Cobalt Complexes Containing Pendant Amines in the Second Coordination Sphere as Electrocatalysts for H2 Production

A series of heteroleptic 17e cobalt complexes, [CpCoII((P2N2Ph)-N-tBu)](BF4), [(CpCoII)-Co-C6F5((P2N2Ph)-N-tBu)](BF4), and [(CpCoII)-Co-C5F4N((P2N2Ph)-N-tBu)](BF4) (where (P2N2Ph)-N-tBu = 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 [CpC5F4NCoII((P2N2Ph)-N-tBu)](+) appears 250 mV positive of the Co-III/II couple of [CpCoII((P2N2Ph)-N-tBu)](+) 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 CpCoI((P2N2Ph)-N-tBu), (CpCoI)-Co-C6F5(PtBu2N2Ph), and (CpCoI)-Co-C5F4N(PtBu2N2Ph), which were also characterized by crystallography. Protonation of these neutral Co-I complexes led to the CoIII hydrides [CpCoIII((P2N2Ph)-N-tBu)H](BF4), [(CpCoIII)-Co-C6F5((P2N2Ph)-N-tBu)H](BF4), and [(CpCoIII)-Co-C5F4N(PtBu2N2Ph)H](BF4), and crystal structures of each of these cobalt hydrides were determined. The cobalt complex with the most electron withdrawing Cp ligand, [(CpCoII)-Co-C5F4N((P2N2Ph)-N-tBu)](+), is an electrocatalyst for production of H-2 using [p-MeOC6H4NH3][BF4] (pKa(MeCN) = 11.86), with a turnover frequency of 350 s1 and an overpotential of 0.86 V at E-cat/2. A pKa value of 15.6 was measured in CH3CN for [(CpCoIII)-Co-C5F4N((P2N2Ph)-N-tBu)H], which was used in conjunction with electrochemical measurements to obtain thermodynamic data for cleavage of the CoH bond.