Synthesis and characterization of monomeric and polymeric Pd(II) and Pt(II) complexes of 3,4-ethylenedioxythiophene-functionalized phosphine ligands

Article abstract of DOI:10.1039/b817210b

Phosphine ligands bearing 3,4-ethylenedioxythiophene (EDOT) groups, in which a Ph₂P group is either connected directly to the thiophene ring [Ph₂P(5-EDOT), 1] or to the EDOT ethylene bridge via a methylene (4) or longer (7) 'spacer',

Full text of DOI:10.1039/b817210b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and characterization of monomeric and polymeric Pd(II) and Pt(II)
complexes of 3,4-ethylenedioxythiophene-functionalized phosphine ligands†
a
Kuhamoorthy Velauthamurty,ab Simon J. Higgins, R. M. Gamini Rajapakse,b John Bacsa,a
*
Harm van Zalinge,a Richard J. Nicholsa and Wolfgang Haissa
Received 2nd October 2008, Accepted 9th December 2008
First published as an Advance Article on the web 4th February 2009
DOI: 10.1039/b817210b
Phosphine ligands bearing 3,4-ethylenedioxythiophene (EDOT) groups, in which a Ph2P group is either
connected directly to the thiophene ring [Ph2P(5–EDOT), 1] or to the EDOT ethylene bridge via
a methylene (4) or longer (7) ‘spacer’, have been prepared, together with their complexes cis-[MCl2(L)2]
(M ¼ Pd and Pt). The electrochemical co-polymerization of the complexes with EDOT was
investigated, with a view to making conducting polymers incorporating covalently-bound metal–
phosphine complexes. Although polymer-coated electrodes were obtained in all experiments, XPS
measurements established that polymers made in the presence of cis-[MCl2(1)2] consisted only of
poly(3,4-ethylenedioxythiophene) (PEDOT), whereas polymers made in the presence of cis-[MCl2(L)2]
(L ¼ 4, 7) were genuine co-polymers containing Pd(II) or Pt(II) complexes.
found that oligothiophenes are interesting substituents for
Introduction
phosphines in several respects. In principle, oligothiophene–
Interest in polymer-supported catalysts and reagents has revived
(metal phosphine complex) dimers, copolymers or metallocycles
in recent years, stimulated by the growth of research into
combinatorial and automated organic synthesis.1–4 At the
could be made directly from Ph2P–(T)n–PPh2 (T ¼ thiophene;
n ¼ 1,2,3) using simple metal complexation reactions,19,22 while
same time, there has been continuing activity in the study of
the electrochemical polymerization of complexes containing
conjugated polymers containing covalently-bound metal
ligands Ph2P–(T)n could also result in dimeric, polymeric or
complexes.5–11 The latter have usually been redox-active (e.g.
2,20-bipyridine derivatives or azamacrocycle complexes), because
network materials in which the metal centres are connected by
dimers, Ph2P–(T)2n–PPh2.15,23 Secondly, oligothiophenes them-
the applications aimed at in such work have included sensing,
selves can participate in the coordination shell of the metal centre
electrocatalysis and electrochromism, all of which would require
in an unusual and interesting manner. For example, Wolf and
that the metal complex has reversible redox activity. Depending
co-workers found that, depending upon the mole ratios used, 30-
upon the application aimed at, the metal complex can either
(Ph2P)-2,20:50,200-terthiophene reacted with PdCl2 to give either
be directly coordinated to the conjugated chain (for instance, by
a conventional trans-[PdCl2(phosphine)2], or a chloro-bridged
the use of 2,20-bipyridine or bithiazole monomer units),8,10,11 or it
can be attached as a pendant group via a ‘spacer’.5,6,10
palladium dimer in which each Pd unit was coordinated to
a phosphine donor and a neighboring cyclometallated thiophene
ring b-position.15 Moreover, when the trans-[PdCl2(phosphine)2]
was treated with two equiv. of AgBF4, this also induced cyclo-
We were intrigued by the possibility of incorporating catalyt-
ically-active metal–phosphine complexes into conducting
polymer supports, with the long-term goal of building new
metallation of one ligand, while the second ligand coordinated in
(electro)catalytic systems. Some time ago, we described a poly-
hemilabile fashion via the phosphine and a neighboring thio-
pyrrole incorporating a pendant redox-active Ru(II)–diphos-
phene S atom. Interestingly, all of the Pd(II) complexes of this
phine complex,12 and other workers prepared polypyrroles
ligand, in which the oligothiophene units have both terminal
bearing pendant diphosphine ligands and their Pd(II) complexes,
a-positions free, could be electrochemically polymerized to give
which are not reversibly redox-active.13 Later, the latter materials
polymer films. The redox activity of these was consistent with
were tested in polymer-supported catalysis.14 More recently,
polyterthiophenes containing intact metal complexes.15
complexes of oligothiophenes bearing –PPh2 groups in terminal
(a) or internal (b) positions have been examined.15–21 It has been
The complex trans-[PdCl2(5-Ph2P-2,20:50,200-terthiophene)2]
(i.e. in which the Ph2P– groups occupy a terminal a-position of
the terthiophenes) has also been electropolymerized.15 Although
aDepartment of Chemistry, University of Liverpool, Crown Street,
little data on the polymer was reported in the latter case, exam-
Liverpool, U.K. L69 7ZD. E-mail: shiggins@liv.ac.uk; Fax: +44-
ination of the voltammogram suggests behaviour consistent with
(0)151-794-3588; Tel: +44-(0)151-794-3512
bDepartment of Chemistry, University of Peradeniya, Peradeniya, Sri
Lanka
a structure in which Ph2P(T)6–PPh2 units alternate with Pd(II)
sites in the polymer. However, other attempts to polymerize
similar complexes with oligothiophenes terminally substituted
with phosphines have been notably less successful. For instance,
a series of octahedral phosphine–capped clusters [W6S8{R2P–
(T)n}6] (R ¼ Ph, Et; n ¼ 1, 2, 3) has been prepared. Attempts to
† Electronic supplementary information (ESI) available: Details of the
synthesis and characterization of precursor 6, cyclic voltammograms
and XPS spectra of copolymers. CCDC reference numbers 699407,
699408 and 701459. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b817210b
1850 | J. Mater. Chem., 2009, 19, 1850–1858
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electropolymerize these complexes failed to yield any redox-
active polymer films.24
for several hours prior to use. The solvent was CH3CN (99.97%
electrochemistry grade). Solvent and electrolyte were handled
under Schlenk conditions. A three-electrode electrochemical cell
having separate compartments for the working (Pt or glassy
carbon, 0.1 mm diameter except for XPS samples; see below),
auxiliary (Pt gauze), and quasi-reference (polypyrrole-coated
Pt wire)34 electrodes was employed. The potentiostat was an
Autolab PG-30, running GPES software (EcoChimie, Nether-
lands). For electrochemical impedance spectra, polymers were
deposited on glassy carbon working electrodes. Potentials are
quoted with respect to the Pt/PPy QRE, which was +0.225 V vs.
standard calomel electrode with this electrolyte.
