Synthesis of novel aqua η4-NNNO/Cu(II) complexes as rapid and selective oxidative catalysts for O-catechol: Fluorescence, spectral, chromotropism and thermal analyses

Abstract

A new tetradentate Schiff base (SB), (E)-4-fluoro-2-(1-((2-(piperazin-1-yl)ethyl)imino)ethyl)phe-nol, was synthesized from condensation of 2-(1-piperazinyl)ethylamine and 5-fluoro-2-hydroxyacetophe-none. This ligand was coordinated with three copper(II) salts (CuCl2, CuBr2 and Cu(NO3)2·3H2 O) separately, giving rise to new neutral water-soluble Cu(II)/η4-NNNO complexes (1–3). The new materials were fully characterized by standard spectroscopic, elemental, thermal, electronic, absorption, and fluorescence analyses. The chromotropism investigation of the aqueous solutions of the complexes revealed notable outcomes. A turn off-on halochromism effect was observed, both in the acidic and basic mediums. The green-colored solution was changed to colorless (off) upon the addition of HCl, while the initial green color was reversibly restored (on) after the addition of NaOH. On the other hand, bathochromic solvatochromism shifts were noticed in various solvents. Interestingly, complex 2 displayed a remarkable blue fluorescence shift (∆λ = 90 nm) when compared to its SB ligand. The oxidation capability of the three complexes was successfully demonstrated for the conversion of o-catechol to o-benzoquinone in aqueous solutions and in the presence of H2 O2, an environmentally friendly oxidant, under mild reaction conditions.