Homoleptic Complexes of Cobalt(0) and Nickel(0,I) with 1,1‘-Bis(diphenylphosphino)ferrocene (dppf): Synthesis and Characterization

Reduction of Co(dppf)Cl2 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 (t1/2 = 21 h at 22 °C).
Similarly to the Rh and Ir congeners, 1 undergoes a one-electron reversible reduction to [Co(dppf)2]-. Attempts to
obtain this d10 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)Cl2 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 d9 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) Å). 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, gx = 2.008, gy = 2.182, gz = 2.326; 4, gx = 2.098, gy = 2.113, gz = 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.