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-P^<i>i</i>Pr₂-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(O^<i>i</i>Pr) (<b>4</b>) was prepared. Upon addition of a π-acidic isocyanide ligand CN^<i>t</i>Bu, a nickel(0) isocyanide species (PP^O<i>i</i>PrP)­Ni­(CN^<i>t</i>Bu) (<b>6b</b>) was generated; P–O bond formation occurred via reductive elimination (RE). When CO is present, migratory insertion (MI) occurs instead. The reaction of <b>4</b> with CO­(g) results in the formation of (PPP)­Ni­(COO^<i>i</i>Pr) (<b>5</b>), representing an alternative pathway. The corresponding RE product (PP^O<i>i</i>PrP)­Ni­(CO) (<b>6a</b>) can be independently produced from the substitution reaction of {(PP^O<i>i</i>PrP)­Ni}₂(μ-N₂) (<b>3</b>) with CO­(g). While two different carbonylation pathways in <b>4</b> 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 <b>3</b> with CO₂(g). The formation of (PPP)­Ni­(OCOO^<i>i</i>Pr) (<b>7</b>) clearly shows that an isopropoxide group migrates onto the bound CO₂ ligand, and a P–Ni moiety is regenerated