Nickel Bis-Diphosphine Complexes: Controlling the Binding and Heterolysis of H2

Two new Ni­(II) complexes, the homoleptic [Ni­(8PCy2NH)2]2+ complex (8PCy2NH = 3,7-cyclohexyl-1-amino-3,7-diphosphacyclooctane) containing two pendant amines on adjacent ligands, and the heteroleptic [Ni­(8PCy2NH)­(dppe)]2+ complex (dppe = bis­(diphenylphosphino)­ethane) containing only a single pendant amine, have been synthesized, and their electrochemical properties are reported. The [Ni­(8PCy2NH)­(dppe)]2+ complex is capable of heterolytically cleaving hydrogen, allowing for the first observation of an endo-protonated nickel hydride related to the [Ni­(PR2NR′2)2]2+ family of complexes. The [Ni­(8PCy2NH)­(dppe)]2+ complex did not exhibit electrocatalytic H2 oxidation activity; however, [Ni­(8PCy2NH)2]2+ is an active electrocatalyst for H2 oxidation with a maximum turnover frequency (kobs) of 18 s–1 under 1 atm H2 at Ecat/2 = −0.71 V versus the ferrocenium/ferrocene (Cp2Fe+/0) couple. In addition to an analysis of the effect of the number of pendant amines on the rates of electrocatalytic H2 oxidation of compounds related to the [Ni­(PR2NR′2)2]2+ family, the effect of the secondary pendant amine on intermolecular deprotonation is discussed.