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 t Bu 2 N Ph 2 )](BF 4 ), [Cp C6F5 Co II (P t Bu 2 N Ph 2 )](BF 4 ), and [Cp C5F4N Co II (P t Bu 2 N Ph 2 )](BF 4 ) (where P t Bu 2 N Ph 2 = 1,5-diphenyl-3,7-di- tert -butyl-1,5-diaza-3,7-diphosphacyclooctane, Cp C6F5 = C 5 H 4 (C 6 F 5 ), and Cp C5F4N = C 5 H 4 (C 5 F 4 N)) 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 C5F4N Co II (P t Bu 2 N Ph 2 )] + appears 250 mV positive of the Co III/II couple of [CpCo II (P t Bu 2 N Ph 2 )] + as a result of the strongly electron withdrawing perfluoropyridyl substituent on the Cp ring. Reduction of these paramagnetic Co II complexes by KC 8 led to the diamagnetic 18e complexes CpCo I (P t Bu 2 N Ph 2 ), Cp C6F5 Co I (P t Bu 2 N Ph 2 ), and Cp C5F4N Co I (P t Bu 2 N Ph 2 ), which were also characterized by crystallography. Protonation of these neutral Co I complexes led to the Co III hydrides [CpCo III (P t Bu 2 N Ph 2 )H](BF 4 ), [Cp C6F5 Co III (P t Bu 2 N Ph 2 )H](BF 4 ), and [Cp C5F4N Co III (P t Bu 2 N Ph 2 )H](BF 4 ), and crystal structures of each of these cobalt hydrides were determined. The cobalt complex with the most electron withdrawing Cp ligand, [Cp C5F4N Co II (P t Bu 2 N Ph 2 )] +, is an electrocatalyst for production of H 2 using [ p -MeOC 6 H 4 NH 3 ][BF 4 ] (p K a MeCN = 11.86), with a turnover frequency of 350 s –1 and an overpotential of 0.86 V at E cat/2 . A p K a value of 15.6 was measured in CH 3 CN for [Cp C5F4N Co III (P t Bu 2 N Ph 2 )H], which was used in conjunction with electrochemical measurements to obtain thermodynamic data for cleavage of the Co–H bond.