Biradical -iminobenzosemiquinonato(1-) complexes of nickel(ii): catalytic activity in three-component coupling of aldehydes, amines and alkynes.

The six-coordinated bis--iminosemiquinone complex, NiL , in which L is the -iminosemiquinone 1-electron oxidized form of the tridentate -aminophenol benzoxazole-based ligand HL, was synthesized and characterized. The crystal structure of the complex reveals octahedral geometry with a NiNO coordination sphere in which Ni(ii) has been surrounded by two tridentate L ligands. This compound exhibits ( = 1) with both spin and orbital contribution to the magnetic moment and antiferromagnetic coupling between two electrons on two L ligands which results in a triplet spin ground state ( = 1). The electronic transitions and the electrochemical behavior of this open-shell molecule are presented here, based on experimental observations and theoretical calculations. The electrochemical behavior of NiL was investigated by cyclic voltammetry and indicates ligand-centered redox processes. Three-component coupling of aldehydes, amines and alkynes (A-coupling) was studied in the presence of the NiL complex, and the previously reported four-coordinated bis--iminosemiquinone NiL . Furthermore, among these two -iminobenzosemiquinonato(1-) complexes of Ni(ii) (NiL and NiL ), NiL was found to be an efficient catalyst in A-coupling at 85 °C under solvent-free conditions and can be recovered and reused for several cycles with a small decrease in activity.