Noninnocent Behavior of Ancillary Ligands: Apparent Trans Coupling of a Saturated N-Heterocyclic Carbene Unit with an Ethyl Ligand Mediated by Nickel

Oxidative addition of the tridentate N-heterocyclic carbene (NHC) diphosphine ligand precursor ([PCP]H)PF6 (1) {[PCP] = o- i Pr2PC6H4(NC3H4N)o-C6H4P i Pr2} to Ni(COD)2 results in the formation of the nickel(II) hydride complex ([PCP]NiH)PF6 (2). This hydride undergoes a rapid reaction with ethylene to generate a nickel(0) complex in which an ethyl group has been transferred to the carbene carbon of the original NHC−diphosphine ligand. If the first intermediate is the anticipated square-planar nickel(II) ethyl species, then the formation of the product would require a process that involves a trans C−C coupling of the NHC carbon and a presumed Ni−ethyl intermediate. Deuterium-labeling studies provide evidence for migratory insertion of the added ethylene into the Ni−H bond rather than into the Ni−carbene linkage; this is based on the observed deuterium scrambling, which requires reversible β-elimination, alkene rotation, and hydride readdition. However, density functional theory studies suggest that a key intermediate is an agostic ethyl species that has the Ni−C bond cis to the NHC unit. A possible transition state containing two cis-disposed carbon moieties was also identified. Such a process represents a new pathway for catalyst deactivation involving NHC-based metal complexes.