontario M5S 3H6, Canada
ReceiVed June 28, 2000
Abstract: This paper describes a general approach to rapid generation and screening of catalytic materials on
electrode surfaces. The properties of the corresponding polymers, including catalytic performance, can be
modulated by varying the monomer feed ratios, monomer concentrations, and applied polymerization potential.
Thus, the generation of the polymeric TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxy) catalysts was performed
by electrochemical copolymerization of 2,2′-bithiophene with the TEMPO catalyst precursors containing pyrrole
side chains. A library of catalyst films was obtained over a wide range of bithiophene/pyrrole ratios upon
repeated scanning of the applied potential from +0.5 to +1.4 V (vs Ag/AgCl). The resulting catalyst films
were utilized in both chemical and electrochemical oxidation of primary alcohols to aldehydes.
Introduction
electropolymerization. The fact that maximum catalytic activity
of a supported catalyst need not correlate with maximum ligand
loading6 warrants screening of surface compositions. This
principle can be extended to generate arrays of copolymer films
combinatorially7-9 to identify the most active catalysts among
them. Noteworthy, electrochemistry was recently introduced into
the field of combinatorial catalysis.9 A contribution from our
lab describes the design and applications of the spatially
addressable electrolysis platform (SAEP)10 for parallel electro-
synthesis which has led to the present idea of parallel electro-
chemical generation of arrays of copolymer film catalysts.
New routes to catalyst immobilization on solid supports have
received considerable attention.1 The ultimate goal in this field
is to combine advantages inherent to homogeneous catalysis
with the many virtues of heterogeneous catalysis.2 The loss of
catalytic activity through poisoning or leaching is the biggest
problem that faces many immobilized catalysts. Thus, there is
a growing need for the new types of catalytic materials that
possess high stability and retain catalytic activity under a wide
range of reaction conditions.
Electrochemistry is at the interface of solution and solid-phase
chemistry, as reactions occur in the diffusion layer formed at
the electrode surface.3 We were intrigued by the possibility of
using electrochemistry to generate copolymer film supports on
electrode surfaces with the catalyst sites attached to the polymer
chains. The resulting catalytic materials can then be used in
either chemically or electrochemically driven redox processes.
It is possible to modulate the composition and properties of the
conducting copolymer by varying (1) monomer feed ratio,4 (2)
monomer concentrations,5 and (3) applied polymerization
potential.4b,c,5 The latter parameter is unique to electrochemistry
and offers a competitive advantage of using different redox-
active monomers at the catalyst preparation stage. If the
oxidation potentials of the monomers are different, it should
be possible to obtain conducting copolymers with a high degree
of variability in the composition of the catalyst matrix by fine-
tuning the feed ratio as well as applied potential during
Experimental Section
General Information. Acrylonitrile, pyrrole, N-hydroxysuccinimide,
4-amino-2,2,6,6-tetramethylpiperidinyloxy (free radical), 4-hydroxy-
2,2,6,6-tetramethylpiperidinyloxy (free radical), and 2,2′-bithiophene
were purchased from Aldrich Chemical Co. Column chromatography
was carried out using 230-400 mesh silica gel. 1H NMR spectra were
referenced to residual CHCl3 (δ 7.26 ppm) and 13C spectra were
referenced to CDCl3 (δ 77.2 ppm). Electrochemical polymerization and
cyclic voltammetry characterization were conducted on a BAS CV-
50W Voltammetric Analyzer (Bioanalytical Systems, Inc.) equipped
with a BAS C3 three-electrode cell stand. Electrochemical oxidation
of benzyl alcohol with the polymer catalyst poly(3-bithiophene) was
performed with use of a laboratory DC power supply (GW, model GRP-
3060D).
Preparation of 3-(Pyrrol-1-yl)propionic Acid. A modified literature
procedure11 was followed. Acrylonitrile (10 mL, 152 mmol) was added
(1) (a) Ribeiro, M. R.; Deffieux, A.; Portela, M. F. Ind. Eng. Chem. Res.
