Synthesis, Structure and Reactivity of Cyclometalated Nickel(II) Complexes: A Review and Perspective

The importance of cyclometalated Ni(II) complex species assumed in catalytic processes related to C-X or C-H activation and functionalisation e.g. in C-C cross coupling reactions stands in stark contrast to the small number of investigations on the synthesis, reactivity, electrochemical and spectroscopic properties of cyclometalated complexes to model catalytically relevant species. This review reveals that the methods to synthesise cyclometalated Ni(II) complexes are rather weakly developed compared with corresponding Pd or Pt derivatives. Furthermore, cyclometalated ligands with their strongly sigma-donating carbanionic functions, their chelate effect and often good pi-accepting properties are also interesting from the general viewpoint of organometallic coordination chemistry and applications beyond catalysis. The recently reported cyclometalated complexes [Ni(Phbpy)(R)] with the carbanionic Phbpy(-) = 6-(phenyl-2-ide)-2,2'-bipyridine ligand (R = Br, CH3, CF3) were described in detail to demonstrate some of the specific properties of such cyclometalated complexes. Further ligand design might allow to synthesise cyclometalated Ni(II) complexes with very interesting properties such as efficient triplet luminescence. What has been achieved in this field with Pt(II) and Pd(II) might be transferred to Ni(II) in the future and some ideas are presented.