Crucial Roles of a Pendant Imidazole Ligand of a Cobalt Porphyrin Complex in the Stoichiometric and Catalytic Reduction of Dioxygen.

A cobalt porphyrin complex with a pendant imidazole base ([(L )Co ]) is an efficient catalyst for the homogeneous catalytic two-electron reduction of dioxygen by 1,1'-dimethylferrocene (Me Fc) in the presence of triflic acid (HOTf), as compared with a cobalt porphyrin complex without a pendant imidazole base ([(L )Co ]). The pendant imidazole ligand plays a crucial role not only to provide an imidazolinium proton for proton-coupled electron transfer (PCET) from [(L )Co ] to O in the presence of HOTf but also to facilitate electron transfer (ET) from [(L )Co ] to O in the absence of HOTf. The kinetics analysis and the detection of intermediates in the stoichiometric and catalytic reduction of O have provided clues to clarify the crucial roles of the pendant imidazole ligand of [(L )Co ] for the first time.