1997, 36, 1224-1237. (b) Clark, J. H.; Macquarrie, D. J. Chem. Commun.
1998, 853-860. (c) Alt, H. G. J. Chem. Soc., Dalton Trans. 1999, 1703-
1709.
(2) Herrmann, W. A.; Cornils, B. Angew. Chem., Int. Ed. 1997, 36,
1048-1067.
(6) Blaser, H.-U.; Jalett, H. P.; Monti, D. M.; Reber, J. F.; Wehrli, J. T.
Stud. Surf. Sci. Catal. 1988, 41, 153.
(7) (a) Jandeleit, B.; Schaefer, D. J.; Powers, T. S.; Turner, H. W.;
Weinberg, W. H. Angew. Chem., Int. Ed. 1999, 38, 2494-2532. (b) Cong,
P.; Doolen, R. D.; Fan, Q.; Ciaquinta, D. M.; Guan, S.; McFarland, E. W.;
Poojary, D. M.; Self, K.; Turner, H. W.; Weinberg, W. H. Angew. Chem.,
Int. Ed. 1999, 38, 484-488. (c) Senkan, S.; Krantz, K.; Ozturk, S.; Zengin,
V.; Onal, I. Angew. Chem., Int. Ed. 1999, 38, 2794-2799. (d) Crabtree, R.
H. Chem. Commun. 1999, 1611-1616.
(8) Reddington, E.; Sapienza, A.; Gurau, B.; Viswanathan, R.; Saran-
gapani, S.; Smotkin, E. S.; Mallouk, T. E. Science 1998, 280, 1735-1737.
(9) Sullivan, M. G.; Utomo, H.; Fagan, P. J.; Ward, M. D. Anal. Chem.
1999, 71, 4369-4375.
(10) Siu, T.; Li, W.; Yudin, A. K. J. Comb. Chem. 2000, 2, 545-549.
(11) Blume, R. C.; Lindwall, H. G. J. Org. Chem. 1945, 10, 255-258.
(3) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications; John Wiley & Sons: New York, 1980.
(4) (a) Bagheri, A.; Nateghi, M. R.; Massoumi, A. Synth. Met. 1998,
97, 85-89. (b) Welzel, H.-P.; Kossmehl, G.; Engelmann, G.; Hunnius, W.-
D.; Plieth, W. Electrochim. Acta 1999, 44, 1827-1832. (c) Wan, X.; Zhang,
W.; Jin, S.; Xue, G.; You, Q.-D.; Che, B. J. Electroanal. Chem. 1999, 470,
23-30.
(5) Latonen, R.-M.; Kvarnstrom, C.; Ivaska, A. Electrochim. Acta 1999,
44, 1933-1943.
10.1021/ja002334u CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/16/2000

11788 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Siu et al.
dropwise over a period of 30 min to a mixture of pyrrole (10 g, 149
mmol) and tetrabutylammonium hydroxide (3.0 mL of 40 wt % aqueous
solution). The temperature was kept at -78 °C. The reaction mixture
was then allowed to warm to room temperature and was stirred for 1
h. The crude nitrile was hydrolyzed by refluxing with a solution of
potassium hydroxide (10 g, 178 mmol) for 1 h in 15 mL of water.
Acidification with 10 mL of 5% hydrochloric acid and extraction with
ether (20 mL × 3) gave 15.3 g (74%) of product.
cm × 2.0 cm) were immersed into the upper layer of the resulting
biphasic mixture. The anode was precoated with the polymer catalyst
poly(3-bithiophene). The mixture was electrolyzed under a constant
current of 20 mA/cm2 with moderate stirring until 2.0 F/mol of
electricity was passed. The organic layer was separated and the aqueous
layer was extracted with CH2Cl2. The combined organic extracts were
dried over MgSO4 and concentrated in vacuo. The crude product was
purified by column chromatography on silica gel with 80:20 hexane/
ethyl acetate as eluent.
