Metal Ion (Ni^II vs Co^II)-Mediated Unusual Amine–Imine Interconversion in Conjugated Amine-ene-imine Ligand: Synthesis, Structure, and Characterization

Abstract

Two configurationally isomeric ligands, namely, 2-((E)-benzylideneamino)-3-((pyridin-2-ylmethyl)amino)maleonitrile (HL1) and 2-(benzylamino)-3-((E)-(pyridin-2-ylmethylene)amino)maleonitrile (HL2), were synthesized and fully characterized, which are malenonitrile-tethered, N atom donors tridentate ligands. Structurally, they differ in the interchangeable position of amine and imine group only. Under the same reaction condition, Ni(II) invoked the transformation of (L1)− to (L2)− via simultaneous oxidation of amine to imine and the reduction of imine to amine. Two sequential 1,5-proton transfer pathways were anticipated for this type of unusual amine-imine oxidation/reduction process under acidic medium. In contrast, Co(II) was silent to such amine-imine interconversion reactions under both HL1 and HL2 ligand environment. The variation in electronic requirement might differentiate between both ligands such that HL2 influenced the arial oxidation of Co(II) to Co(III) but HL1 could not. The redox chemistry of Co(II)/Co(III) complexes with either HL1 or HL2 was explained using cyclic voltammetry and UV–vis spectroscopy.