Electropolymerization of a cyclometalated terthiophene: A hybrid material with a palladium-carbon bond to the backbone [4]

Full text of DOI:10.1021/ja002258v

10456
J. Am. Chem. Soc. 2000, 122, 10456-10457
Electropolymerization of a Cyclometalated
Terthiophene: A Hybrid Material with a
Palladium-Carbon Bond to the Backbone
Olivier Clot, Michael O. Wolf,* and Brian O. Patrick
Department of Chemistry
The UniVersity of British Columbia
VancouVer, British Columbia, Canada V6T 1Z1
ReceiVed June 22, 2000
Polythiophenes have been widely investigated for their interest-
ing and potentially useful properties, which include high con-
ductivity and luminescence.1,2 Many polythiophene derivatives
have been prepared in order to introduce new functionalities to
the material, such as side groups that allow a selective response
to specific analytes for sensor applications.3 Conjugated polymers
that bear pendant metal complexes are of particular importance
due to the possibility of integrating the chemical, optical, and
redox properties of the metal group and the conjugated backbone
to yield new hybrid materials.3-5 Several approaches have been
taken to attach metal centers to polythiophene derivatives; for
example bipyridine or phenanthroline binding sites have been
introduced into the backbone, and used to coordinate Cu, Zn, and
Ru.6-8 Recently, an interesting system was reported in which
Ru(II) is coordinated in a bidentate fashion by a pendant phos-
phine group and a backbone sulfur.9 Polythiophene may be viewed
as a cis polyacetylene structure stabilized by the sulfur atom;2
therefore, bonding a metal center to the carbon backbone is
expected to directly influence the electronic properties of the
polymer. We report herein the first example of such a material,
in which Pd(II) is σ-bonded to a backbone carbon atom.
Electropolymerization has been widely used for the preparation
of conductive polymer thin-films,2,3 and we have elected to
synthesize a metal-containing monomer for polymerization via
this method. The monomer 2 was synthesized via the reaction of
3′-diphenylphosphino-2,2′:5′,2′′-terthiophene (1)10 with 1 equiv
of PdCl2.11 The solid-state structure of 2 was unambiguously
established by determination of its X-ray crystallographic structure
(Figure 1), and consists of a dimer in which Pd is square planar
with bridging chlorides and 1 binding via the phosphine and C3
of the terthiophene.12 Cyclometalation of thiophene derivatives
Figure 1. X-ray crystal structure of 2. The hydrogen atoms are omitted
for clarity, and thermal ellipsoids are drawn at 30% probability. Selected
bond lengths (Å) and angles (deg): Pd(1)-C(3) 2.002(3); Pd(1)-P(1)
2.2171(6); Pd(1)-Cl(1) 2.4184(7); P(1)-Pd(1)-C(3) 92.03(8); Cl(1)-
Pd(1)-C(3) 176.55(8).
is well-known and has been observed with nitrogen-containing
chelating ligands such as 2-(2-thienyl)pyridine13-15 and 6-(2-
thienyl)-2,2′-bipyridine16 on Pd and Pt and with diphenyl-2-
thienylphosphine in a Ru cluster.17
The 31P NMR spectrum of 2 in CDCl3 contains two closely
spaced resonances at δ 17.7 and 16.6 in a 0.7:1 ratio, likely due
to the presence of both cis and trans isomers of the complex in
solution as has been previously observed for other halo-bridged
Pd dimers.18,19 Reagants such as phosphines and isonitriles are
known to cleave dihalo bridges in related compounds,20 and we
observe similar behavior for 2. For example, 2 reacts with tert-
butyl isocyanide (t-BuNC) at 25 °C in solution to give the
monomeric complex 3, which exhibits a characteristic terminal
NC stretch at 2209 cm-1 in the infrared region. The isocyanide
is most likely cis to the σ-bonded carbon in 3 as is typically the
case in related compounds.21 Other reagents such as PPh3,
P(OMe)3, PPh2Me, and PhNC also react with 2, whereas no
reaction occurs with MeCN.
* To whom correspondence should be addressed. E-mail: mwolf@
chem.ubc.ca.
(1) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of
Conducting Polymers, 2nd ed.; Marcel Dekker: New York, 1998.
(2) Roncali, J. Chem. ReV. 1992, 92, 711-738.
The cyclic voltammogram (CV) of 2 in CH2Cl2 containing 0.1
M (n-Bu)4NPF6 shows the presence of both an oxidation wave at
+1.20 V (Figure 2, first scan) and an irreversible reduction wave
(3) Roncali, J. J. Mater. Chem. 1999, 9, 1875-1893.
(4) Kingsborough, R. P.; Swager, T. M. Prog. Inorg. Chem. 1999, 48, 123-
231.
(5) Wolf, M. O.; Zhu, Y. AdV. Mater. 2000, 12, 599-601.
