Mechanistic Study on C-C Bond Formation of a Nickel(I) Monocarbonyl Species with Alkyl Iodides: Experimental and Computational Investigations

An open-shell reaction of the nickel(I) carbonyl species (PNP)Ni-CO (1) with iodoalkanes has been explored experimentally and theoretically. The initial iodine radical abstraction by a nickel(I) carbonyl species was suggested to produce (PNP)Ni-I (4) and the concomitant alkyl radical, according to a series of experimental indications involving stoichiometric controls employing iodoalkanes. Corresponding alkyl radical generation was also confirmed by radical trapping experiments using Gomberg's dimer. Molecular modeling supports that the nickel acyl species (PNP)Ni-COCH₃ (2) can be formed by a direct C-C bond formation between a carbonyl ligand of 1 and a methyl radical. As an alternative pathway, the five-coordinate intermediate species (PNP)Ni(CO)(CH₃) (5) that involves both CO and CH₃ binding at a nickel(II) center is also suggested with a comparable activation barrier, although this pathway energetically favors the formation of (PNP)Ni-CH₃ (3) via a barrierless elimination of CO over a CO migratory insertion. Thus, our present work supports that the direct C-C bond coupling occurs between an alkyl radical and the carbonyl ligand at a monovalent nickel center in the generation of an acyl product.