Rare [(NHC)₂Ni-OH]-Type Terminal Nickel Hydroxo and [(NHC)₂Ni]-Type Complexes of N/O-Functionalized N-Heterocyclic Carbenes as Precatalysts for Highly Desirable Base-Free Michael Reactions in Air at Ambient Temperature

Highly convenient base-free Michael reaction of β-dicarbonyl, β-keto ester, and α-cyano ester compounds with activated ethylene compounds in air at ambient temperature catalyzed by a series of new nickel complexes, 1b−4b, of N/O-functionalized N-heterocyclic carbenes (NHC) are reported. Quite interestingly, of these nickel complexes, 1b and 2b were found to be of the [(NHC)₂Ni-OH] type bearing a terminal [Ni-OH] moiety and are among the few known structurally characterized examples of monomeric terminal [Ni-OH] complexes. Compounds 3b and 4b, on the other hand, were found to be of the commonly encountered [(NHC)₂Ni]-type complexes. More significantly, both the [(NHC)₂Ni-OH]-type complexes 1b and 2b and the [(NHC)₂Ni]-type complexes 3b and 4b were synthesized using same synthetic methodology by sheer modulation of the N-substituents on the N-heterocyclic carbene ring by the reaction of the imidazolium chloride salts with NiCl₂·6H₂O in the presence of K₂CO₃ as a base.