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

Two new tetraphosphine ligands, P<sup><i>n</i>C‑PPh<sub>2</sub></sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub> (1,5-diphenyl-3,7-bis­((diphenylphosphino)­alkyl)-1,5-diaza-3,7-diphosphacyclooctane; alkyl = (CH<sub>2</sub>)<sub>2</sub>, <i>n</i> = 2 (L2); (CH<sub>2</sub>)<sub>3</sub>, <i>n</i> = 3 (L3)), have been synthesized. Coordination of these ligands to cobalt affords the complexes [Co<sup>II</sup>(L2)­(CH<sub>3</sub>CN)]<sup>2+</sup> and [Co<sup>II</sup>(L3)­(CH<sub>3</sub>CN)]<sup>2+</sup>, which are reduced by KC<sub>8</sub> to afford [Co<sup>I</sup>(L2)­(CH<sub>3</sub>CN)]<sup>+</sup> and [Co<sup>I</sup>(L3)­(CH<sub>3</sub>CN)]<sup>+</sup>. Protonation of the Co<sup>I</sup> complexes affords [HCo<sup>III</sup>(L2)­(CH<sub>3</sub>CN)]<sup>2+</sup> and [HCo<sup>III</sup>(L3)­(CH<sub>3</sub>CN)]<sup>2+</sup>. The cyclic voltammetry of [HCo<sup>III</sup>(L2)­(CH<sub>3</sub>CN)]<sup>2+</sup>, analyzed using digital simulation, is consistent with an E<sub>r</sub>C<sub>r</sub>E<sub>r</sub> reduction mechanism involving reversible acetonitrile dissociation from [HCo<sup>II</sup>(L2)­(CH<sub>3</sub>CN)]<sup>+</sup> and resulting in formation of HCo<sup>I</sup>(L2). Reduction of HCo<sup>III</sup> also results in cleavage of the H–Co bond from HCo<sup>II</sup> or HCo<sup>I</sup>, leading to formation of the Co<sup>I</sup> complex [Co<sup>I</sup>(L2)­(CH<sub>3</sub>CN)]<sup>+</sup>. Under voltammetric conditions, the reduced cobalt hydride reacts with a protic solvent impurity to generate H<sub>2</sub> in a monometallic process involving two electrons per cobalt. In contrast, under bulk electrolysis conditions, H<sub>2</sub> formation requires only one reducing equivalent per [HCo<sup>III</sup>(L2)­(CH<sub>3</sub>CN)]<sup>2+</sup>, indicating a bimetallic route wherein two cobalt hydride complexes react to form 2 equiv of [Co<sup>I</sup>(L2)­(CH<sub>3</sub>CN)]<sup>+</sup> and 1 equiv of H<sub>2</sub>. These results indicate that both HCo<sup>II</sup> and HCo<sup>I</sup> can be formed under electrocatalytic conditions and should be considered as potential catalytic intermediates.