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.