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.