(6) Zhu, S. S.; Swager, T. M. J. Am. Chem. Soc. 1997, 119, 12568-12577.
(7) Zhu, S. S.; Kingsborough, R. P.; Swager, T. M. J. Mater. Chem. 1999,
9, 2123-2131.
(12) Crystal data for 2: monoclinic, yellow block, space group P21/c, a )
16.0489(5) Å, b ) 9.4521(3) Å, c ) 15.9728(6) Å, â ) 116.707(2)°, T )
173 K, Z ) 2, R (refined on F, I > 3σ(I)) ) 0.026, Rw ) 0.036, GOF 1.08
(13) Giordano, T. J.; Rasmussen, P. G. Inorg. Chem. 1975, 14, 1628-
1634.
(8) Vidal, P. L.; Divisia-Blohorn, B.; Bidan, G.; Kern, J. M.; Sauvage, J.
P.; Hazemann, J. L. Inorg. Chem. 1999, 38, 4203-4210.
(9) Weinberger, D. A.; Higgins, T. B.; Mirkin, C. A.; Liable-Sands, L.
M.; Rheingold, A. L. Angew. Chem., Int. Ed. Engl. 1999, 38, 2565-2568.
(10) Clot, O.; Wolf, M. O.; Yap, G. P. A.; Patrick, B. O. Dalton Trans.
2000, 2729-2737.
(11) Synthesis of 2: A warm solution of 1 (108 mg, 0.25 mmol) in ethanol/
acetonitrile (10/3 mL) was slowly added at 50 °C to PdCl2 (50 mg, 0.28 mmol)
and concentrated HCl (0.05 mL) in water (3 mL). A yellow precipitate formed
immediately, and the mixture was stirred for 2 h at 50 °C, cooled, and filtered
to yield a yellow powder. The solid was washed with water (2 × 10 mL),
ethyl ether (1 × 5 mL), and hexanes (2 × 20 mL) and dried in air.
Recrystallization from a methylene chloride/diethyl ether/hexanes mixture (1/
1/1) yielded a light orange solid. Yield: 112 mg (70%). 1H NMR (200 MHz,
CDCl3) δ 7.78-7.59 (m, 4H), 7.57-7.25 (m, 17H), 7.21-6.90 (m, 9H), 6.59-
6.48 (m, 2H). 31P{1H} NMR (81 MHz, CDCl3) δ 17.72, 16.61. Anal. Calcd
for CH32Cl2P2PdS6: C 50.27, H 2.79%. Found: C 50.39, H 2.85%. UV-vis:
λmax (nm) (ꢀ (M-1cm-1)) 388 (6.64 × 104), 288 (sh) (3.84 × 104).
(14) Kulikova, M. V.; Kvam, P.-I.; Songstad, J.; Balashev, K. P. Russ. J.
Gen. Chem. 1997, 67, 967-968.
(15) Balashev, K. P.; Puzyk, M. V.; Kanhukaeva, O. P.; Antonov, N. V.;
Ivanov, A. Y. Russ. J. Gen. Chem. 1998, 68, 1670-1671.
(16) Constable, E. C.; Henney, R. P. G.; Raithby, P. R.; Sousa, L. R. J.
Chem. Soc., Dalton Trans. 1992, 2251-2258.
(17) Deeming, A. J.; Jayasuriya, S. N.; Arce, A. J.; De Sanctis, Y.
Organometallics 1996, 15, 786-793.
(18) Alonso, M. T.; Juanes, O.; de Mendoza, J.; Rodr´ıguez-Ubis, J. C. J.
Organomet. Chem. 1992, 430, 349-355.
(19) Hockless, D. C. R.; Gugger, P. A.; Leung, P.-H.; Mayadunne, R. C.;
Pabel, M.; Wild, S. B. Tetrahedron 1997, 53, 4083-4094.
(20) Dupont, J.; Pfeffer, M. J. Chem. Soc., Dalton Trans. 1990, 3193-
3198.
(21) Hayashi, Y.; Isobe, K.; Nakamura, Y.; Okeya, S. J. Organomet. Chem.
1986, 310, 127-134.
10.1021/ja002258v CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/10/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 42, 2000 10457
between poly-2 and the monomer are expected to cause some
changes in the infrared spectrum, as has been previously observed
for other thiophene oligomers and polymers.23
The reactivity of poly-2 films is very similar to that of 2.
Exposure of a poly-2 film to a solution of t-BuNC in hexanes at
25 °C resulted in a rapid reaction. The surface reflectance IR
spectrum of the reacted film shows a new, strong band at 2208
cm-1 similar to the -NC stretch in 3 and the region between
1500 and 600 cm-1 also shows good correlation with the
transmission spectrum of 3. When a film of poly-2 is treated with
a solution of phenyl isocyanide (PhNC) in hexanes at 25 °C, the
IR spectrum of the resulting film also contains a new band at
2188 cm-1 due to a terminal isocyanide group. After reaction of
the poly-2 films with either isocyanide, the resulting films are
insoluble in hexanes but highly soluble in CH2Cl2, in marked
contrast to poly-2 which is completely insoluble in this solvent.
