Mechanistic study of a reversible conversion between phosphinite-ni(0) and phosphide-ni(II) via unique metal-ligand cooperation

Abstract

Recently, metal-ligand cooperation has attracted much attention in multi electron catalytic reac-tions. Especially, non-innocent behaviors of the $\pi$ -conjugated ligands have been researched as a new method-ology to expand the role of transition metal. Most of known systems employ a noninnocent motif such as aryl-substituted bis(imino)pyridine and pyridine/acridine studied by the Chirik group as well as aromatiza-tion/dearomatization to tightly couple with the redox change of metal studied by the Milstein group which is therefore known as metal-ligand cooperation. Although various phosphine ligands have been widely utilized in organometallic chemistry, examples of a noninnocent phosphine behavior are relatively rare. Recently, the Thomas group has reported a central cationic N-heterocyclic phosphenium (NHP) ligand which has a planar three-coordinate phosphorus center. We have established the interconversion between $(PPP)Ni(II) (PPP^{-} = P[2-P^{i}Pr^{2}-C_6H_4]_2^{-})$ and $(PP^{OR}P)Ni(0)$ via P-O bond formation and cleavage. Dinitrogen species, $(PP^{OMe}P)Ni(N_2)$ and ${(PP^{OMe}P)Ni}_2(\mu -N_2)$ were synthesized from the reaction of (PPP)NiCl with a methoxy anion. Under CO(g) atmosphere, a dinitrogen ligand was substituted by a carbonyl ligand at a nickel(0) center producing a monocarbonyl nickel complex $(PP^{OMe}P)Ni(CO)$. However, by exposing $CO_2(g)$, metal-ligand cooperation occurs to generate (PPP)Ni(OCOOMe) via a P-O bond cleavage followed by a migratory methoxy group transfer. In this study, the mechanisms of a reversible conversion between phosphide-nickel(II) and phosphinite-nickel(0) via P-O bond formation and cleavage and methylcarbonate generation with two equivalent of iodomethane were explained. Dinitrogen species, $(PP${OMe}P)Ni(N_2)$ and ${(PP%{OMe}P)Ni}_2(\mu -N_2) were produced from the reaction of (PPP)NiCl with a methoxy anion generated from the solution of tetramethylammonium hydroxide in methanol. Under Ar(g) atmosphere, $(PPP)Ni(II)-OR (R = Me and ^{i}Pr)$ species were generated and converted to the corresponding $(PP^{OR}P)Ni(0)-L$ by $N_2$ and CO exposure, respec-tively. To understand the reactivity of phosphide-Ni(II) species, several reactions of $(PPP)Ni(O^{i}Pr)$ with organic azides and isocyanides were conducted.