Preparation, voltammetry, coulometry and electrochemical quartz crystal gravimetry of [Os(bipy)2(PVPS)nClRu(edta)] redox polymers

Abstract

Mixed-metal polymers [Os(bipy)₂(PVPS)_nClRu(edta)](n= 10,15,20) have been prepared by reaction of [Os(bipy)₂(PVPS)_nCl]Cl coated electrodes with K[Ru(edta)(H₂O)](n= 10,15,20) in solutions of various electrolytes (PVPS = polyvinylpyridine–co-styrene, 10% styrene; edta = ethylenediaminetetraacetato; bipy = 2,2′-bipyridyl). Incorporation of the [Ru(edta)]^– unit and binding to the pyridine ligands of P VPS takes place with displacement of water from the coordination sphere of the Ru centre. The resulting [Os(bipy)₂(PVPS)_nClRu(edta)](n= 10,15,20) polymers are reasonably stable, although reduction at –0.4 V causes gradual loss of Ru centres over 1 h. Slow-scan voltammetry (5 mV s^–1) of thin films in aqueous and acetonitrile solvents confirms the approximate 1:1 Os: Ru stoichiometry. Charge-transport characteristics have been investigated by chronocoulometry and rotating disk voltammetry. The behaviour of [Os(bipy)₂(PVPS)_nCl]X towards other anionic complexes has been investigated: [Mo(CN)₈]^4– and [Fe(bathophen)₃]^4–(bathophen = 4,7-diphenyl-l,10-phenanthrolinedisulfonate) bind electrostatically to [Os(bipy)₂(PVPS)_nCl]X. Ion-exchange studies of the mixed Os-Ru polymers towards complex anions and cations shows that methyl viologen (MV²⁺) induces a decrease in the electroactivity of the covalently bound [Ru(edta)]^– units, and additionally, a decrease in the electroactivity of the Os sites. The fraction of electroactive Os sites in the thick films is increased by the presence of coordinated Ru owing to an enhanced rate of charge transport. No evidence was found that reduction of the [Ru(edta)]^– units of the mixed Os-Ru polymer induces incorporation of MV²⁺; instead, loss of [Ru(edta)(H₂O)]^– from the polymer rather than MV²⁺ incorporation dominates. Oxidation of the [Os(bipy)₂Cl]⁺ units of the mixed Os-Ru polymer does, however, induce incorporation of [Fe(bathophen)₃]^4–; alternatively, quaternization of the polymer dramatically increases its affinity for anions. It is possible to incorporate MV²⁺ and [Fe(bathophen)₃]^4– simultaneously, yielding a multiple redox polymer containing six different redox states. In aqueous 0.1 mol dm^–3 NaNO₃ solution in the absence of added Ru complex, quartz crystal microbalance experiments show rapid loss of the Ru complex from the polymer, leaving an Os polymer which takes up anions on oxidation. In acetonitrile solvent the stability of the attached Ru is much greater and quartz crystal microbalance experiments suggest that the mixed Os–Ru polymer shows a mass increase on Ru reduction, associated with both cation ingress and anion egress. The Ru^III/II potential in acetonitrile becomes more negative, indicating stronger preferences of the polymer and cations for the oxidized Ru ^IIhalf of the redox couple.