Biradicalo-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,NiL2BIS, in whichLBISis theo-iminosemiquinone 1-electron oxidized form of the tridentateo-aminophenol benzoxazole-based ligandH2LBAP, was synthesized and characterized. The crystal structure of the complex reveals octahedral geometry with a NiN4O2coordination sphere in which Ni(ii) has been surrounded by two tridentateLBISligands. This compound exhibits (SNi= 1) with both spin and orbital contribution to the magnetic moment and antiferromagnetic coupling between two electrons on twoLBISligands 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 ofNiL2BISwas 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 theNiL2BIScomplex, and the previously reported four-coordinated bis-o-iminosemiquinoneNiL2NIS. Furthermore, among these twoo-iminobenzosemiquinonato(1−) complexes of Ni(ii) (NiL2NISandNiL2BIS),NiL2NISwas 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.