Synthesis and Electrochemical Studies of Cobalt(III) Monohydride Complexes Containing Pendant Amines

Two new tetraphosphine ligands, 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
synthesized. Coordination of these ligands to cobalt affords the complexes
[CoII(L2)­(CH3CN)]2+ and [CoII(L3)­(CH3CN)]2+, which are reduced by KC8 to afford [CoI(L2)­(CH3CN)]+ and [CoI(L3)­(CH3CN)]+. Protonation
of the CoI complexes affords [HCoIII(L2)­(CH3CN)]2+ and [HCoIII(L3)­(CH3CN)]2+. The cyclic voltammetry of [HCoIII(L2)­(CH3CN)]2+, analyzed using digital simulation, is consistent
with an ErCrEr reduction mechanism
involving reversible acetonitrile dissociation from [HCoII(L2)­(CH3CN)]+ and resulting in formation of
HCoI(L2). Reduction of HCoIII also results in
cleavage of the H–Co bond from HCoII or HCoI, leading to formation of the CoI complex [CoI(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 [HCoIII(L2)­(CH3CN)]2+, indicating a bimetallic
route wherein two cobalt hydride complexes react to form 2 equiv of
[CoI(L2)­(CH3CN)]+ and 1 equiv of
H2. These results indicate that both HCoII and
HCoI can be formed under electrocatalytic conditions and
should be considered as potential catalytic intermediates.