Reductive electropolymerization

In comparatively rare cases reductive electropolymerization can also be used. In this case the monomers usually contain halogen substituents at the positions where the coupling takes place. In contrast to oxidative coupling, some catalyst is added to enhance the coupling process. Figure 7b shows the stoichiometry of reductive coupling. Reductive electropolymerization has been used for the preparation of certain thiophene and pyridine derivatives. 

Electrochemical polymerization (ECP) has such a central role in preparing conductive polymers that it deserves a special chapter, although the main principles have been described previously. In this section we concentrate on oxidative electropolymerization. Reductive electropolymerization is usable only in rare cases. 

Reductive Electropolymerization of Polypyridyl Complexes. Good examples of transition-metal coordination complexes that could have potential use in ECDs include the [M (bipy)3l + (M = iron, ruthenium, osmium bipy = 2,2 -bipyridine) series which are respectively red, orange, and green in the M(II) redox state, due to the presence of an intense MLCT absorption band (26). Elec-trochromicity results from loss of the MLCT absorption band on switching to the M(III) redox state. 

Methyl-4 -vinyl-2,2 -bipyridyl was copolymerized with styrene and then treated with c/5-Ru(bpy)2Cl2 to form pendant Ru(bpy)3 45 (see [3,6,37,204,205] and literature cited therein). In order to study ionic domains around the Ru complex also copolymers with acrylic acid were synthesized. In solution or as thin films photophysical properties and photo-induced electron transfer were investigated. Photoluminescence properties were also studied for polysiloxane pendant Ru(bpy)3 prepared from the corresponding substituted dihydroxysilanes [206]. Reductive electropolymerization of Fe(II), Ru(II) complexes containing 2,2 -bipyridyl and others in acetonitrile in the presence of Et4NC104 (as electrolyte) on Pt results in films of <1 pm thickness [207]. 

Reductive Electropolymerization. Besides the oxidative anodic electropolymerization of the monomer, which is the most convenient and the most widely used method, polythiophene can also be prepared by a cathodic route involving the electroreduction of the complex Ni(2-bromo-5-thienyl)(PPh3)4Br in acetonitrile. This method, initially proposed for the synthesis of poly(p-phenylene) [374-376], has been extended to polythiophene [520]. The major drawback is that the polymer is produced in its neutral insulating form, which leads rapidly to a passivation of the electrode and limits the attainable film thickness to approximately 100 nm. On the other hand, this technique presents the advantage of being applicable to electrode materials subject to anodic corrosion such as small-bandgap semiconductors [521]. 

Huang S-C, Lin C-Y (2015) Reductive electropolymerization of N-methyl-3-pyridylethynyl-porphyrins. Chem Commun 51 519-521... 

Fig. 7 Scheme showing the mechanism of reductive electropolymerization of 10,20-bis-(A -methyl-3-pyridylethynyl)-5,15-biphenylporphinato zinc(II) species... 

Dloxygen reduction electrocatalysis by metal macrocycles adsorbed on or bound to electrodes has been an Important area of Investigation (23 ) and has achieved a substantial molecular sophistication in terms of structured design of the macrocyclic catalysts (2A). Since there have been few other electrochemical studies of polymeric porphyrin films, we elected to inspect the dloxygen electrocatalytic efficacy of films of electropolymerized cobalt tetraphenylporphyrins. All the films exhibited some activity, to differing extents, with films of the cobalt tetra(o-aminophenylporphyrin) being the most active (2-4). Curiously, this compound, both as a monomer In solution and as an electropolymerized film, also exhibited two electrochemical waves... 

The [Co(terpy)2]+ ion (terpy = 2,2 6, 2"-terpyridine) has been found to catalyze the reduction of C02 when the divalent precursor is immobilized on electrode surfaces. The vinyl-substituted terpy analog was electropolymerized to give a film that reduced the overpotential for C02 reduction in DMF and MeCN by more than 1V.104 The activity of this surface-confined species was superior to that of the complexes in solution.105 Similarly, in water, the [Co(terpy)2]2+ ion immobilized and reduced in situ within a Nafion film also catalyzes the reduction of C02106 and H+ ions.107... 

Furthermore, it has been demonstrated that the successful electrocatalytic reduction of C02 with [Ru(bpy)2(CO)2]2+ in aqueous MeCN is mainly due to the formation of a polymeric electroactive film, which occurs during the reduction of the complex.91 This film is composed of an open cluster polymer [Ru(bpy)(CO)2]ra (Scheme 6) based upon extended Ru°—Ru° bonds. Electropolymerization of [Ru(bpy)2(CO)2]2+ results from the overall addition of two electrons per mole of [Ru(bpy)2(CO)2]2+ and is associated with the decoordination of one bpy ligand (Equation (33)). 

Complex (21), a Con-cyclam analogue, is very active for the reduction of N02 and NH2OH intermediates and catalyzes the complex electrochemical conversion of N03 to NH3 325 Gold electrodes modified with cobalt-cyclam incorporated in Nafion films,324 or by electropolymerization of the pyrrole-substituted cobalt cyclam (22)326 have shown catalytic activity for the reduction of nitrate in strongly basic media. 

In contrast to Co-porphyrin complexes, the direct four-electron reduction of 02 has been only very rarely claimed to be catalyzed by a cobalt phthalocyanine 404 407 In particular cofadal binuclear Co-Pc complexes immobilized on pyrolytic graphite catalyze only the two-electron electroreduction of 02 to H202.408,409 However, recent work has established that an electropolymerized Co-Pc derivative provides a stable four-electron reduction pathway over a wide pH range 410... 

Metallopolymer films have also been prepared by oxidative polymerization of complexes of the type [M(phen)2(4,4 -bipy)2]2+ (M = Fe, Ru, or Os phen= 1,10-phenanthroline, 4,4 -bipy = 4,4 -bipyridine).23 Such films are both oxidatively and reductively electrochromic reversible film-based reduction at potentials below —IV lead to dark purple films,23 the color and potential region being consistent with the viologen dication/radical cation electrochromic response. A purple state at high negative potentials has also been observed for polymeric films prepared from [Ru(L13)3]2+.24 Electropolymerized films prepared from the complexes [Ru(L16)-(bipy)2][PF6]22 and [Ru(L17)3][PF6]226,27 exhibit reversible orange/transparent electrochromic behavior associated with the Run/Ruin interconversion. 

An alternate method to produce templated electrodes is the use of chemical reduction of the monomer in the presence of a track-etched or alumina membrane. Parthasarathy et al. [46] have produced enzyme-loaded nanotubules by a combination of both electrochemical and chemical deposition. Initially, the alumina membrane was sealed at one end with a thick Au film (Figure 1.9a), after which the membrane was placed into a mixture of pyrrole and Et4NBF4. The pyrrole was then electropolymerized to form a small plug of polypyrrole at the closed end of the alumina membrane (Figure 1.9b). Subsequently, the membrane was placed into a... 

Fig. 9.9 CV and frequency-potential curves for the oxidation and re-reduction processes of the electropolymerized polyaniline film [26] (a) in 0.5 M LiCICVAN, and (b) in aqueous 0.5 M NaCl04+HCIO4 (pH = 1). Voltage sweep rate 5 mVs-1, quantity of film deposition 0.4Ccm-2, and SSCE = saturated NaCl calomel electrode.

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