Poly(3,4–ethylenedioxythiophene) (PEDOT) is a particularly
stable polythiophene derivative.25,26 The electron rich EDOT unit
greatly lowers the redox potential of both monomer and polymer
compared with thiophenes. For the (co)polymerization of metal
complexes with pendant thiophene units, the main advantage
is that the likelihood of oxidative degradation of the complex is
lessened.27 Secondly, the blocking of potentially reactive thio-
phene b-positions in EDOT prevents the formation of conjuga-
tion-limiting a,b0 links in the polymer during electrodeposition,
and avoids complications due to cyclometallation etc. Thirdly,
PEDOT is extremely chemically robust. For example, it has even
been used as a catalyst support material in the aggressive envi-
ronment of the direct methanol fuel cell anode.28 We were
interested in the possibility of using phosphine-substituted 3,4-
ethylenedioxythiophenes to achieve the goal of preparing robust
conducting materials incorporating covalently-attached metal–
phosphine complexes. This could afford a route to electro-
polymerized, catalytically-active polymer films for organic
synthesis in automated systems. While complexes of many
different ligands bearing pendant EDOTs (e.g. pyridines, 2,20-
bipyridyl derivatives, tetraazamacrocycles) have been made and
successfully electropolymerized,27,29 there are only two reports
of EDOT-containing phosphines. Ligands Ph2P(5–EDOT) and
(R2P)(5-biEDOT) (R ¼ Ph, Et) were prepared by Perruchas
et al, and used in their unsuccessful attempts to polymerize
[W6S8(phosphine)6],24 and ligands Ph3ꢀnP(5–EDOT)n were
prepared by Chahma et al, along with their complexes
[Mo(CO)4(L)2].30 Attempts to electropolymerize the free ligands
and their Mo(0) complexes were all unsuccessful.
X-Ray photoelectron spectroscopy
For XPS experiments, polymer samples were electrodeposited
upon 1.1 ꢁ 1.1 cm gold on Cr-primed glass slides. A gold wire
was attached to one corner of the slide to act as a contact. The
gold slides were annealed in a butane flame prior to polymer
electrodeposition. Spectra were acquired using a Scienta ESCA
300 spectrometer (NCESS, Daresbury Laboratory, UK) at 90ꢂ
take-off angle. The spectra were referenced to the main C–H 1s
peak at 285.0 eV. In all cases, a survey scan (0–1300 eV) was run,
followed by high-resolution scans at characteristic peak positions
for the metal, Cl and P. In one case, data for S and O were also
obtained at high resolution, but time limitations prevented this
being done for all samples.
Syntheses
(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)diphenylphosphine,
Ph2P(5–EDOT), 1. To a solution of 3,4-ethylenedioxythiophene
(0.682 g, 4.8 mmol) in anhydrous THF (10 cm3) was added
lithium diisopropylamideꢂ (0.524 g, 4.9 mmol) dropwise
over a period of 1 h at 0 C. The reaction mixture was stirred
for 2 h. The mixture was then cooled to ꢀ78 ꢂC and chloro-
diphenylphosphine (1.10 g, 5.0 mmol) was added dropwise. The
mixture was stirred for 2 h and gradually brought to room
temperature. Degassed distilled water (10 cm3) was added and
the product was extracted with degassed CH2Cl2. The organic
layer was dried over MgSO4, filtered, and the solvent was
removed under reduced pressure to yield an off-white solid. This
was pure enough to be used for complex formation. Yield 1.23 g,
78%. Anal. Calcd for C18H15O2PS: C, 66.25; H, 4.63%. Found:
C, 67.01; H, 4.96%. MS (CI+, NH3): m/z calcd for (M + H)+:
327.06086. Found: 327.06008. 1H NMR (400 MHz, CDCl3):
d 4.19 (m, 4H, –OCH2CH2O–), 7.31–7.46 (m, 10H, Ph), 6.57 (d,
1H, JPH 0.9 Hz, thienyl H). 13C{1H} NMR (100 MHz): d 65.07,
65.33, 99.90, 106.59, 128.61, 129.23, 132.13, 137.10, 142.17,
146.93. 31P{1H} NMR (162 MHz): d ꢀ28.0.
In this paper, we describe the syntheses of two new phosphine
ligands bearing pendant EDOT groups in which both EDOT
a-positions are free for polymerization, with little chance that
any part of the EDOT unit will participate in coordination to the
metal. We describe their complexes [MCl2(L)2] (M ¼ Pd, Pt),
and the successful electrochemical copolymerization of these
complexes with EDOT. For comparison, we have also prepared
related complexes with Ph2P(5-EDOT), and we describe attempts
to copolymerize these complexes with EDOT.
Experimental section
Syntheses and structures of ligands and complexes
General methods and instrumentation used were as recently
described in other papers from this laboratory.31 Reagents
and solvents were reagent grade from Aldrich Chemical
Company, unless otherwise specified. Reactions involving free
phosphines were routinely carried out using Schlenk techniques
under Ar. 2-(Chloromethyl)-2,3-dihydrothieno[3,4-b][1,4]diox-
ine (EDOTCH2Cl, 3) was prepared by a literature route.32 2-[(6-
Bromohexyloxy)methyl]-2,3-dihydrothieno[3,4-b][1,4]dioxine
(EDOTCH2O(CH2)6Br, 6) was prepared as described previously
for the bromopentyl analog;33 full details are in the ESI.†
trans-[PdCl2(1)2], 2a. To a 100 cm3 round-bottom flask fitted
with a magnetic stirrer bar and a reflux condenser was added
degassed CH2Cl2 (20 cm3) and [PdCl2(CH3CN)2] (0.492 g,
1.9 mmol). The solution was brought to reflux and 1 (1.226 g,
3.8 mmol) in hot degassed CH2Cl2 (30 cm3) was added dropwise.
The reaction mixture was left stirring for 1 h, after which time
the solution was allowed to cool to room temperature and the
solvent was removed in vacuo. The resulting oil was washed with
Polymer preparation and characterization
For all electrochemistry experiments, the electrolyte was
Bu4NBF4. This was recrystallised and dried under high vacuum
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diethyl ether (10 cm3), whereupon it crystallized. The yellow
crystals were filtered off and washed with Et2O. Yield 1.16 g,
74.2%. Anal. Calcd for C36H30Cl2O4P2PdS2: C, 52.09; H, 3.64%.
