Abstract
Nickel is one of the first-row transition metals used as alternatives in homogeneous catalysts using precious metals for organic transformations. Nickel(0) and (II) complexes are generally involved as intermediates in catalytic cross-coupling reactions. However, a handful of recent studies showed that mechanisms can exist involving other oxidation states of nickel, such as nickel(I) and (III). Different pathways could provide different selectivity in products and/or starting materials. However, it is still not clear how these reactions proceed, so it is not possible to predict the reaction pathways from the structure of nickel complexes and/or reaction conditions. Unfortunately, it is not easy to clarify the true active species, such as “paramagnetic” monovalent and trivalent nickel species, so elucidation of the reaction mechanism needs various spectroscopic and/or theoretical techniques. In this account, the possibility of nickel(I) and/or nickel(III) active species involving in catalysis is discussed, based on our recent results on the experimental and theoretical mechanistic studies on N-heterocyclic carbene(NHC)-ligated nickel system. The characteristics and future perspectives of cross-coupling reactions catalyzed by the NHC-nickel(I) complexes are also described.