General Procedure for Synthesis of Monomers 1-3. 3-(Pyrrol-
1-yl)propionic acid (0.70 g, 5.0 mmol) and 4-hydroxy-2,2,6,6-tetra-
methylpiperidinyloxy (0.86 g, 5.0 mmol) were stirred in 20 mL of ethyl
acetate at 0 °C. N,N′-Dicyclohexylcarbodiimide (DCC, 1.03 g, 5.0
mmol) in 10 mL of ethyl acetate was added slowly to the above mixture.
A white precipitate of dicyclohexylurea was formed within 30 min.
The reaction mixture was allowed to warm to room temperature and
was stirred overnight. The crude product was filtered and the filtrate
was purified using a silica gel column and 60:40 hexane/ethyl acetate
as eluent. Monomer 3 appeared as orange crystals (67% yield).
Monomers 1 and 2 were prepared by a similar procedure using the
corresponding TEMPO precursors.
Results and Discussion
Polypyrrole and polythiophene conduct electricity in the
oxidized state and are normally obtained via chemical or
electrochemical oxidative polymerization of pyrrole and thiophene,
respectively.12 The electrochemical polymerization method
provides conducting polymers in the form of films adhered to
the electrode surfaces. A number of catalysts were found among
the polypyrroles derivatized with metal complexes.13 We now
wish to demonstrate implementation of a novel technique for
conducting copolymer catalyst generation and screening. A
specific example illustrates high-throughput generation and
identification of new heterogeneous oxidation catalysts on
electrode surfaces.
Our studies commenced with the generation of suitable
conducting polymer precursors for heterogeneous TEMPO
(2,2,6,6-tetramethylpiperidin-1-yloxy) oxidation catalysts.14 Pyr-
role-based monomers 1-3 were synthesized from 3-(pyrrol-1-
yl)propionic acid.11 It was envisaged that the pyrrole 1 could
be utilized for post-polymerization catalyst loading while the
pyrroles 2 and 3 could introduce catalyst sites via direct
electropolymerization.
3-(Pyrrol-1-yl)propionic Aacid 2,5-Dioxopyrrolidin-1-yl Ester (1).
1H NMR (CDCl3) δ 6.70 (t, 2H, J ) 2.0 Hz), 6.17 (t, 2H, J ) 2.0 Hz),
4.30 (t, 2H, J ) 7.2 Hz), 3.07 (t, 2H, J ) 7.2 Hz), 2.84 (s, 4H). 13C
NMR (CDCl3) δ 168.87, 166.41, 120.62, 109.13, 44.39, 33.76, 25.68.
4-(3-(Pyrrol-1-yl)propionylamino)-2,2,6,6-tetramethylpiperidin-
1-yloxy (2). IR (KBr) 3312 (NH), 2975 (CH), 1651 (CdO), 1538 (NH),
728 (pyrrole) cm-1. MS, m/e (rel intensity) 293 (43), 154 (40), 139
(80), 124 (78), 109 (82), 80 (100). HRMS 293.1857 (Calcd for
C16H25N2O3: 293.1865).
4-(3-(Pyrrol-1-yl)propionyloxy)-2,2,6,6-tetramethylpiperidin-1-
yloxy (3). IR (KBr) 2974, 2935 (CH), 1734 (CdO), 1166 (C-O), 727
(pyrrole) cm-1. MS, m/e (rel intensity) 292 (11), 206 (18), 154 (21),
139 (19), 124 (33), 94 (63), 84 (100). HRMS 292.2142 (Calcd for
C16H26N3O2: 292.2127).
