Disproportionation Reactions of an Organometallic Ni(I) Amidate Complex: Scope and Mechanistic Investigations

The N-heterocyclic carbene supported three-coordinate Ni(I) complex (IPr)Ni(kappa(1)-N-N(Pr-i)C(O)(t) Bu)(CNXyl) ({[}2-CNXyl]; CNXyl = 2,6-dimethylphenyl isocyanide) undergoes disproportionation with 1 equiv of CNXyl to afford an equimolar mixture of Ni(0) and Ni(II) complexes (IPr)Ni(CNXyl)(3) ({[}3-CNXyl]) and (IPr)Ni(kappa(2)-N,O-N(Pr-i)CO(Bu-t))(2) ({[}4]), respectively. Complexes {[}3-CNXyl] and {[}4] are also produced when 2 equiv of the isonitrile CNXyl is added to (IPr)Ni(kappa(3)-N,bis(H2CMe)-N(Pr-i)C(O)Bu-t) ({[}1]). Furthermore, we show that reactions with pi-acceptors CO and select alkenes also result in disproportionation. In the case of CO we show that reactions in the solid state afford selectivity for and isolation of the reaction intermediate (IPr)Ni(kappa(l)-N-N(Pr-i)C(O)Bu-t)(CO) ({[}2-CO]), which is not isolable by solution methods. Kinetic analysis of the disproportionation reaction with alkenes suggests that the reaction is first order in both nickel and alkene. Preliminary trapping experiments rule out amidyl radicals as intermediates in these reactions. We propose instead that a bimolecular ligand transfer is operative, in part due to the hemilabile nature of amidate ligands.