Synthesis, characterization and study on the dissimilar reactivity of a Ni(II)-bis(iminosemiquinone) complex core to ligand-appended hemilabile –CH2OH and –CH2NH2 units

Abstract

Noninnocent ligand H3LCH2OH reacted with NiCl2·6H2O under aerial atmosphere and provided corresponding complexes 1 {[NiII(HLCH2OH)2]0} and 1a {[NiII(LCHO)2]0} in the presence of Et3N and NaOH, respectively. Ligand H4LCH2NH2 provided complex 2 {[NiII(HL3]0} in the presence of NiCl2·6H2O and Et3N under air. All the complexes were characterized by various spectroscopic and spectrometric techniques, which included FT-IR, mass, UV-vis/NIR, 1H NMR, cyclic voltammetry and single crystal X-ray diffraction methods. Molecular structural analyses on complexes 1 and 1a revealed that both four-coordinate complexes were neutral in charge and consisted of two ligand-centered iminosemiquinone radicals and a Ni(II) ion. While, the hemilabile –CH2OH group remained unaffected and did not make any coordination with the central Ni(II) ion in complex 1, two-electron oxidation of each –CH2OH group to –CHO group was realized in complex 1a. In complex 2, the ligand backbone was an innocent tetradentate salen unit, which was formed by a complete modification of the initial noninnocent ligand H4LCH2NH2. To find out the mechanistic path for the formation of complex 2, ligand H3L1 and H4L2 were also introduced and it was presumed that transimination took place in the ligand backbone during complex 2 formation.