An unusual Cu(II) complex of a Schiff-base ligand of 3-aminoquinoline with a hydronated pyridine unit: Synthesis, characterizations, and computational studies

A very unusual Cu(II) compound of formula [Cu(mqmp)2(ClO4)2] was synthesized in situ from a one-pot reaction between copper(II) perchlorate and a Schiff base ligand, 2-methoxy-6-((quinolin-3-ylimino)methyl)phenol, abbreviated mqmp. The resulting compound, bearing positively charged side groups was characterized using single-crystal X-ray diffraction, elemental analysis, electrospray-ionization mass spectrometry (ESI-MS), infrared spectroscopy (IR), ligand-field (LF) spectroscopy, and powder electron paramagnetic resonance (EPR) spectroscopy. The coordination chromophore consists of tetragonal [CuN2O2O′2] units. The compound shows a step-like molecular assembly assisted by hydrogen-bond interactions from –NH+ to ClO4− between the neighbouring molecules. The presence of pyridinic nitrogens bearing a H+ (“hydron”) on the quinoline unit is quite unusual, and was validated with computational studies which clearly suggested that this hydronated nitrogen stabilizes the structure by 164 kJ mol−1 compared to the hydronated phenolic version of the structure.