Metal-Ligand Cooperation and Migratory Insertion Induced by π-acidic Ligands at a Single Nickel Center Supported by a PPP Ligand

Abstract

Metal-ligand cooperation mediated by non-innocent ligands has become famous recently as a unique methodology for various transition metal based catalyses. Unlike well-known examples possessing a π-conjugated system as a non-innocent motif such as aryl-substituted bis(imino)pyridine and pyridine/acridine, there are relatively few studies in phosphine-based ligands. We recently reported non-innocent activities of an anionic PPP (PPP– = P[2-PiPr2-C6H4]2–) ligand with a single nickel ion revealing the forward and backward transformation between a phosphide-nickel(II) alkoxide and a phosphinite-nickel(0) species via a P-O bond formation/cleavage. In order to further expand this unusual metal-ligand cooperation, we have tried to generate a nickel(II)-alkoxide complex which can be converted to a Ni(0)-phosphinite species in the presence of a π-acidic ligand. With an isopropoxide anion, (PPP)NiOiPr was synthesized and its reactions with carbon monoxide, isocyanide were examined. Upon addition of CO(g), surprisingly a nickel(II) acyl species (PPP)NiCOOiPr was produced via CO migratory insertion. This result is unexpected when the reaction is compared with previous examples. Analogous phenoxide and phenylthiolate congeners reveal the formation of (PPEPhP)NiCO (E = O, S) from their reactions with CO(g). However, upon addition of CNtBu, P-O bond formation occurs which is coupled with two electron reduction of a nickel center resulted in the formation of a Ni(0) species (PPOiPrP)NiCNtBu.