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

Two new Ni(II) complexes, the homoleptic [Ni(8P^Cy ₂N^H)₂]²⁺ complex (8P^Cy ₂N^H = 3,7-cyclohexyl-1-amino-3,7-diphosphacyclooctane) containing two pendant amines on adjacent ligands, and the heteroleptic [Ni(8P^Cy ₂N^H)(dppe)]²⁺ complex (dppe = bis(diphenylphosphino)ethane) containing only a single pendant amine, have been synthesized, and their electrochemical properties are reported. The [Ni(8P^Cy ₂N^H)(dppe)]²⁺ complex is capable of heterolytically cleaving hydrogen, allowing for the first observation of an endo-protonated nickel hydride related to the [Ni(P^R ₂N^R′ ₂)₂]²⁺ family of complexes. The [Ni(8P^Cy ₂N^H)(dppe)]²⁺ complex did not exhibit electrocatalytic H₂ oxidation activity; however, [Ni(8P^Cy ₂N^H)₂]²⁺ is an active electrocatalyst for H₂ oxidation with a maximum turnover frequency (k_obs) of 18 s^-1 under 1 atm H₂ at E_cat/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(P^R ₂N^R′ ₂)₂]²⁺ family, the effect of the secondary pendant amine on intermolecular deprotonation is discussed.