General Procedure for Electrochemical Polymerization. Electro-
chemical polymerization and characterizations were performed in a one-
compartment cell with a platinum disk (0.07 cm2) as working electrode
and a platinum wire as counter electrode, respectively. A Ag/AgCl
electrode was used as reference electrode. All electrochemical polym-
erization experiments were performed in acetonitrile solution with 0.1
M total monomer concentration and 0.1 M tetrabutylammonium
tetrafluoroborate as supporting electrolyte. Repetitive cyclic voltam-
metric scans between +0.5 and +1.5 V led to the formation of a black
electroactive film at the working electrode. Characterizations (by cyclic
voltammetry) of the copolymer films were conducted in acetonitrile
solutions with 0.1 M tetrabutylammonium tetrafluoroborate. Preparative
scale copolymer films for catalytic reaction were obtained with a pair
of platinum foil (5 cm2 each) electrodes as working and counter
electrodes and a Ag/AgCl reference electrode.
General Procedure for Alcohol Oxidation with Polymer Cata-
lysts. The reaction was performed in a 20 mL glass vial cooled in an
ice-water bath. The platinum foil, covered with the polymer catalyst
according to the procedure described above, was placed in the vial. A
dichloromethane solution (5 mL) of the alcohol (0.2 M) and tetralin
(0.1 M; as GC internal standard) were added followed by KBr (24 mg,
0.2 mmol). After the above mixture was cooled to 0 °C, 12 mL of
aqueous NaOCl (diluted to a final concentration of 0.1 M and buffered
by addition of NaHCO3 to a pH of 9.1) was added. The reaction mixture
was then vigorously stirred for 1 h. The organic phase was separated,
and the aqueous phase was extracted with dichloromethane. The
combined organic phases were dried over MgSO4. After analysis by
GC, the solvent was removed in vacuo, and the product was purified
by column chromatography on silica gel with 80:20 hexane/ethyl acetate
as eluent. The polymer film was washed three times with water,
methanol, and dichloromethane (5 mL each), air-dried, and reused as
such.
Direct electrochemical polymerization of 1 on a platinum
electrode in acetonitrile produced a film, while monomers 2
and 3 could not be polymerized electrochemically. It is well-
known that certain functional groups on the pyrrole side chain
can prohibit polymerization.15 Thus, we turned to copolymer-
ization with bithiophene, known for its ability to initiate
electrochemical polymerizations due to its low oxidation
potential (+1.3 V vs SCE).16 Gratifyingly, stable catalyst films
of generic composition 4 were obtained over a wide range of
bithiophene/pyrrole ratios upon scanning the potential from +0.5
to + 1.4 V (vs Ag/AgCl). A typical time for catalyst generation
(12) (a) Warren, L. F.; Anderson, D. P. J. Electrochem. Soc. 1987, 134,
101-105. (b) Armes, S. P. Synth. Met. 1987, 20, 365-371. (c) Roncali, J.
J. Mater. Chem. 1999, 9, 1875-1893.
(13) (a) Murray, R. W. Acc. Chem. Res. 1980, 13, 135-141. (b)
Deronzier, A.; Moutet, J.-C. Coord. Chem. ReV. 1996, 147, 339-371.
(14) de Nooy, A. E. J.; Besemer, A. C.; van Bekkum, H. Synthesis 1996,
1153-1174.
(15) (a) Morse, N. J.; Rosseinsky, D. R. J. Electroanal. Chem. 1988,
255, 119-141. (b) Bartlett, P. P.; Chung, L.-Y.; Moore, P. Electrochim.
Acta 1990, 35, 1273-1278.
(16) Peters, E. M.; van Dyke, J. D. J. Polym. Sci. A: Polym. Chem.
1991, 29, 1379-1385.
