Metal-ligand cooperation (MLC) has recently been considered as an important concept to expand the role of metal complexes in catalytic reactions, since redox and chemically active ligands assist transition metals to achieve catalysis for various bond-forming reactions. Several groups apply such MLC for various organometallic and catalytic reactions. Our group recently reported a unique example of MLC by employing a phosphide-based PPP ligand, (PPP– = $P[2-P^iPr_2-C_6H_4]_2^–$). Interconversion between a phosphide-Ni(II) alkoxide and a phosphinite-Ni(0) species via a reversible P–O bond formation occurs with geometric change of the central metal from a square planar to a tetrahedral geometry. In order to understand mechanistic steps of this unusual MLC process, a series of (PPP)Ni(ER) with various ER groups (E = O or S, R = aryl or alkyl) and its reactivity toward a \pi-acidic ligand such as CO and isocyanide were investigated.
Herein, we will discuss the mechanism of the MLC reaction occurring at the nickel center supported by the PPP ligand. The mechanistic insight will be provided by the reaction of para-substituted nickel(II)-phenoxide complexes having electron-withdrawing and -donating substituents, $(PPP)Ni(OC_6H_4X)$ (X = OMe, H, Me, $CF_3$), with CO. In order to maximize electron-withdrawing effect, $(PPP)Ni(OC_6F_5)$ is also applied. The nickel(II) complexes were converted to the corresponding zerovalent nickel monocarbonyl complexes in the presence of CO and reaction rates were obtained from monitoring the reaction by UV-Vis spectroscopy. The resulting kinetic data showed clean pseudo first-order fits over 3 half-lives and indicates the rate of phenoxide group transfer increases as the electron-withdrawing ability increases. The increased reactivity of the complex with electron-withdrawing group can be explained by a structural analysis and a DFT investigation. In order to investigate the reaction mechanism, the reactions of (PPP)Ni(II) complexes with phenoxy radical were explored and transition state calculations were conducted.