Oxidative Addition of a Diphosphine Anhydride to Iron(0) and Nickel(0): A Simple Approach to Installing Four Ligands

The diphosphine anhydride (PCO)2O was prepared in high yield by DCC coupling of 2-diphenylphosphinobenzoic acid. The diphosphine oxidatively adds to Fe(0) carbonyls to give the acyl carboxylate Fe(PCO)(PCO2)(CO)2 (1). The complex adopts a distorted octahedral geometry with trans phosphines and cis carbonyl ligands. Treatment of Ni(COD)2 with (PCO)2O afforded the square-planar acyl carboxylate Ni(PCO)(PCO2) (2) with trans phosphine ligands. Upon treatment with PMe3, 1 gives the monocarbonyl Fe(PCO)(PCO2)(CO)(PMe3), whereas 2 and PMe3 react to give the pentacoordinate adduct 2PMe3. Both 1 and 2 give 1:1 adducts with B(C6F5)3 (BPhF3). Crystallographic analysis revealed that the borane binds to the noncoordinated carboxylate oxygen in both cases. The value of νCO for the carboxylato ligand decreased by 45 cm−1, and in the case of 1BPhF3, νCO for the CO ligands increased by 15 cm−1. In solution, 1BPhF3 exists as an equilibrium mixture of three isomers, including a kinetic isomer and another isomer that selectively crystallizes.