Synthesis and electrochemical studies of cobalt(III) monohydride complexes containing pendant amines

Two new tetraphosphine ligands, P(nC-PPh2)2N(Ph)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 synthesized. Coordination of these ligands to cobalt affords the complexes [Co(II)(L2)(CH3CN)](2+) and [Co(II)(L3)(CH3CN)](2+), which are reduced by KC8 to afford [Co(I)(L2)(CH3CN)](+) and [Co(I)(L3)(CH3CN)](+). Protonation of the Co(I) complexes affords [HCo(III)(L2)(CH3CN)](2+) and [HCo(III)(L3)(CH3CN)](2+). The cyclic voltammetry of [HCo(III)(L2)(CH3CN)](2+), analyzed using digital simulation, is consistent with an ErCrEr reduction mechanism involving reversible acetonitrile dissociation from [HCo(II)(L2)(CH3CN)](+) and resulting in formation of HCo(I)(L2). Reduction of HCo(III) also results in cleavage of the H-Co bond from HCo(II) or HCo(I), leading to formation of the Co(I) complex [Co(I)(L2)(CH3CN)](+). Under voltammetric conditions, the reduced cobalt hydride reacts with a protic solvent impurity to generate H2 in a monometallic process involving two electrons per cobalt. In contrast, under bulk electrolysis conditions, H2 formation requires only one reducing equivalent per [HCo(III)(L2)(CH3CN)](2+), indicating a bimetallic route wherein two cobalt hydride complexes react to form 2 equiv of [Co(I)(L2)(CH3CN)](+) and 1 equiv of H2. These results indicate that both HCo(II) and HCo(I) can be formed under electrocatalytic conditions and should be considered as potential catalytic intermediates.