Reaction of poly-2 films with PPh3, P(OMe)3, or PPh2Me also
resulted in an increase in the solubility in CH2Cl2, whereas MeCN
did not react.
The characterization and reactivity of poly-2 all support the
conclusion that the coordination at the Pd center is retained upon
polymerization. Thiophene oligomers polymerize primarily by
C-C coupling at the R position,2 and we therefore propose that
the films consist primarily of R,R-coupled 2, although R,â- or
â,â-coupled material may also be present. Reaction of poly-2 with
isocyanides and phosphines cleaves the dichloro bridges, thus
eliminating intrachain cross-linking in the material and adding
solubilizing groups to the backbone. These factors together are
responsible for the increase in solubility.
The UV-vis spectrum of a neutral film of poly-2 shows a
strong absorption band with λmax at 522 nm, which shifts to 808
nm upon oxidation. The positions of these bands are comparable
to those observed in substituted polythiophenes,24 and the red-
shift of the absorption band relative to 2 is consistent with a
significant increase in conjugation length upon polymerization.
Upon reaction of the film with t-BuNC or PhNC, the absorption
band red-shifts to λmax ) 528 nm for poly-2/t-BuNC and 542 nm
for poly-2/PhNC. These shifts are indicative of the electronic
effect on the backbone caused by the reaction at the pendant metal,
and the differences in λmax observed for reactions with two similar
isonitriles indicates how sensitive the electronic structure of the
conjugated backbone is to the ligands at the Pd center.
Figure 2. Cyclic voltammetry of 2 in CH2Cl2 containing 0.1 M
(n-Bu)4NPF6. Multiple scans from -0.25 to +1.75 V, scan rate ) 200
mV/s (the oxidation wave at 0 V in the first scan is due to added
decamethylferrocene, used as an internal reference). Inset: scan of a
poly-2 film on a gold electrode in 0.1 M (n-Bu)4NPF6 in CH2Cl2 (scan
rate ) 200 mV/s).
at -1.31 V vs SCE (not shown). The wave at -1.31 V is assigned
to a Pd-based reduction as 1 does not exhibit any reduction waves
between 0 and -1.5 V. The anodic wave has two associated
cathodic features which appear on the return scan. The wave at
0.73 V results from the reduction of surface-confined material
formed upon oxidation, and the shoulder to higher potential is
likely due to reduction of shorter oligomers. Repeated scanning
from -0.25 to +1.75 V results in the deposition of a red film of
poly-2 on the electrode surface, along with an increase in the
current in the CV (Figure 2). The CV of the film after removal
of the electrode from the polymerization solution shows broad
oxidation and reduction waves between -0.25 and +1.75 V vs
SCE (Figure 2, inset), similar to CV scans observed for other
polythiophene derivatives.2 The conductivity of the film when
In conclusion, we have prepared, by electropolymerization of
the cyclometalated Pd dimer 2, the first conductive polythiophene
derivative in which a metal is σ-bonded to the conjugated
backbone. The Pd moiety is stable under the electropolymerization
conditions; however, it undergoes subsequent reactions with
isocyanides and phosphines. The proximity of the Pd to the
conjugated backbone in this new binding mode, combined with
the reactivity of the Pd center, suggests that these materials may
be useful in transition-metal based conductive polymer sensors.
oxidized is 10-3 S cm-1
.
Poly-2 films were characterized by several methods. The
elemental composition of the films was determined by energy-
dispersive X-ray (EDX) analysis to be the same as that of the
monomer within experimental error. X-ray photoelectron spec-
troscopy (XPS) analysis shows a Pd (3d5/2) peak at 337.3 eV,
consistent with Pd(II) (no Pd(0) or (IV) was detected). The P 2p
peak in the XPS appears at 131 eV, as expected for a coordinated
phosphine.22 Careful inspection of the surface reflectance IR
spectrum of poly-2 reveals that many of the same bands are
present as in a solid-state transmission spectrum of 2; however,
other bands are subdued or absent. The structural differences
Acknowledgment. We thank the Natural Sciences and Engineering
Research Council of Canada for support of this research and Dr. K-C.
Wong and Professor K. Mitchell for the XPS measurements.
Supporting Information Available: Preparative details for 3, table
of EDX data, IR and XPS spectra (PDF). An X-ray crystallographic file
(CIF) for 2. This material is available free of charge via the Internet at
http://pubs.acs.org.
JA002258V
(23) Furukawa, Y.; Akimoto, M.; Harada, I. Synth. Met. 1987, 18, 151-
156.
(24) McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Anderson, D. L. J.
Org. Chem. 1993, 58, 904-912.
(22) Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy;
Perkin-Elmer Corporation: New York, 1979.