Found: C, 52.29; H, 3.84%. MS (ES+, MeOH, 30 V) m/z:
[PtCl2(4)2], 5b. This was prepared as for 2b from
[PtCl2(PhCN)2] (0.495 g, 1.05 mmol) and 7 (0.714 g, 2.10 mmol)
in refluxing CH2Cl2 (50 cm3). After 4 h the reaction was worked
up as for 2b. The resulting oil was washed with diethyl ether
(10 cm3), whereupon it crystallized. The greenish yellow crystals
were filtered off and washed with Et2O. Yield 0.736 g, 74.1%.
Anal. Calcd for C38H34Cl2O4P2PtS2$0.5CH2Cl2: C, 46.75; H,
795 [M–Cl]+. IR: 3056, 2937, 1058, 1475 cmꢀ1. H NMR (400
1
0
MHz): d 4.20 (m, 8H, –OCH2CH2O–), 6.69 (t, 2H, |JPH + JP H
|
3.8 Hz, thienyl H), 7.36 (m, 16H, Ph), 7.81 (m, 4H, Ph). 13C{1H}
NMR (162 MHz): d 63.0, 63.5, 101.4, 106.5, 126.9, 128.2, 132.6,
135.0, 140.8, 145.0. 31P{1H} NMR (162 MHz): d 11.62.
3.57%. Found: C, 46.69; H, 3.42%. MS (ES+, MeOH:CH2Cl2
+
:
2:1, 0.1% HCOOH, 35 V) m/z: calcd for C38H3435ClO4P2195PtS2
910.0752. Found: 911.0744. IR: 3060, 1430, 1097 cmꢀ1. 1H NMR
(400 MHz): d 2.30, 3.10 (complex m), 3.90, 4.25 (complex m),
5.10 (m), 5.98, 6.29 (m0s, thienyl H), 6.95, 7.20, 7.45, 7.50, 7.65,
7.80 (m0s, Ph). 13C{1H} NMR (101 MHz): d 53.85, 65.05, 65.39,
99.29, 129.101, 130.94, 132.44, 133.00, 135.17, 138.30, 142.0.
31P{1H} NMR (162 MHz): d 9.13 (1JPtP 3664 Hz), 10.76 (1JPtP
3669 Hz).
trans-[PtCl2(1)2], 2b. A solution of [PtCl2(PhCN)2] (0.8456 g,
1.79 mmol) in degassed CH2Cl2 (20 cm3) was refluxed for 15 min.
A solution of 1 (1.173 g, 3.6 mmol) in hot degassed CH2Cl2
(30 cm3) was added dropwise. The reaction mixture was stirred
for 4 h, then the solution was allowed to cool to room temper-
ature, and the solvent was removed in vacuo. The resulting oil
was washed with diethyl ether (10 cm3), whereupon the oil
crystallized. The greenish yellow crystals were filtered off and
washed with diethyl ether. Yield 1.176 g, 71.2%. Anal. Calcd for
C36H30Cl2O4P2PtS2: C, 47.07; H, 3.29%. Found: C, 46.69; H,
3.42%. MS (ES+, MeOH, 0.5% HCOOH, 40 V) m/z: 883
[M–Cl]+. IR: 3073, 2940, 1062, 1467 cmꢀ1. 1H NMR (400 MHz):
{6-[(2,3-Dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy]hexyl}-
diphenylphosphine, 7. To a solution of 6 (1.05 g, 3.1 mmol) in
anhydrous THF (20 cm3) at ꢀ30 ꢂC was added LiPPh2 (0.61 g,
3.2mmol) in THF (10 cm3) dropwise over a period of 30 min.
The reaction was stirred for 2 h and gradually warmed to room
temperature. Degassed distilled water (10 cm3) was added and
the product was extracted with CH2Cl2. The organic layer was
dried over MgSO4, filtered, and the solvent was removed in
vacuo to yield yellow, air sensitive oil. This was used without
further purification. Yield 1.286 g, 91.4%. Anal. Calcd for
C25H29O3PS: C, 68.16; H, 6.64 %. Found: C, 69.16; H, 6.33 %.
MS (ES+, MeOH/10 % HCOOH) m/z: calcd for [M + H]+:
0
d 4.23 (m, 8H, –OCH2CH2O–), 6.67 (t, 2H, |JPH + JP H| 4.2 Hz,
thienyl H), 7.40 (m, 16H, Ph), 7.83 (m, 4H, Ph). 13C NMR (400
MHz): d 63.0, 63.5, 101.4, 106.5, 126.6, 128.3, 129.3, 133.4, 140.9,
145.8. 31P{1H} NMR (162 MHz): d 9.79 (1JPtP 2670 Hz).
[(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methyl]diphenylphos-
phine, EDOTCH2PPh2, 4. To a solution of 3 (0.40 g, 2.1 mmol)
in anhydrous THF (20 cm3) at ꢀ30 ꢂC was added LiPPh2 (0.422
g, 2.2 mmol) in THF (10 cm3) dropwise over a period of 30 min.
The reaction was stirred for 2 h and allowed to warm to room
temperature. Degassed distilled water (10 cm3) was added, the
solution was extracted with CH2Cl2 and the organic layer was
dried over MgSO4, filtered, and evaporated to dryness yielding
a yellow air sensitive oil, 0.584 g, 82%. Anal. Calcd for
C19H17O2PS: C, 67.25; H, 5.05%. Found: C, 67.04; H, 5.03%.
MS (CI+, NH3) m/z: calcd for [M + H]+: 341.07691. Found:
341.07626. 1H NMR (400MHz): d 2.32, 2.56 (m0s, –CH2P),
3.70 (m, –CHCH2P), 3.96, 4.20 (m0s, –OCH2CH), 6.29 (m,
thienyl H), 7.35, 7.45 (m0s, Ph), 7.70 (m, Ph). 13C{1H} NMR
(162 MHz): d 32.75, 65.98, 69.27, 100.06, 106.59, 129.10,
130.94, 132.44, 133.00, 140.90, 141.50. 31P{1H} NMR
(162 MHz): d ꢀ22.58.
1
441.1653. Found: 441.1661. H NMR (400 MHz): d 1.36 (m,
2H), 1.46 (m, 4H), 1.57 (m, 2H, J 7.1 Hz), 2.07 (t, 2H, PCH2),
3.48 (t, 2H, –OCH2(CH2)5Br), 3.59 (m, 2H, –CH(O-thie-
nyl)CH2OCH2–, JAB 10.4, JHH 5.0), 4.06, 4.25 (m0s, 2H,
–OCH2CH(R)O–, JAB 11.6 Hz), 4.18–4.25 (m, 1H, –OCH2-
CH(R)O–), 6.34 (s, thienyl H), 7.25–7.6, 7.7–7.8 (m0s, 10H, Ph).
