Nickel(II) complexes of bidentate N-heterocyclic carbene/phosphine ligands: Efficient catalysts for suzuki coupling of aryl chlorides

Nickel(II) complexes of bidentate N-heterocyclic carbene (NHC)/phosphane ligand L were prepared and structurally characterized. Unlike palladium, which forms [PdCl₂(L)], the stable nickel product isolated is the ionic [Ni(L)₂]Cl₂. These Ni^II complexes are highly robust in air. Among different N-substituents on the ligand framework, the nickel complex of ligand L bearing N-1-naphthylmethyl groups (2 a) is a highly effective catalyst for Suzuki cross-coupling between phenylboronic acid and a range of aryl halides, including unreactive aryl chlorides. The activities of 2a are largely superior to those of other reported nickel NHC complexes and their palladium counterparts. Unlike the previously reported [NiCl₂(dppe)] (dppe = 1,2-bis(diphenylphosphino)ethane), 2a can effectively catalyze the cross-coupling reaction without the need for a catalytic amount of PPh ₃, and this suggests that the PPh₂ functionality of hybrid NHC ligand L can partially take on the role of free PPh₃. However, for unreactive aryl chlorides at low catalyst loading, the presence of PPh ₃, accelerates the reaction.