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

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 {E1/2 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 (E1/2 = -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.