Alkoxide Migration at a Nickel(II) Center Induced by a π-Acidic Ligand: Migratory Insertion versus Metal-Ligand Cooperation.

Two pathways of alkoxide migration occurring at a nickel(II) center supported by a PPP ligand (PPP- = P[2-PiPr2-C6H4]2-) are presented in this Article. In the first route, the addition of a π-acidic ligand to a (PPP)Ni alkoxide species reveals the formation of a P-O bond. This reaction occurs via metal-ligand cooperation (MLC) involving a 2-electron reduction at nickel. To demonstrate a P-O bond formation, a nickel(II) isopropoxide species (PPP)Ni(OiPr) (4) was prepared. Upon addition of a π-acidic isocyanide ligand CNtBu, a nickel(0) isocyanide species (PPOiPrP)Ni(CNtBu) (6b) was generated; P-O bond formation occurred via reductive elimination (RE). When CO is present, migratory insertion (MI) occurs instead. The reaction of 4 with CO(g) results in the formation of (PPP)Ni(COOiPr) (5), representing an alternative pathway. The corresponding RE product (PPOiPrP)Ni(CO) (6a) can be independently produced from the substitution reaction of {(PPOiPrP)Ni}2(μ-N2) (3) with CO(g). While two different carbonylation pathways in 4 seem feasible, C-O bond forming migratory insertion singly occurs. Regeneration of a (PPP)Ni moiety via a P-O bond cleavage was demonstrated by treating 3 with CO2(g). The formation of (PPP)Ni(OCOOiPr) (7) clearly shows that an isopropoxide group migrates onto the bound CO2 ligand, and a P-Ni moiety is regenerated.