Procedure for Electrochemical Oxidation of Benzyl Alcohol with
Polymer Catalyst. A solution of benzyl alcohol (108 mg, 1.0 mmol)
in CH2Cl2 (5 mL) was placed in a 20 mL glass vial. To this solution
was added 10 mL of an aqueous solution of NaBr (25%) buffered by
addition of solid NaHCO3 to pH 8.6. Two platinum foil electrodes (2.5

General Approach to Rapid Generation of Catalytic Materials
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11789
Figure 1. Cyclic voltammogram of the copolymer of 1 and bithiophene
Figure 3. TEMPO content of the copolymer film as a function of the
2/bithiophene feed ratio in solution.
in acetonitrile (9:1 feed ratio of 1/bithiophene) with 0.1 M Bu4N+BF4
.
-
Table 1. Oxidation of Primary Alcohols to Aldehydes with the
TEMPO Catalyst Poly(2-bithiophene) Obtained from 2 and
Bithiophene (2:1 feed ratio)
Figure 2. Bithiophene composition in the catalyst matrix as a function
of bithiophene fraction in solution.
is 1-2 min. The cyclic voltammogram (Figure 1) of the
copolymer film, obtained from a 9:1 mixture of 1 and
bithiophene, displays a broad redox band extending from +1.0
to +0.6 V, where no distinctive redox bands for polypyrrole
(+0.2 to 0.0 V) and polythiophene (+1.4 to +1.0 V) could be
identified, indicating the polymer film is a copolymer instead
of a polymer blend.4c The appearance of a single redox couple
for the conducting copolymer has been correlated with uniform
redox properties.4c Most importantly, in the case of the film
produced from 1 and bithiophene, elemental analyses of the S/N
ratios indicate that the bithiophene/pyrrole ratio in solution is
in linear correlation with the film composition (Figure 2). This
correlation provides a convenient way of varying the content
of catalyst sites in the copolymer matrix when making an array
of catalysts. In addition to the feed ratio as a means of generating
diverse copolymeric materials, changing the position of the
potential window during potentiodynamic electropolymerization
can be used to vary the compositions of the resulting films.
However, preliminary results indicate that the corresponding
catalytic materials have low mechanical stability when potentials
above +1.5 V vs Ag/AgCl are being applied.17
In the case of electrochemical copolymerization of bithiophene
with the pyrrole monomer 2, we have observed an interesting
nonlinear behavior between the TEMPO content in the copoly-
mer film and the fraction of 2 in solution (Figure 3). The
TEMPO content was determined by correlating the integrated
CV curve of the copolymer backbone with the CV curve of the
TEMPO redox region. We believe that two effects, i.e., the
initiating/promoting effect of bithiophene and inhibiting effect
of the monomer 2, determine this electrochemical polymeriza-
tion behavior. Presumably, at high bithiophene/pyrrole feed
ratios, the initiating/promoting effect dominates and the TEMPO
content in the film correlates with the 2/bithiophene ratio. When
more monomer 2 is present in solution, its inhibiting effect
becomes predominant, and the TEMPO content decreases with
increasing 2/bithiophene feed ratio. Similar inhibiting behavior
of one of the monomers in electrochemical copolymerization
was previously observed.18 The nonlinear behavior in the present
case validates the combinatorial approach to conducting film
catalysts for those precursors that require bithiophene “activa-
tion” for efficient film formation and compositional screening.
To illustrate the utility of the electrochemically generated
materials in heterogeneous catalysis, we chose the TEMPO-
catalyzed oxidation of primary alcohols to aldehydes with
sodium NaClO/NaBr (Table 1).19 The 2:1 feed ratio of
2/bithiophene corresponds to the most active catalyst which was
identified during initial catalyst screening. The typical amount
of catalyst on a platinum foil electrode of 5 cm2 area is
(18) Mazeikiene, R.; Malinauskas, A. Synth. Met. 1998, 92, 259-263.
(19) Bolm, C.; Fey, T. Chem. Commun. 1999, 1795-1796.
(17) Siu, T.; Yudin, A. K. Unpublished results.

11790 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Siu et al.
of the ester-based catalyst is most likely due to better mechanical
properties of the adhered polymer.
