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 (pKa = 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 pKa 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 pKa 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 pKa 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 Me2PCH2PMe2) illustrate the importance of tetrahedral distortions about Ni in determining the hydride acceptor ability of Ni(II) complexes.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry