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

Abstract

The six-coordinated bis-o-iminosemiquinone complex, NiL₂^BIS, in which L^BIS is the o-iminosemiquinone 1-electron oxidized form of the tridentate o-aminophenol benzoxazole-based ligand H₂L^BAP, was synthesized and characterized. The crystal structure of the complex reveals octahedral geometry with a NiN₄O₂ 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 NiL₂^BIS was investigated by cyclic voltammetry and indicates ligand-centered redox processes. Three-component coupling of aldehydes, amines and alkynes (A³-coupling) was studied in the presence of the NiL₂^BIS complex, and the previously reported four-coordinated bis-o-iminosemiquinone NiL₂^NIS. Furthermore, among these two o-iminobenzosemiquinonato(1−) complexes of Ni(II) (NiL₂^NIS and NiL₂^BIS), NiL₂^NIS was found to be an efficient catalyst in A³-coupling at 85 °C under solvent-free conditions and can be recovered and reused for several cycles with a small decrease in activity.