Cobalt and nickel complexes of 2,2′ : 6′,2″ : 6″,2‴-quaterpyridine as catalysts for electrochemical reduction of carbon dioxide

Abstract

The complexes [Co(qtpy)(OH₂)₂][ClO₄]₂ and [Ni(qtpy)(MeCN)₂][ClO₄]₂(qtpy = 2,2′ : 6′,2″ : 6′,2‴-quaterpyridine) were prepared by treating qtpy with Co(ClO₄)₂·6H₂O and Ni(ClO₄)·6H₂O respectively in acetonitrile. In acetonitrile both complexes show one reversible couple assignable as the [M(qtpy)L₂]^2+/+ couple {E_1/2 for [Co(qtpy)(OH₂)₂]^2+/+=–0.67 V; [Ni(qtpy)(MeCN)₂]^2+/+=–0.79 V vs. saturated calomel electrode, SCE}, one quasi-reversible couple assignable as [M(qtpy)]^+/0(E_1/2=–0.96 for Co, –1.15 V for Ni) and one reduction wave at about –1.98 V vs. SCE assignable as reduction of [M(qtpy)] to [M(qtpy)]^–. Constant-potential electrolysis of a 0.2–0.4 mmol dm^–3 solution of [Co(qtpy)(OH₂)₂]²⁺ in the presence of CO₂ at –1.7 V resulted in the production of CO with a current efficiency of about 80%. At bulk concentration of [Co(qtpy)(OH₂)₂]²⁺ 0.2 mmol dm^–3, electrodeposition of the cobalt complex occurs at –1.65 V vs. SCE to give a dark brown electroactive film on the electrode surface. The cobalt-modified electrode is catalytically active towards the electrochemical reduction of carbon dioxide both in acetonitrile and in water. Constant-potential electrolysis with the cobalt-modified electrode at –1.7 V in acetonitrile and at –1.3 V in water resulted in the production of CO with current efficiencies of about 35%. No film formation on the electrode was observed for [Ni(qtpy)(MeCN)₂]²⁺ and its catalytic activity towards reduction of CO₂ is much less than that of the cobalt complex.