Bimetallic Cooperativity in Proton Reduction with an Amido-Bridged Cobalt Catalyst

摘要

The bimetallic catalyst [Co~(II) 2(L~1)(bpy)_2]ClO_4 (1), in which L~1 is an [NN'2O_2] fused ligand, efficiently reduced H~+ to H_2 in CH_3CN in the presence of 100 equiv of HOAc with a turnover number of 18 and a Faradaic efficiency of 94% after 3 h of bulk electrolysis at -1.6 V (vs. Ag/AgCl). This observation allowed the proposal that this bimetallic cooperativity is associated with distance, angle, and orbital alignment of the two Co centers, as promoted by the unique Co-N_(amido)-Co environment offered by L~1. Experimental results revealed that the parent [Co~(II)Co~(II)] complex undergoes two successive metal-based 1 e-reductions to generate the catalytically active species [Co~ICo~I], and DFT calculations suggested that addition of a proton to one CoI triggers a cooperative 1 e-transfer by each of these CoI centers. This 2 e-transfer is an alternative route to generate a more reactive [Co~(II)(Co~(II)-H~-)] hydride, thus avoiding the Co~(II)I-H-required in monometallic species. This [Co~(II)(Co~(II)-H~-)] species then accepts another H~+ to release H_2.