13C{1H} NMR (101 MHz): d 26.01, 26.10, 29.67, 29.76, 32.16,
66.61, 69.91, 72.20, 73.03, 99.94, 100.06, 128.74, 129.38, 132.10,
133.187, 142.00. 31P{1H} NMR (162 MHz): d ꢀ14.98.
[PdCl2(7)2], 8a. This was prepared as for 2b using
[PdCl2(CH3CN)2] (0.464 g, 1.8 mmol) and 7 (1.58 g, 3.6 mmol) in
hot CH2Cl2 (50 cm3). The reaction mixture was stirred for 1 h,
and worked up as for 2b. The yellow crystals were filtered off
and washed with Et2O. Yield 1.254 g, 65.9 %. Anal. Calcd
for C50H58O6S2P2PdCl2: C, 56.74; H, 5.52 %. Found: C, 56.49;
H, 5.35 %. MS (ES+, MeOH, 45 V) m/z: calcd for
C50H58O6P2S235Cl106Pd+: 1021.1873. Found: 1021.1881. 1H
NMR (400 MHz): d 1.30, 1.38 (m0s, 8H total) 1.48 (m, 4H),
1.65 (br m, 4H), 2.45 (br m, 4H, –PCH2), 3.42 (t, 4H,
–OCH2(CH2)5P), 3.55, 3.63 (m0s, 4H, –CH(O-thie-
nyl)CH2OCH2–, JAB 10.4, JHH 5.0), 4.01, 4.20 (m0s, 4H,
–OCH2CH(R)O–, JAB 11.6 Hz), 4.25 m, 2H, –OCH2CH(R)O–),
6.31 (s, thienyl H), 7.26–7.42 (m0s, 20H). 31P{1H} NMR
(162 MHz): d 17.60.
[PdCl2(4)2], 5a. This was prepared as for 2a using
[PdCl2(CH3CN)2] (0.22 g, 0.85 mmol) and 4 (0.58 g, 1.70 mmol)
in CH2Cl2 (30 cm3). Workup was as for 2a. The resulting oil was
washed with diethyl ether (10 cm3), whereupon it crystallized.
The yellow crystals were filtered off and with washed with Et2O.
Yield 0.516 g, 70.8%. Anal. Calcd for C38H34Cl2O4P2PdS2: C,
53.19; H, 3.99%. Found: C, 52.80; H, 3.93%. MS (ES+, MeOH,
35V) m/z: calcd for C38H3435ClO4P2105PdS2+: 820.0113. Found:
820.0152. IR: 2902, 1058, 1479 cmꢀ1 1H NMR (400 MHz):
.
d 2.67, 2.96 (m0s, –CH2P), 3.92, 4.11, 4.67 (m0s –CHCH2P and
–OCH2CH), 5.91, 6.25 (m, thienyl H), 7.35, 7.45, 7.72 (m, Ph).
13C{1H} NMR (101 MHz): d 53.8, 68.8, 70.9, 128.6, 131.2, 133.6,
134.99, 99.99, 100.5, 141.0, 141.6. 31P{1H} d 15.4, 18.3 (s,
3:1 ratio).
cis-[PtCl2(7)2], 8b. This was prepared as for 2b using
[PtCl2(PhCN)2] (0.5664 g, 1.20 mmol) and 7 (1.056 g, 2.4 mmol)
in hot CH2Cl2 (50 cm3). The reaction mixture was stirred for 4 h,
and worked up as for 2b. The off-white crystals were filtered off
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and washed with Et2O. Yield 1.26 g, 91.6 %. Anal. Calcd for
C50H58Cl2O6P2PtS2: C, 52.35; H, 5.10 %. Found: C, 51.72; H,
5.15 %. MS (ES+, 1:1 MeOH:CH3CN, 0.1 % HCOOH) m/z: calc
for C50H5835ClO6P2194PtS2+: 1109.2465. Found 1109.2458. 1H
NMR (400 MHz): d 1.22, 1.46, 1.55 (m0s, 16 H), 2.25, 2.45 (br
m0s, 4H, –PCH2), 3.41 [t, 4H, –OCH2(CH2)5P], 3.55, 3.62 (m0s,
4H, –CH(O-thienyl)CH2OCH2–, JAB 10.4, JHH 5.0), 4.02, 4.21
(m0s, 4H, –OCH2CH(R)O–, JAB 11.6 Hz), 4.18–4.29 [m, 2H,
–OCH2CH(R)O–], 6.31 (s, thienyl H), 7.24–7.46 (m0s, 20H).
13C{1H} NMR (101 MHz): d 22.90, 23.07, 25.45, 25.79, 29.67,
32.01, 66.59, 69.51, 72.19, 73.03, 99.92, 100.06, 128.66, 129.85,
132.55, 133.82, 141.67, 142.00. 31P{1H} NMR (162 MHz): d 8.67
(1JPtP 3657 Hz).
Results and discussion
Synthesis and characterization of EDOT-functionalized ligands
and complexes
To test the possibility of using phosphine-substituted 3,4-ethyl-
enedioxythiophenes to achieve the goal of preparing robust
polythiophene materials incorporating covalently-attached
metal–phosphine complexes, the most straightforward route is
to use terminally-substituted EDOT-phosphines since these are
easily prepared. EDOT is readily deprotonated using LiNiPr2 in
THF, and reaction of LiEDOT with Ph2PCl gave Ph2P(5–
EDOT), 1. Reaction of 2 equiv. of this with [PdCl2(CH3CN)2] or
[PtCl2(PhCN)2] in CH2Cl2 gave square planar [PdCl2(1)2] (2a)
and [PtCl2(1)2] (2b) respectively. For 2b, the geometry is clearly
1
Electrochemical (co)polymerization
trans as JPtP is 2670 Hz. Moreover, the thienyl H resonance is
a triplet owing to long-range coupling in a second-order AA0XX0
Repetitive scan cyclic voltammetry at 100 mV sꢀ1 was used to
grow the polymer films, using a solution of EDOT (0.01 M) or of
the complex together with EDOT itself (1:5 mole ratio; 0.01 M
total monomer) in 0.1 M Bu4NBF4–CH3CN. Ten cycles between
ꢀ0.1 and +2.0 V were used to grow all polymer films. After film
growth, the electrode was held at 0 V for ten minutes, then
removed from the organic electrolyte, washed with CH3CN,
dried and transferred to fresh 0.1 M Bu4NBF4–CH3CN. Cyclic
voltammetry and EIS characterization of the polymer films was
then undertaken in the absence of monomers.
