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 (CoIITpivPP) 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 × 103 M-1 and 1.7 × 1020 M-2. The cyclic voltammetric responses were affected by the rates of the ligand association/dissociation reactions. Formation of the O2 adduct of the Co(II) porphyrin resulted in only small changes in the cyclic voltammetric pattern. Comparison between CoIITpivPP and cobalt tetraphenylporphyrin (CoIITPP) as catalysts for the electroreduction of O2 showed the latter to be the superior catalyst both when adsorbed on graphite electrodes and when dissolved in thin layers of acidified benzonitrile where CoIITpivPP exhibited unimpressive catalytic activity compared with CoIITPP, which appeared to catalyze the reduction of O2 by significantly more than 2 electrons per molecule.