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

Two cobalt(tetraphosphine) complexes [Co(P nC‑PPh2 2NPh 2)(CH3CN)](BF4)2 with a tetradentate phosphine ligand (P nC‑PPh2 2NPh 2 = 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 (TOF) of 980 s–1 with an overpotential at E cat/2 of 1210 mV was measured for [CoII(L2)(CH3CN)]2+, and a TOF of 980 s–1 with an overpotential at E cat/2 of 930 mV was measured for [CoII(L3)(CH3CN)]2+. Addition of water increases the TOF of [CoII(L2)(CH3CN)]2+ to 18,000 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 density functional theory (DFT). Notably, HCoI(L2) and HCoI(L3) 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 nonproductive exoisomer to the productive endo isomer.