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

Abstract

Two pathways of alkoxide migration occurring at a nickel(II) center supported by a PPP ligand (PPP- = P[2-(PPr2)-Pr-i-C6H4](2) ) are presented in this Article. In the first route, the addition of a pi-acidic ligand to a (PPP)Ni alkoxide species reveals the formation of a PO bond. This reaction occurs via metalligand cooperation (MLC) involving a 2-electron reduction at nickel. To demonstrate a PO bond formation, a nickel(II) isopropoxide species (PPP)Ni((OPr)-Pr-i) (4) was prepared. Upon addition of a pi-acidic isocyanide ligand (CNBu)-Bu-t, a nickel(0) isocyanide species ((PPP)-P-OiPr)Ni((CNBu)-Bu-t) (6b) was generated; PO 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((COOPr)-Pr-i) (5), representing an alternative pathway. The corresponding RE product ((PPP)-P-OiPr)Ni(CO) (6a) can be independently produced from the substitution reaction of {((PPP)-P-OiPr)Ni}(2)(mu-N-2) (3) with CO(g). While two different carbonylation pathways in 4 seem feasible, CO bond forming migratory insertion singly occurs. Regeneration of a (PPP)Ni moiety via a PO bond cleavage was demonstrated by treating 3 with CO2(g). The formation of (PPP)Ni((OCOOPr)-Pr-i) (7) clearly shows that an isopropoxide group migrates onto the bound CO2 ligand, and a PNi moiety is regenerated.