0
system to both P atoms (|JPH + JP H| ¼ 4.2 Hz), whereas the
corresponding resonance in the free ligand is a doublet (JPH
0.9 Hz). This is diagnostic of trans geometry.35 Complex 2a
¼
0
also has a triplet thienyl H resonance (|JPH + JP H| ¼ 3.8 Hz)
suggesting that this, too, is trans. X-Ray crystal structure
determinations confirm these assignments. Fig. 1 shows the
molecular structure of complex 2b.‡Note that there are two
slightly different independent molecules in the unit cell; bond
lengths and angles for only one of these is given in the caption.
Although somewhat long, the metal–ligand bond lengths lie
within the usual range for complexes of the type trans-
[PtCl2(PAr3)2] (checked via the Cambridge Crystallographic
Database),36 especially where the aryl groups are bulky. For
Electrochemical impedance spectroscopy
EIS spectra were collected in background electrolyte at selected
DC bias potentials, each with a 10 mV AC potential in the
frequency range between 0.1 Hz and 10 MHz in the single sine
wave AC mode with 10 s equilibration time.
ꢀ
example, in trans-[PtCl2(PPh3)2], Pt–P is 2.3163(11) A and Pt–Cl
is 2.2997(11) A,37 while in trans-[PtCl2(P{2-iPrC6H4}3)2], Pt–P is
ꢀ
38
ꢀ
ꢀ
2.3436(11) A and Pt–Cl is 2.3039(10) A. A dataset was also
obtained for 2a (deposited with CCDC; reference 699407).†
However, the data were very weak and the diffraction peaks
broad owing to small crystal size and low crystal quality. Due to
the problematic data, anisotropic refinement resulted in physi-
cally implausible ellipsoids for the C and O atoms and a low
data to parameter ratio. Therefore these atoms were refined
isotropically (resulting in a higher than normal final R-factor ca.
11 %). Nevertheless, the coordination geometry at the Pd atoms
is unequivocally established as being trans, with two half mole-
cules of 2a (and a chloroform of solvation) in the asymmetric
unit, and with the Pd atoms lying on inversion centres.
Attempts to electropolymerize these complexes intact, either
alone or as copolymers with unsubstituted EDOT, all failed as
judged by a combination of cyclic voltammetry, EIS and XPS
characterization of the resulting polymers (q.v.). We therefore
decided to prepare phosphine ligands bearing pendant EDOT
Fig. 1 Thermal ellipsoid plot (50% probability ellipsoids) of the
molecular structure of complex 2b. There are two independent half
molecules in the asymmetric unit, and in each case the Pt atom lies on an
inversion centre. One of these two molecules is shown. The inversion
related P10 and Cl10 atoms are labelled in this figure. Significant bond
distances and angles are as follows. Pt(1)–P(1) 2.3126(15), Pt(1)–Cl(1)
‡ Crystal data for 2b, C36H30Cl2O4P2PtS2$C4H10O, Mr 992.77, triclinic,
ꢂ
ꢀ
ꢁ
P1 a ¼ 11.379(2), b ¼ 12.109(2), c ¼ 15.524(3) A, a ¼ 72.370(4) , b ¼
ꢂ
ꢂ
3
ꢀ
89.055(5) , g ¼ 66.038(5) , V ¼ 1848.9(6) A , T ¼ 100 K, Z ¼ 2,
100003 measured reflections, 4981 independent reflections, R1 (I >
2s(I)) 0.0623, wR2 (all data) 0.0955, CCDC 699408. Crystal data for
ꢁ
5a, C38H34Cl2O4P2PdS2, Mr 858.01 triclinic, P1 a ¼ 9.244(5), b ¼
ꢂ
ꢂ
ꢀ
ꢀ
10.182(5), c ¼ 10.679(6) A, a ¼ 107.001(8) , b ¼ 96.758(9) , g ¼
ꢀ
2.354(2), P(1)–C(1) 1.787(6), P(1)–C(13) 1.810(6), P(1)–C(7) 1.832(6) A;
ꢂ
3
106.538(8) , V ¼ 899.0(8) A , T ¼ 100 K, Z ¼ 1, 3759 measured
reflections, 3054 independent reflections, R1 (I > 2s(I)) 0.0417, wR2 (all
data) 0.1035, CCDC 701459.
P(1)–Pt(1)–P(10) 180.0ꢂ, P(1)–Pt(1)–Cl(1) 92.32(5), P(1)–Pt(1)–Cl(10)
87.68(5), Cl(1)–Pt(1)–Cl(1) 180.0ꢂ.
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Steric crowding is well known to inhibit electropolymerization
for thiophenes in which a bulky substituent is close to the
monomer unit, whereas with a longer ‘spacer’ between the bulky
substituent and the thiophene ring, electropolymerization
proceeds satisfactorily.39 Accordingly, we synthesized ligand 7 to
complement 4 (Scheme 1). Reaction of glycerine acetone ketal
with excess 1,6-dibromohexane, followed by hydrolysis, gave the
required functionalized diol, and acid-catalyzed ether exchange
with 3,4-dimethoxythiophene gave 633 (for details see the ESI)†
which was then treated with LiPPh2 in THF to give 7 as
a viscous, air-sensitive oil. Reaction of 7 with [PdCl2(CH3CN)2]
or [PtCl2(PhCN)2] in CH2Cl2 gave complexes, 8a (M ¼ Pd) and
8b (M ¼ Pt) respectively. Microanalytical and mass spectrometry
data were consistent with the expected formulae, [MCl2(7)2].
Complexes 8a and 8b show singlets in their 31P{1H} spectra, with
1JPtP (3657 Hz) consistent with cis geometry for 8b, in spite of the
fact that these are also diasteromeric mixtures (the asymmetric
centres are far from the coordination sphere).
Scheme 1 Syntheses of ligands 4 and 7.
groups, in which both of the EDOT a-positions are free for
oxidative polymerizations (Scheme 1).
