Cobalt complexes containing pendant amines in the second coordination sphere as electrocatalysts for H₂ production

A series of heteroleptic 17e cobalt complexes, [CpCo^II(P^tBu₂N^Ph₂)](BF₄), [Cp^C6F5Co^II(P^tBu₂N^Ph₂)](BF₄), and [Cp^C5F4NCo^II(P^tBu₂N^Ph₂)](BF₄) (where P^tBu₂N^Ph₂ = 1,5-diphenyl-3,7-di-tert-butyl-1,5-diaza-3,7-diphosphacyclooctane, Cp^C6F5 = C₅H₄(C₆F₅), and Cp^C5F4N = C₅H₄(C₅F₄N)) were synthesized, and the structures of all three were determined by X-ray crystallography. Electrochemical studies showed that the Co^III/II couple of [Cp^C5F4NCo^II(P^tBu₂N^Ph₂)]⁺ appears 250 mV positive of the Co^III/II couple of [CpCo^II(P^tBu₂N^Ph₂)]⁺ as a result of the strongly electron withdrawing perfluoropyridyl substituent on the Cp ring. Reduction of these paramagnetic Co^II complexes by KC₈ led to the diamagnetic 18e complexes CpCo^I(P^tBu₂N^Ph₂), Cp^C6F5Co^I(P^tBu₂N^Ph₂), and Cp^C5F4NCo^I(P^tBu₂N^Ph₂), which were also characterized by crystallography. Protonation of these neutral Co^I complexes led to the Co^III hydrides [CpCo^III(P^tBu₂N^Ph₂)H](BF₄), [Cp^C6F5Co^III(P^tBu₂N^Ph₂)H](BF₄), and [Cp^C5F4NCo^III(P^tBu₂N^Ph₂)H](BF₄), and crystal structures of each of these cobalt hydrides were determined. The cobalt complex with the most electron withdrawing Cp ligand, [Cp^C5F4NCo^II(P^tBu₂N^Ph₂)]⁺, is an electrocatalyst for production of H₂ using [p-MeOC₆H₄NH₃][BF₄] (pK_a^MeCN = 11.86), with a turnover frequency of 350 s^-1 and an overpotential of 0.86 V at E_cat/2. A pK_a value of 15.6 was measured in CH₃CN for [Cp^C5F4NCo^III(P^tBu₂N^Ph₂)H], which was used in conjunction with electrochemical measurements to obtain thermodynamic data for cleavage of the Co-H bond.