Electrochemical polymerization of a bis(thienyl)bithiazole osmium complex
Brian J. MacLean and Peter G. Pickup*
Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X7.
E-mail: ppickup@morgan.ucs.mun.ca
Received (in Columbia, MO, USA) 27th July 1999, Accepted 1st November 1999
A novel and easily n-doped conjugated metallopolymer has
been prepared by the electrochemical polymerization of
Os(bpy)2[5,5A-bis(2-thienyl)-2,2A-bithiazole]2+.
passivating layer can mediate the electrochemistry of Os(ii) in
solution, their redox potential is not high enough to mediate the
oxidation/polymerization of the bthbthz ligand.
Reasoning that the passivating nature of the film formed by 1
was due to nucleophilic attack of residual water‡ on oxidized
segments of the bithiophene-bithiazole backbone,8 we added
BF3·OEt2 (Aldrich) to the acetonitrile–0.1 mol dm23 NEt4ClO4
electrolyte solution. This reagent has been reported to be an
excellent solvent system for the electrochemical synthesis of
polythiophene.9 The rationale for its use here is the deactivation
of residual water by complexation with the BF3. As shown in
Fig. 2, compound 1 undergoes facile and sustained electro-
chemical polymerization in the BF3·OEt2 containing electrolyte
when the potential is cycled into the ligand oxidation wave at
ca. 1.65 V. The Os(iii/ii) wave at a formal potential (E°A) of
+0.85 V increases monotonically with cycling of the potential
into the ligand oxidation wave, and new waves appear in the
region between the Os and ligand waves. The number,
positions, and intensities of these new waves are somewhat
variable. They are in the expected potential region for oxidation
and re-reduction of a conjugated polymer backbone formed
from polymerization of the bthbthz ligand at the 5-positions of
the thiophene end groups, and therefore provide evidence that
the complex polymerizes in this way. A deep red–purple film
was observed on the electrode following the cyclic voltammetry
shown in Fig. 2.
2,2A-Bithiazole is an attractive component for conjugated
metallopolymers1 because of its structural similarity to bithio-
phene and high p-electron density relative to bipyridine. Thus,
unlike (bi)pyridine containing polymers, bithiazole based
materials can be stable when p-doped and can exhibit relatively
high conductivities.2–5 An appealing approach to well defined
bithiazole based metallopolymers is via the electrochemical
polymerization of complexes of bis(thienyl)bithiazoles such as
1. However, we have found that such complexes {Os(bpy)2L2+
and Ru(bpy)2L2+, where bpy = 2,2A-bipyridine and L = 5,5A-
bis(2-thienyl)-2,2A-bithiazole (bthbthz), 5,5A[bis(2-thienyl)-
4,4A-dimethyl-2,2A-bithiazole, or 5,5A-bis[2-(3-methoxythie-
nyl)]-4,4A-dimethyl-2,2A-bithiazole} can not be polymerized
under conventional conditions. Zhu and Swager have found that
complexes of the analogous bipyridine ligand, 5,5A-bis(2-
thienyl)-2,2A-bipyridine, are also resistant to electrochemical
polymerization.6,7 These authors circumvented the problem by
extending the terminal thiophenes to bithiophenes, which gave
polymerizable complexes.
The reason for the failure of complexes with single thiophene
rings on the ligand to polymerize appears to be due to instability
of the bithiophene linkages that are formed at the high potentials
needed to drive the polymerization. As can be seen in Fig. 1, the
oxidation of the bthbthz ligand of 1,† does not begin until ca.
+1.55 V vs. SSCE (Epa = 1.68 V). This wave decreases rapidly
with cycling (not shown), indicating that a passivating film is
produced on the electrode. Interestingly, the Os(iii/ii) wave
remains unchanged with extensive cycling, indicating that the
passive layer contains electroactive Os centres. Indeed, a small
Os(iii/ii) wave can still be seen when a passivated electrode is
transferred to a blank electrolyte. Although the Os centres in the
Fig. 2 Cyclic voltammetry (100 mV s21) of 1 in acetonitrile containing 0.1
mol dm23 NEt4ClO4 and 0.45 mol dm23 BF3·OEt2. Arrows indicate
changes with cycling.
Fig. 3 shows cyclic voltammograms of a poly-1 coated
electrode in the absence of 1 in solution. A reversible Os(iii/ii)
wave appears at E°A = 0.865 V and a series of reduction waves
are seen in the region 20.5 to 22.0 V. Based on the first three
redox potentials for reduction of Os(bpy)32+ (21.26, 21.45 and
21.76 V10), which are for ligand based reductions,10 the most
positive reduction wave (E°A = 20.79 V) can be assigned to n-
doping of the polymer backbone, while those at E°A = 21.62
and 21.85 V can be assigned to bpy based reductions. The mild
potential and reversibility of the polymer based reduction, and
the good stability of the n-doped form (indicated by the lack of
significant decay of peak heights on multiple scans), open up the
possibility of applications in molecular electronics and the
Fig. 1 Cyclic voltammetry (100 mV s21) of 1 in acetonitrile containing 0.1
mol dm23 NEt4ClO4.
Chem. Commun., 1999, 2471–2472
This journal is © The Royal Society of Chemistry 1999
2471
MacLean, Brian J.
