Electropolymerization of an N-heterocyclic carbene-gold(I) complex

Article abstract of DOI:10.1021/ja908969q

(Figure Presented) We report the first example of a polymer in which each monomer contains an N-heterocyclic carbene (NHC) orthogonally connected to its main chain. A polymerizable imidazolylidene-AuCl complex containing pendant bithiophene moieties was p

Full text of DOI:10.1021/ja908969q

Published on Web 11/25/2009
Electropolymerization of an N-Heterocyclic Carbene-Gold(I) Complex
Adam B. Powell, Christopher W. Bielawski, and Alan H. Cowley*
Department of Chemistry and Biochemistry and Center for Nano and Molecular Science and Technology,
The UniVersity of Texas at Austin, 1 UniVersity Station, A5300, Austin, Texas 78712-0165
Received October 20, 2009; E-mail: cowley@mail.utexas.edu
In contrast to the large number of useful polymeric materials
that contain tethered N-heterocyclic carbenes (NHCs),1 there are
relatively few examples of polymers in which the NHCs are integral
components of the main chain.2 Several of these polymeric NHCs
exhibit sensitivity to air and moisture, severely limiting their utility.3
Furthermore, in all of these examples, the carbenes are essential
for maintaining the polymer structure and generally not available
for ligation or further reactivity.
lengths and angles were similar to those of known NHC-Au
complexes,7 the packing diagram revealed a one-dimensional series
of Au · · · Au contacts [3.2262(4) Å] between neighboring units of
6 that were arranged in a head-to-head fashion (Figure 1, right).
We envisioned that electrochemical polymerization of an ap-
propriately substituted thiophene-based monomer would constitute
a new and promising approach to a polymer that contains an NHC
orthogonally positioned with respect to its main chain. Arrange-
ments of this type are of considerable interest for their ability to
attach readily accessible entities with catalytic or biological
properties or enable the addition of electronically tunable features
to these systems. Furthermore, such an approach should be general
and readily extended to incorporate a wide variety of NHC-metal
complexes into polymeric materials. In view of the rapidly
expanding interest in both the catalytic4 and the medicinal applica-
tions of gold,5 the efforts described herein focused on the
polymerization of an NHC-AuCl complex.
Figure 1. (left) POV-Ray view of 6 showing 50% probability thermal
ellipsoids and selected atom labels. (right) Packing diagram for 6 showing
Au· · ·Au contacts. Hydrogen atoms have been omitted for clarity.
Scheme 2. Electrochemical Polymerization of
Bithiophene-Substituted Gold(I) Carbene Monomer 6
Scheme 1. Synthesis of Gold(I) Carbene Complex 6
Electropolymerization of a 1 mM solution of 6 (in CH2Cl2) onto
a platinum disk or an indium tin oxide glass slide afforded poly(6),
as illustrated in Scheme 2.8 The reaction was performed in a drybox
using a three-electrode cell containing a silver wire quasi-reference
electrode and [(n-Bu)4N+][PF6-] as the supporting electrolyte.8 As
shown in the top panel of Figure 2, the polymerization proceeded
cleanly and in the anticipated fashion within a window of -0.2 to
1.5 V.9 The polymer film oxidation wave grew with subsequent
scans at 1.1 V and continued to increase for over 20 cycles,
indicating good conductivity through the film. The shift to higher
peak potentials with additional scans is typical and may originate
from heterogeneous transfer kinetics and/or polymer growth on the
electrode.8 Similar to other thiophene-based polymerizations, the
reduction peak (0.9 V) shifted cathodically with increasing number
of scans.10 Subsequent oxidation or reduction of the deposited film
(in monomer-free solutions) resulted in no decrease in peak height,
reflecting the high stability of the polymer.11 Examination of the
scan-rate dependence (Figure 2, bottom) revealed that the peak
current for the electrodeposited film of poly(6) in a solution of
CH2Cl2/[(n-Bu)4N+][PF6-] increased with scan rate, as expected
on the basis of the Cottrell equation, and was in accord with related
examples in the literature.10 Scan rates above 100 mV/s resulted
in distortion of the electrochemical profiles.
As summarized in Scheme 1, monomer 6 was synthesized using
a five-step procedure in an overall yield of ∼60%. Bithiophene
monoimine 1, which was prepared by condensing bithiophene
carboxaldehyde with 4-n-butylaniline (92% yield), was dimerized
to the corresponding ene-diamine 2 (structure not shown) via a
NaCN-catalyzed aldimine coupling reaction.6 Subsequent oxidation
of 2 with p-benzoquinone afforded the desired diimine 3, which,
when treated with paraformaldehyde and anhydrous HCl, resulted
in 4. Treatment of the latter with Ag2O afforded AgCl complex 5
(structure not shown). Finally, transmetalation of 5 with Au(tht)Cl
(tht ) tetrahydrothiophene) afforded the desired AuCl complex 6
(diagnostic 13C NMR signal: δ ) 172 ppm in CD2Cl2).7 Further
confirmation of this structural assignment was obtained via X-ray
diffraction (XRD) analysis (Figure 1, left). Although the key bond
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10.1021/ja908969q  2009 American Chemical Society

C O M M U N I C A T I O N S
the diimine moiety. Upon cyclization to the imidazolium chloride
4, λmax hypsochromically shifted to 326 nm, an observation that is
consistent with an increase of charge on the complex. Upon
polymerization of 6, however, λmax shifted bathochromically to 406
nm and the absorption signature broadened, as expected for a
polymeric system. On the basis of these results, we conclude that
the spectroscopic character of 3-6 is dominated by ligand π-π*
transitions. The electrochemical, XPS, and UV-vis spectroscopic
studies collectively support the structural assignment of poly(6).
In summary, the first example of a polymer containing an NHC
orthogonally connected to its main chain has been synthesized.
Because of this structural arrangement, the carbene moiety in this
material is poised to ligate transition metals or facilitate other
carbene-based reactivities. The methodology presented may be
general and readily extended to access a variety of materials
containing NHC-based complexes connected directly to the main
chains of polymers.
Acknowledgment. We are grateful to the Robert A. Welch
Foundation (Grants F-0003 and F-1621) for support of this work.
We also thank Dr. V. Lynch for his assistance and the National
Science Foundation (Grant 0618242) for the X-ray photoelectron
spectrometer used in this work.
Supporting Information Available: Additional experimental, NMR,
electrochemical, XPS, UV-vis, and X-ray crystallographic data and a
CIF file for 6. This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 2. (top) Electropolymerization of 6. Inset: plot of linear current
increase vs number of scans. (bottom) Poly(6) scan-rate dependence. Inset:
plot of linear current increase vs scan rate.
References
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Films of poly(6) were also examined by X-ray photoelectron
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experimental error12 of the values anticipated for the structure of
poly(6) shown in Scheme 2. The S 2p signal featured a shoulder
that was assigned to sulfur atoms in slightly different chemical
environments. Because the monomer contains distinct thiophene
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gold deposition may be present in the analyzed samples, since the
Au 4f signals revealed small shoulders at lower binding energies.
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Finally, the photophysical properties of poly(6) were explored
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3, λmax for 3 was 376 nm and attributed to the π-π* transitions of
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