Using literature methods,32 3,4-dimethoxythiophene was
prepared from 3,4-dibromothiophene, and an acid-catalyzed
ether exchange reaction of this with (racemic) 3-chloropropane-
Electrochemical synthesis of poly(3,4-ethylenedioxythiophene)s
copolymers with EDOT-phosphine complexes
1,2-diol
gave 2-(chloromethyl)-2,3-dihydrothieno[3,4-
When contemplating the electrochemical polymerization
of EDOT-containing phosphine complexes, it is necessary to
consider the possible redox activity of the metal centre as well as
that of the EDOT unit(s). There have been several studies of the
irreversible redox behavior of cis- and trans-[PtCl2(PR3)2],
although most of the attention has focussed on reduction to
Pt(0).40–46 Similarly, because of the importance of [Pd(PR3)2] and
related species as intermediates in Pd-catalyzed coupling cycles,
b][1,4]dioxine (EDOTCH2Cl; 3). Reaction of 3 with LiPPh2 in
THF gave EDOTCH2PPh2, 4 as an air-sensitive oil. Spectro-
scopic and analytical data indicated that it was sufficiently pure
to use as prepared. Reaction of 4 with [PdCl2(CH3CN)2] or
[PtCl2(PhCN)2] in CH2Cl2 gave sparingly soluble complexes, 5a
(M ¼ Pd) and 5b (M ¼ Pt) respectively. Microanalytical and
mass spectrometry data were consistent with the expected
formulae, [MCl2(4)2]. The proton NMR spectra of 5a and 5b
show complex and quite broad alkyl EDOT unit resonances.
Interestingly, while the thienyl proton resonances for 4 are at
closely similar chemical shifts with a very small HH coupling,
both 5a and 5b show two distinct sets of thienyl H resonances,
both of which are complex multiplets. The 31P{1H} NMR
spectrum of 5b shows two sharp resonances of near–identical
intensity, at 9.13 and 10.76 p.p.m., with 1JPtP satellites at almost
identical separations, 3664 and 3669 Hz respectively, consistent
with cis geometry. Since ligand 4 is a mixture of enantiomers,
the most likely explanation for the complexity of the 1H and 31
P
NMR spectra is that the resulting two diastereomeric complexes
1
comprising 5b give distinct sets of H and 31P{1H} resonances.
To add to the complexity, two much less intense resonances, at
12.0 and 13.2 p.p.m., were seen in the 31P{1H} spectrum of 5b.
1
The signal to noise ratio was insufficient to allow JPtP to be
determined for these minor resonances, but it is possible that
they are due to the two diasteromeric trans isomers. The 31P{1H}
NMR spectrum of 5a showed two singlets of equal intensity at
15.38 and 15.44 p.p.m., and a minor peak at 18.29 p.p.m.
Repeated attempts at recrystallization of 5a eventually yielded
crystals suitable for X-Ray analysis, and these proved to be
of the trans isomer. The molecular structure is illustrated in
Fig. 2.‡
Fig. 2 Thermal ellipsoid plot (50% probability ellipsoids) of molecule of
5a. The Pd atom lies on an inversion centre, resulting in trans-stereo-
chemistry and the inversion related P10 and Cl10 atoms are labelled in this
figure. Significant bond distances and angles are as follows. Pd(1)–P(1)
2.3027(15), Pd(1)–Cl(1) 2.2791(16), P(1)–C(7) 1.848(ꢂ4), P(1)–C(8)
Bond lengths and angles are typical of other examples of trans-
[PdCl2(Ar2PR)2].36 Preparative-scale recrystallization, to sepa-
rate either the cis and trans isomers, or the diastereomers,
was unsuccessful for 5a and 5b. We therefore decided to attempt
the electrochemical polymerization of these complexes as isomer
mixtures.
ꢀ
1.822(4), P(1)–C(14) 1.821(3) A; P(1)–Pd(1)–P(1a) 180.0 , P(1)–Pd(1)–
Cl(1) 86.45(3)ꢂ, P(1)–Pd(1)–Cl(1a) 93.55(3)ꢂ, Cl(1)–Pd(1)–Cl(1a) 180.0ꢂ.
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there has been intensive electrochemical study of Pd(0)/Pd(II)
interconversion involving phosphine ligands.47–50 In contrast,
however, few studies have appeared on M(II)/M(IV) couples. In
early work, the oxidative behavior of cis-[PtCl2(PPh3ꢀnRn)2] (R
¼ Me; n ¼ 0–3 and R ¼ Et, n ¼ 3) was reported.46 The complex
cis-[PtCl2(PPh3)2] showed a small irreversible anodic wave at EP
¼ +1.78 V, and a much larger irreversible anodic wave at EP ¼
+2.23 V. These were assigned to the oxidation of, respectively,
the trans- and cis-isomers; it was assumed that the former
originated from the latter by isomerisation. Later, a detailed
electrochemical study of the oxidation of cis- and trans-
[PtCl2(PEt2)3] was published.40 Both isomers showed very similar
voltammetric waves (Pt working electrode, 0.08 M Bu4NClO4–
CH3CN), with a small anodic peak near +1.7 V and a larger
anodic peak near +2.1 V. On reversing the scan a small cathodic
Fig. 3 Repetitive scan cyclic voltammetry experiments to grow PEDOT
homopolymer (A) and an EDOT:2a copolymer (B). Both experiments
used a scan rate of 100 mV sꢀ1, 0.1M Bu4NBF4–CH3CN electrolyte and
0.01 M total monomer concentration. The copolymer experiment used
a 5:1 ratio of EDOT:2a.
ꢂ
peak was seen at ca. +0.8 V. Upon cooling to ꢀ10 C both the
small anodic wave at +1.7 V and the cathodic wave at +0.8 V
disappeared; the origin of these waves is still unclear, since the
irreversible reduction of cis-[PtCl4(PEt3)2] (studied indepen-
dently) occurred at ca. 0.0 V. Attempts to carry out bulk
electrolyses at +2.1 V were hampered by very low current flow,
indicating that the electrooxidation was probably a CE or EC
process with a slow chemical step. Coulometry could not be
performed, and 31P{1H} NMR spectroscopic characterization of
the products of such electrolyses (incomplete even after several
hours) revealed mainly unreacted [PtCl2(PEt3)2], with some
[PtCl4(PEt3)2], and oxidised phosphine.
Fig. 4 Repetitive scan cyclic voltammetry copolymerizations of EDOT
with (A) 5a and (B) 5b. Conditions as in caption to Fig. 3.
In spite of the longer ‘spacer’ for the ligand 7, we were not able
to electropolymerize the complexes 8a or 8b alone, and again
resorted to copolymerization. The voltammograms of the
EDOT:8 copolymers are very similar in profile to those of the
corresponding EDOT:5 copolymers (ESI, Fig. S1).† After
electrodeposition, all polymer films were characterized by cyclic
voltammetry in background electrolyte, electrochemical imped-
ance spectroscopy and X-Ray photoelectron spectroscopy.
In view of these results, we anticipated that because our Pt(II)
complexes involve less electron-donating ligands of type Ph2PAr
or Ph2PR, at potentials negative of +2 V (the positive limit in our
electropolymerizations), the redox activity of the metal centers
during repetitive scan cyclic voltammetry would be limited. No
related study of corresponding complexes [PdCl2(PPh3–nRn)2]
has been performed, but since known examples of Pd(IV)–
phosphine complexes are less thermodynamically stable than the
corresponding Pt(IV) complexes,51,52 it is unlikely that our Pd(II)
complexes would be significantly oxidised at potentials less
positive than the Pt(II) examples.
