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 ([(L₁)Co^II]) is an efficient catalyst for the homogeneous catalytic two-electron reduction of dioxygen by 1,1′-dimethylferrocene (Me₂Fc) in the presence of triflic acid (HOTf), as compared with a cobalt porphyrin complex without a pendant imidazole base ([(L₂)Co^II]). The pendant imidazole ligand plays a crucial role not only to provide an imidazolinium proton for proton-coupled electron transfer (PCET) from [(L₁)Co^II] to O₂ in the presence of HOTf but also to facilitate electron transfer (ET) from [(L₁)Co^II] to O₂ in the absence of HOTf. The kinetics analysis and the detection of intermediates in the stoichiometric and catalytic reduction of O₂ have provided clues to clarify the crucial roles of the pendant imidazole ligand of [(L₁)Co^II] for the first time.