Three neutral Cu(II)/ƞ4-NNNO Schiff base complexes (1-3) were prepared from the (E)-4-nitro-2-(((2-(piperazin-1-yl)ethyl)imino)methyl)phenol, tetradentate Schiff base (SB) ligand, and the corresponding copper(II) salts. The new SB and its complexes were fully characterized by CHN-EA, standard spectroscopic, thermal, and fluorescence analyses. The formation of the complexes was monitored by EDX, FT-IR, and UV-Vis. The chromotropism studies of the complexes reflected remarkable findings, in which bathochromic solvato- and thermochromism shifts were detected. The turn-off-on halochromism phenomena were observed in the acidic and basic medium. On the other hand, the fluorescence of the free SB ligand was turned off via complexation to the Cu(II) center. In the presence of H2O2 as green oxidant and under mild oxidation catalytic condition, the three complexes successfully catalyzed the formation benzaldehyde from benzyl alcohol.

A new tetradentate Schiff base (SB), (E)-4-fluoro-2-(1-((2-(piperazin-1-yl)ethyl)imino)ethyl)phenol, was synthesized from condensation of 2-(1-piperazinyl)ethylamine and 5-fluoro-2-hydroxyacetophenone. This ligand was coordinated with three copper(II) salts (CuCl2, CuBr2 and Cu(NO3)2⸱3H2O) separately, giving rise to new neutral water-soluble Cu(II)/ƞ4-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 (Δλ = 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 H2O2, an environmentally friendly oxidant, under mild reaction conditions.