XPS characterization of the copolymers
The electrochemical oxidation of 2a or 2b alone did not result
in the formation of redox-active polymer films on the electrode
surface. Co-polymerization with unsubstituted monomer is
a strategy that has often succeeded in cases where a functional-
ized monomer does not electropolymerize alone,6,12,29,53 and we
therefore attempted to grow polymer films from electrolyte
solutions containing both 2a (or 2b) and EDOT (5:1 mole ratio;
0.01 M total monomer concentration), using repetitive scan
cyclic voltammetry. In these experiments, redox-active polymer
films were deposited on the electrodes. The cyclic voltammo-
grams obtained during electrodeposition (Fig. 3) were similar in
profile to those obtained during the electrodeposition of PEDOT
itself, under the same conditions.
The XPS spectra of polymers prepared using mixtures of 2a
(or 2b) and EDOT showed that any metal or chlorine present
were below the detection limit. However, small but significant P
2p peaks were apparent, at 131.9 eV. For 2a, the S:P ratio was
>20:1, and for complex 2b the S:P ratio was >10:1. This suggests
that electrochemical oxidation of complexes 2a and 2b results in
decomposition, with liberation of 1 (or its oxidation product)
that is subsequently incorporated into the growing polymer film.
This would explain why the voltammograms in background
electrolyte (see below) are characteristic of a polymer somewhat
less conjugated than PEDOT (although, interestingly, the
electrochemical impedance spectra were very similar). Failure to
incorporate intact metal complexes with this ligand into conju-
gated polymers is in agreement with the failure of earlier attempts
to polymerise Mo(0)30 and W cluster complexes24 of this and
related ligands.
Similarly, we could not prepare redox-active polymer films by
electrooxidation of 5a or 5b alone. Cyclic voltammograms of
the copolymerization of, respectively, 5a and 5b with EDOT are
shown in Fig. 4. On the first scan, there are two anodic peaks at
+1.38 and +1.90 V, and the subsequent evolution of these peaks
on repeated scanning is again significantly different from that
of the corresponding peaks at +1.36 and +2.00 V seen in the
first scan of EDOT itself.
In contrast, the complexes with ligands in which EDOT units
are present as a pendant group (5a, 5b, 8a, 8b) were successfully
copolymerized with EDOT. The data are summarized in Table 1.
Characteristic peaks are observed for Pd (3d5/2; 5a and 8a) and Pt
(4f7/2; 5b and 8b) at binding energies that are very similar to those
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Table 1 XPS data for complex copolymers. Note that time constraints
prevented collection of high-resolution data for Cl 2p (8b), S 2p and O 1s
(5a, 5b, 8b respectively), although these elements were all observed in the
survey scans
Copolymer with 5a
Pd 3d5/2, 3d3/2 337.9, 343.2 eV
Cl 2p 198.3, 199.9 eV
P 2p 132.0 eV
Pd:P 0.63
Pd:Cl 0.52
Copolymer with 5b
Pt 4f7/2, 4f5/2 73.1, 76.4 eV
Cl 2p 198.2, 199.8 eV
P2p 131.6 eV
Pt:P 0.49
Pt:Cl 0.51
Fig. 6 Cyclic voltammograms (100 mV sꢀ1) of PEDOT homopolymer
(A) and ‘EDOT:2a copolymer’ (B) in 0.1 M Bu4NBF4–CH3CN.
Copolymer with 8a
Pd 3d5/2, 3d3/2 338.1, 343.3 eV
Cl 2p 198.2, 199.9 eV
P 2p 131.5 eV
S 2p 163.8, 164.8 eV
O 1s 532.4 eV
Copolymer with 8b
Pd:P 0.66
Pd:Cl 0.61
Pd:S 0.33
Pd:O 0.2
that the voltammograms of the copolymers (see below) are so
different from that of PEDOT itself.
The cyclic voltammograms of PEDOT and of a ‘copolymer’
grown using complex 2a and EDOT are shown in Fig. 6. The
PEDOT voltammogram (Fig. 6, (A)) resembles those observed
previously for films grown and cycled in polar aprotic solvents
with small, non-nucleophilic anion electrolytes.26,60 The ‘copoly-
mer’ obtained using complex 2a (Fig. 6, (B)) shows somewhat
more positive onset and peak potentials for oxidation compared
with PEDOT itself, suggesting that the polymer is less conjugated
than PEDOT. This is consistent with the XPS evidence that
although there is no Pd or Cl in the film, there is some P owing to
incorporation of (oxidised) 1.
Pt 4f7/2, 4f5/2 73.0, 76.4 eV
P 2p 131.6 eV (Cl 2p not determined)
Pt:P 0.41
reported for other neutral Pd(II) or Pt(II) complexes.54,55 Peaks
due to P 2p and Cl 2p are also observed, in approximately the
correct element ratios, and at binding energies consistent with
coordinated PR3 and Clꢀ respectively.56,57 Fig. 5 shows the data
obtained for the EDOT: 8a copolymer.
The XPS data supports the contention that the complexes of
phosphines with ‘pendant’ EDOT units are incorporated intact
into these copolymers. In particular, irreversible oxidation of the
metal centers to M(IV) would result in significant binding energy
increases. For example, for K2[PtCl6], the 4f7/2 peak is reported
to occur at 75.7 eV,58 and for the trans-[PdCl2(en)2]2+ unit (in
halogen-bridged Pd(II)/Pd(IV) mixed valence complexes), the
3d5/2 peak is at 340.6 eV.59 For the copolymer of EDOT with 8a,
high-resolution data were also obtained for S (2p). Interestingly,
the mole ratio Pd:S was ca. 1:3. This is consistent with a ca.
1:1 ratio of EDOT and 8a in the copolymer, a considerably
higher proportion of complex than was used in the monomer
feed. A high proportion of incorporated metal complex would
lead to a heavily cross-linked polymer, a large volume fraction of
which is metal complex, and this might explain the observation
Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy is a powerful yet simple
tool to study electronic and ionic resistances associated with
electroactive materials. The technique has been extensively used
to gather electrical information on electronically conductive
polymers. Among the various theories developed to interpret
EIS data of electronically conductive polymers, the dual-rail
transmission line model proposed by Albery et al.61–64 has been
widely used to extract electronic and ionic resistances of such
materials as polypyrrole, polyaniline and PEDOT in combina-
tion with various anions. In this model, the motion of electrons
along the polymer film is described by one resistive rail, R1, while
the motion of the counter ions in the pores of the polymer con-
taining the electrolyte is described by a second resistive rail, R2.
