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

The bimetallic catalyst [Co^II₂ (L¹ )(bpy)₂ ]ClO₄ (1), in which L¹ is an [NN'₂ O₂ ] fused ligand, efficiently reduced H⁺ to H₂ in CH₃ CN 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 L¹ . 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^I Co^I ], and DFT calculations suggested that addition of a proton to one Co^I triggers a cooperative 1 e^- transfer by each of these Co^I centers. This 2 e^- transfer is an alternative route to generate a more reactive [Co^II (Co^II -H^- )] hydride, thus avoiding the Co^III -H^- required in monometallic species. This [Co^II (Co^II -H^- )] species then accepts another H⁺ to release H₂ .