Enhanced catalytic activity towards one-pot hydroxylation of phenol with hydrogen peroxide by nickel(II) complex encompassing 3-formylchromone-S-methylisothiosemicarbazone derivatives

Abstract

The new template reaction between 3-formylchromone-S-methylisothiosemicarbazone and substituted 2-hydroxy benzaldehydes in presence of Ni(OAc)2·4H2O salts afforded four new complexes (1–4) with general molecular formula [NiL1–4] (L1=(N1-3-formylchromone)(N2-(2-hydroxybenzylidene)-S-methylisothiosemicarbazone, L2=(N1-3-formylchromone)(N2-(2-hydroxy-1-naphthalene)-S-methylisothiosemicarbazone, L3=(N1-3-formylchromone)(N2-(5-chloro-2-hydroxybenzylidene)-S-methylisothiosemicarbazone, L4=(N1-3-formylchromone)(N2-(2-hydroxy-3-methoxybenzylidene)-S-methylisothiosemicarbazone). The compounds are remarkably stable and were obtained in good yields. Structural elucidation was carried out thoroughly in solid and solution-states by elemental analysis and various spectroscopy techniques (IR, UV–Vis, 1H NMR) as well as ESI-mass spectrometry. The molecular structure of the complex 1 was investigated by single-crystal X-ray diffraction. The analysis showed tetradentate coordination of the complexes 1–4, in which the metal atom adopts a square planar geometry with the ligand coordinating ONNO tetradentate fashion and the complex 1 [NiL1] units are forming dimerized chains through Ni(1)⋯Ni(2) interactions (3.408Å). The new complexes were examined for their catalytic performances in one-pot hydroxylation of phenol in presence of H2O2 into catechol and hydroquinone.