Examination of Cobalt "Picket Fence" Porphyrin and Its Complex with 1-Methylimidazole as Catalysts for the Electroreduction of Dioxygen

Abstract

The electrochemical behavior of cobalt 5,10,15,20-tetrakis(α,α,α,α-2-pivalamidophenyl)porphyrin (Co^(II)TpivPP) was examined in benzonitrile solutions. The electrochemical responses were changed significantly by the addition of 1-methylimidazole which coordinates once to the Co(II) and twice to the Co(III) porphyrin. The corresponding equilibrium binding constants were estimated as 2.9 × 10^3 M^(-1) and 1.7 × 10^(20) M^(-2). The cyclic voltammetric responses were affected by the rates of the ligand association/dissociation reactions. Formation of the O_2 adduct of the Co(II) porphyrin resulted in only small changes in the cyclic voltammetric pattern. Comparison between Co^(II)TpivPP and cobalt tetraphenylporphyrin (Co^(II)TPP) as catalysts for the electroreduction of O_2 showed the latter to be the superior catalyst both when adsorbed on graphite electrodes and when dissolved in thin layers of acidified benzonitrile where Co^(II)TpivPP exhibited unimpressive catalytic activity compared with Co^(II)TPP, which appeared to catalyze the reduction of O_2 by significantly more than 2 electrons per molecule.

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