Structural and Thermodynamic Reactivity Studies of Nickel Complexes Containing a Combination of Tridentate and Monodentate Phosphine Ligands



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Transition metal hydrides are crucial intermediates in many catalytic processes, including CO2 hydrogenation and H2 activation. These complexes can act as either a hydride or proton donors, which can be thermodynamically categorized according to their pKa or ΔG°H– parameters for acidities and hydricities, respectively. This study will elaborate on the structural and thermodynamic properties of nickel complexes containing a combination of one tridentate triphosphine (PP2) or diphosphinoamine (PNP) ligand and one monophosphine (PR3) ligand. The thermodynamic hydride donor abilities of the triphosphine complexes were determined to be 55.6 kcal/mol and 57.6 kcal/mol for [HNi(PP2)(PMe3)]+ and [HNi(PP2)(PEt3)]+, respectively, based on studies of their heterolytic activation of H2. The pKa value of the PMe3 complex was found to be approximately 20 based on proton transfer equilibria with NEt3. The value for the PEt3 complex was larger and the value for the PPh3 complex was lower, but proton-transfer equilibria for these species could not be accurately measured. The structures of the Ni(0) complexes, the Ni(II) dications, and the Ni(II) hydrides were studied by X-ray crystallography. The structures of the four-coordinate Ni(II) dications are square planar and those of the five-coordinate Ni(II) hydrides are trigonal bipyramidal with the hydride in an axial position, analogous to their previously studied Co(I) and Rh(I) analogues. The structures of the Ni(0) PMe3 and PEt3 complexes are tetrahedral, comparable to those of the previously studied PPh3 complex. For the complexes with a tridentate PNP ligand, the Ni(0) complexes were found to be unstable, and studies focused on reactions of the Ni(II) hydride species. The hydride ion in this species could be transferred to hydride acceptors, including a phosphine ligand containing an organic hydride acceptor. When no hydride acceptor was present, the hydride appeared to transfer intramolecularly to the amine in the form of a proton upon addition of a ligand.



Chemistry, Inorganic