The electrons are driven along the rail by the Nernst potential
between the solution phase in the pores and the polymer inter-
face. The Donnan potential between the solution–polymer
interface drives the ions into and out of the polymer. The origin
of the distributed capacitance, Cp, between the two lines has been
clearly explained by Feldberg.65 (See ESI, Fig. S5.)†
The detailed analysis of the model for the charge transport in
electronically conducting polymers will yield the following
impedance plot for electronically conducting polymers. At the
highest frequency there is a simple resistance, RS, due to the
resistance of the bulk electrolyte. As the frequency is lowered,
a semicircular section is found which is caused by the resistance,
Re, of the whole polymer coat transmission line in parallel with
the capacitance CE. Under these conditions the polymer carries
charge to the polymer–electrolyte interface, where a small part of
the distributed capacitance is charged and discharged. When the
semicircle is complete the corresponding resistance in the real
axis of the Argand diagram is equal to the sum of the solution
Fig. 5 XPS data for the copolymer made using EDOT and complex 8a.
Clockwise from top: Pd 3d, Cl 2p, S 2p, and P 2p.
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Table 2 Values of electronic resistance Re and ionic resistance RI for ‘copolymer’ films from EIS spectra as a function of electrode potential
PEDOT
Re, RI/U
EDOT/2a
Re, RI/U
EDOT/2b
Re, RI/U
EDOT/5a
Re, RI/U
EDOT/5b
Re, RI/U
EDOT/8a
Re, RI/U
EDOT/8b
Re, RI/U
E/mV
–100
+100
+300
+800
253, 747
225, 663
170, 512
157, 460
335, 870
181, 534
210, 613
174, 410
263, 688
145, 281
178, 526
102, 196
383, 1038
357, 1057
265, 774
323, 937
760, 1672
697, 2016
413, 1140
281, 764
n.d.
352, 1037
670, 1950
529, 1550
287, 848
546, 1445
339, 940
269, 670
resistance and the electronic resistance of the polymer. Thus the
diameter of the semicircle provides the electronic resistance, Re,
of the polymer. At moderate frequencies there exists a 45ꢂ
‘Warburg’ region characteristic of the more resistive solution
phase of the transmission line. As the frequency is further low-
ered, the involvement of the distributed capacitance becomes
prominent and when the frequency is less than (RSCP)ꢀ1, the
characteristic time constant of the transmission line, the whole
of the distributed capacitance is involved. This results in
a vertical straight line in the Argand diagram. The difference
in Zreal between the extremes of the Warburg region gives one
third of the ionic resistance of the polymer, RI (ESI Fig. S5).†
The Re and RI values extracted from the EIS spectra at selected
applied DC potentials for the polymers, together with the
respective conductivities, are shown in Table 2. Interestingly,
the electrochemical impedance data for the polymers made in the
presence of 2a or 2b are little different from that of PEDOT itself.
The electronic resistances at a given potential are similar to, or
even a little lower, than the values for the pure PEDOT film.
+0.28 V in the copolymer with 5b. The cyclic voltammogram of
the EDOT:5a copolymer shows behavior intermediate between
that of the PEDOT homopolymer and the EDOT:5b copolymer.
This suggests that the copolymers have a shorter mean conju-
gation length (shortest for the EDOT:5b copolymer), consistent
with the higher electronic resistance values at a given potential
from the EIS spectra for these materials (Table 2). Also, whereas
there is a significant cathodic current in the PEDOT voltam-
mogram almost immediately after the scan direction is reversed,
the re-reduction of the copolymer with 5b does not commence
until +1.2 V, and the main cathodic peak is at a more negative
potential (ꢀ0.2 V) than that of PEDOT (+0.1 V). Hence, the
kinetics of the polymer redox process(es) are, once more, clearly
slower in the copolymers.
The voltammograms of copolymers of complexes 8 with
EDOT (Fig. 7C and 7D) are also different in profile from that of
PEDOT itself, and are likewise consistent with the copolymers
having shorter mean conjugation lengths than PEDOT itself, and
slower kinetics of redox switching, as expected for such heavily
cross-linked materials bearing a large volume of redox-inactive
complex.
Electrochemistry of copolymers
Interestingly, although the relative peak heights are different,
the peak potentials for the oxidation processes seen for the 5a
and 8a copolymers are similar. However, the 5b copolymer CV is
quite different in profile to that of 5b.
The voltammograms of copolymers of 5 and 8 with EDOT
recorded in the absence of monomer are again notably different
from those of the PEDOT homopolymer (Fig. 7). The onset of
PEDOT polymer oxidation seen at ca. ꢀ0.3 V in PEDOT (Fig. 6,
(A)) is shifted positive, to almost 0.0 V, and the peak potential
of the first anodic wave is shifted from ꢀ0.05 V in PEDOT to
Conclusions
We have shown that to build PEDOT materials containing
covalently-bound square planar Pd(II) and Pt(II) phosphine
complexes, it is necessary to use EDOT monomer units with both
thiophene a-positions free, and bearing the phosphine moiety as
a pendant group, such as 4 or 7. Complexes of ligands in which
the thiophene ring of EDOT is a phosphorus substituent, such as
1, do not electropolymerize intact, instead undergoing oxidative
decomposition with at least partial incorporation of the ligand
(or its oxidation product) into the growing polymer.
We are currently extending this work to redox-active Ru(II)
complexes, and we are studying applications of the materials
in (electro)catalytic reactions. Additionally, we are constructing
bidentate and chiral phosphine ligands with pendant EDOT
groups.
Acknowledgements
We thank the University of Jaffna, Sri Lanka (QEF grant-batch
2, under the IRQUE project) for a scholarship (KV), the EPSRC
for partial funding of this work (grant EP/C00678X) and for
funding the NCESS and Chemical Database Services, and
Fig. 7 Cyclic voltammograms (100 mV sꢀ1) of EDOT:5a copolymer (A),
EDOT:5b copolymer (B), EDOT:8a copolymer (C) and EDOT:8b
copolymer (D) in 0.1 M Bu4NBF4/CH3CN.
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32 J. L. Segura, R. Gomez, E. Reinold and P. Bauerle, Org. Lett., 2005,
7, 2345–2348.
33 F. Mouffouk and S. J. Higgins, Electrochem. Commun., 2006, 8,
15–20.
Dr Graham Beamson (NCESS, Daresbury) for experimental
assistance and advice.
34 J. Ghilane, P. Hapiot and A. J. Bard, Anal. Chem., 2006, 78, 6868–
6872.
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