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

Two new tetraphosphine ligands, P nC-PPh_{2 2}N ^Ph₂ (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 E_rC_rE_r 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.