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

The six-coordinated bis- o -iminosemiquinone complex, NiL 2 BIS , in which L BIS is the o -iminosemiquinone 1-electron oxidized form of the tridentate o -aminophenol benzoxazole-based ligand H 2 L BAP , was synthesized and characterized. The crystal structure of the complex reveals octahedral geometry with a NiN 4 O 2 coordination sphere in which Ni(ii) has been surrounded by two tridentate L BIS ligands. This compound exhibits ( S Ni = 1) with both spin and orbital contribution to the magnetic moment and antiferromagnetic coupling between two electrons on two L BIS ligands which results in a triplet spin ground state ( S = 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 2 BIS was investigated by cyclic voltammetry and indicates ligand-centered redox processes. Three-component coupling of aldehydes, amines and alkynes (A 3 -coupling) was studied in the presence of the NiL 2 BIS complex, and the previously reported four-coordinated bis- o -iminosemiquinone NiL 2 NIS . Furthermore, among these two o -iminobenzosemiquinonato(1-) complexes of Ni(ii) (NiL 2 NIS and NiL 2 BIS ), NiL 2 NIS was found to be an efficient catalyst in A 3 -coupling at 85 °C under solvent-free conditions and can be recovered and reused for several cycles with a small decrease in activity.