[Ni(Et2PCH2NMeCH2PEt2)2]2+ as a functional model for hydrogenases

The reaction of Et2PCH2N(Me)CH2PEt2 (PNP) with [Ni(CH3CN)(6)](BF4)(2) results in the formation of [Ni(PNP)(2)](BF4)(2), which possesses both hydride- and proton-acceptor sites. This complex is an electrocatalyst for the oxidation of hydrogen to protons, and stoichiometric reaction with hydrogen forms [HNi(PNP)(PNHP)](BF4)(2), in which a hydride ligand is bound to Ni and a proton is bound to a pendant N atom of one PNP ligand. The free energy associated with this reaction has been calculated to be -5 kcal/mol using a thermodynamic cycle. The hydride ligand and the NH proton undergo rapid intramolecular exchange with each other and intermolecular exchange with protons in solution. [HNi(PNP)(PNHP)](BF4)(2) undergoes reversible deprotonation to form [HNi(PNP)(2)](BF4) in acetonitrile solutions (pK(a) = 10.6). A convenient synthetic route to the PF6- salt of this hydride involves the reaction of PNP with Ni(COD)(2) to form Ni(PNP)(2), followed by protonation with NH4PF6. A pK(a) of value of 22.2 was measured for this hydride. This value, together with the half-wave potentials of [Ni(PNP)(2)](BF4)(2), was used to calculate homolytic and heterolytic Ni-H bond dissociation free energies of 55 and 66 kcal/mol, respectively, for [HNi(PNP)(2)](PF6). Oxidation of [HNi(PNP)(2)](PF6) has been studied by cyclic voltammetry, and the results are consistent with a rapid migration of the proton from the Ni atom of the resulting [HNi(PNP)(2)](2+) cation to the N atom to form [Ni(PNP)(PNHP)](2+). Estimates of the pK(a) values of the NiH and NH protons of these two isomers indicate that proton migration from Ni to N should be favorable by 1-2 pK(a) units. Cyclic voltammetry and proton exchange studies of [HNi(depp)(2)]-(PF6) (where depp is Et2PCH2CH2CH2PEt2) are also presented as control experiments that support the important role of the bridging N atom of the PNP ligand in the proton exchange reactions observed for the various Ni complexes containing the PNP ligand. Similarly, structural studies of [Ni(PNBuP)(2)](BF4)(2) and [Ni(PNP)(dmpm)](BF4)(2) (where PNBuP is Et2PCH2N(Bu)CH2PEt2 and dmpm is Me2PCH2Me2) illustrate the importance of tetrahedral distortions about Ni in determining the hydride acceptor ability of Ni(II) complexes.