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Title: Thermochemical and Mechanistic Studies of Electrocatalytic Hydrogen Production by Cobalt Complexes Containing Pendant Amines

Two cobalt(tetraphosphine) complexes [Co(PnC-PPh22NPh2)(CH3CN)](BF4)2 with a tetradentate phosphine ligand (PnC-PPh22NPh2 = 1,5-diphenyl-3,7-bis((diphenylphosphino)alkyl)-1,5-diaza-3,7-diphosphacyclooctane; alkyl = (CH2)2, n = 2 (L2); (CH2)3, n = 3 (L3)) have been studied for electrocatalytic hydrogen production using 1:1 [(DMF)H]+:DMF. A turnover frequency of 980 s–1 with an overpotential of 1210 mV was measured for [CoII(L2)(CH3CN)]2+, and a turnover frequency of 980 s–1 with an overpotential of 930 mV was measured for [CoII(L3)(CH3CN)]2+. Addition of water increases the turnover frequency of [CoII(L2)(CH3CN)]2+ to 19,000 s–1. The catalytic wave for each of these complexes occurs at the reduction potential of the corresponding HCoIII complex. Comprehensive thermochemical studies of [CoII(L2)(CH3CN)]2+ and [CoII(L3)(CH3CN)]2+ and species derived from them by addition/removal of protons/electrons were carried out using values measured experimentally and calculated using DFT. Notably, HCoI(L2) and HCoI(L2) were found to be remarkably strong hydride donors, with HCoI(L2) being a better hydride donor than BH4-. Mechanistic studies of these catalysts reveal that H2 formation can occur by protonation of a HCoII intermediate, and that the pendant amines of these complexes facilitate proton delivery to the cobalt center. The rate-limiting step for catalysis is a net intramolecular isomerization of the protonated pendant amine from the non-productive exo-isomer to the productive endomore » isomer. We thank Dr. Shentan Chen for many helpful discussions. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Computational resources were provided at the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.« less
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Journal Article
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Journal Name: Inorganic Chemistry, 52(24):14391-14403
Research Org:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
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Country of Publication:
United States
cobalt; electrocatalysis; hydrogen production