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

5 years ago

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

Kenneth K. Kpogo, Cláudio N. Verani, Adam T. Fiedler, Denan Wang, H. Bernhard Schlegel, Shivnath Mazumder

The bimetallic catalyst [CoII2(L1)(bpy)2]ClO4 (1), in which L1 is an [NN′2O2] fused ligand, efficiently reduced H+ to H2 in CH3CN 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−Namido−Co environment offered by L1. Experimental results revealed that the parent [CoIICoII] complex undergoes two successive metal-based 1 e− reductions to generate the catalytically active species [CoICoI], 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 [CoII(CoII−H−)] hydride, thus avoiding the CoIII−H− required in monometallic species. This [CoII(CoII−H−)] species then accepts another H+ to release H2. More cobalt, more fun: The bimetallic cobalt complex 1 is investigated to assess its possible cooperative catalytic activity towards H+ reduction to H2 in CH3CN. Mechanistically, complex 1 displays the protonation of a CoI center prompting a sequential 2 e− transfer, in which each of the two CoI centers donates an electron to the H+.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201701982