To validate electrochemical oxidation as a complementary
screening tool for catalytic activity, we explored the TEMPO
modified platinum supports as anodes in the oxidation of
alcohols. Direct oxidation of 1-octanol on a poly(3-bithiophene)-
modified electrode in the presence of 2,6-lutidine20 was unsuc-
cessful. The peak current corresponding to TEMPO oxidation
and reduction decreased significantly upon adding 2,6-lutidine
to the alcohol solution in acetonitrile, indicating passivation of
the electrode surface. However, the indirect, mediatory elec-
trooxidation method21 gave encouraging results. For the NaBr-
mediated electrooxidation of benzyl alcohol in the CH2Cl2/H2O
two-phase system on a modified platinum anode (3/bithiophene
feed ratio ) 2:1), the isolated yield of benzaldehyde was 63%.
When an unmodified platinum electrode was used the isolated
yield of benzaldehyde was 7% and the starting material was
recovered.
In summary, we have developed a new approach to rapid
generation and screening of diverse catalytic materials on
electrode surfaces. Copolymerization of various ratios of film
precursors yields diverse surfaces with modulated catalytic
properties. In theory, the number of possible combinations is
infinite. Bithiophene solves the problem of poor polymerizability
of some pyrrole-substituted catalyst precursors. Rapid generation
of modified electrodes should be made straightforward by
copolymerizing libraries of pyrroles with bithiophene on arrays
of electrodes.9 The resulting materials can be applied in
processes that are driven by either electrochemical or chemical
redox systems. We are currently exploring the SAEP instru-
ment10 for parallel screening of film compositions for high-
throughput generation of electrocatalysts.
Figure 4. Recyclability of the poly(2-bithiophene) (dark) and poly-
(3-bithiophene) (light) film catalysts for oxidation of benzyl alcohol
to benzaldehyde (GC yield, %).
approximately 1.5 µmol, estimated from the TEMPO content
in the copolymer by integrating the CV curve. Thus, the catalyst
loading is approximately 1.5 mol %, which corresponds to a
minimum turnover frequency of 60/h for benzyl alcohol
oxidation, a result comparable with the silica-supported TEMPO
catalysts (90/h).19 The catalytic activities of the films obtained
using other feed ratios were also screened under the same
alcohol oxidation conditions. For the films that correspond to
9:1 and 1:9 feed ratios of 2 and bithiophene, the respective yields
for benzyl alcohol oxidation were 51% and 48%, consistent with
the TEMPO loadings calculated from the CV experiments. Thus,
the cyclic voltammetry can be used as a reliable screening tool.
In a control experiment, a copolymer film of 1-butylpyrrole and
bithiophene (1:1) was used for oxidation of benzyl alcohol. Less
than 5% of benzaldehyde was detected by GC and 90% of
starting material was recovered.
Acknowledgment. We thank the National Science and
Engineering Research Council (NSERC), Canada Foundation
for Innovation (CFI), the Connaught Fund, the Research
Corporation, and the University of Toronto for financial support.
We also thank Dr. Wei Li for helpful discussions.
The recyclability of poly(2-bithiophene) catalyst was quite
low: the yield of subsequent oxidation under the same condi-
tions dropped to 19% after four runs. We were pleased to find
that, by changing the amide bond in 2 into an ester bond
(monomer 3), the heterogenized catalyst poly(3-bithiophene),
obtained with the optimized 3/bithiophene feed ratio of 2:1,
showed a significant improvement in catalytic performance. The
catalytic activity of the film remained high after four runs
(Figure 4). The yield dropped insignificantly (to 88%) after eight
consecutive catalytic experiments. The improved performance
JA002334U
(20) Semmelhack, M. F.; Chou, C. S.; Cortes, D. A. J. Am. Chem. Soc.
1983, 105, 4492-4494.
(21) Inokuchi, T.; Matsumoto, S.; Torii, S. J. Org. Chem. 1991, 56,
2416-2421.