Homoleptic complexes of cobalt(0) and nickel(0, I) with 1,1 '-bis(diphenylphosphino)ferrocene (dppf): Synthesis and characterization

Reduction of Co(dppf)Cl-2 with 2 equiv of sodium naphthalenide in THF, in the presence of dppf, affords the homoleptic complex Co(dppf)(2), 1, isolated in 65% yield as a brick red solid, extremely air sensitive. In solution, under inert atmosphere, 1 slowly decomposes into Co and dppf, following a first-order kinetic law (t(1/2) = 21 h at 22 degrees C). Similarly to the Rh and Ir congeners, 1 undergoes a one-electron reversible reduction to [Co(dppf)(2)](-). Attempts to obtain this d(10) species by chemical as well as electrochemical reduction of 1 lead to the hydride HCo(dppf)(2), 2, as the only product that can be isolated. Reduction of Ni(dppf)Cl-2 with sodium in the presence of dppf and catalytic amounts of naphthalene affords Ni(dppf)(2), 3, isolated in 60% yield as a yellow air stable solid. The stoichiometric oxidation of 3 with [FeCp2]PF6 forms the d(9) complex [Ni(dppf)(2)]PF6, 4, which represents the second example of a structurally characterized Ni(I) complex stabilized by phosphines. A single-crystal X-ray analysis shows for the metal a distorted tetrahedral environment with a dihedral angle defined by the planes containing the atoms P(1), Ni, P(2) and P(3), Ni, P(4) of 78.2 and remarkably long Ni-P bond distances (2.342(3) - 2.394(3) angstrom). The EPR spectroscopic properties of 1 (at 106 K in THF) and 4 (at 7 K in 2-methyl-THF) have been examined and g tensor values measured (1, g(x) = 2.008, g(y) = 2.182, g(z) = 2.326; 4, g(x) = 2.098, g(y) = 2.113, g(z) = 2.332). A linear dependence between the hyperfine constants and the Ni-P bond distances has been evidenced. Finally, the change with time of the EPR spectrum of 4 indicates that it very slowly releases dppf.