Crystal structure, thermal, redox, and EPR studies of copper(II) complexes with catechol-based quaternary ammonium salts

Quaternary ammonium salts (QASs), acylhydrazones and catechols are well known for their antimicrobial, antioxidant and antitumor properties. Complexation of acylhydrazones and catechols with Cu(II) ions potentially leads to obtaining compounds with promising biological properties. In this paper, we present a synthesis of two binuclear copper(II) complexes based on redox-active QASs with acylhydrazone and catechol fragments and a study of their structure in the solid state. The copper(II) complex with a tributylammonium fragment crystallizes in the triclinic space group  with the following lattice parameters: a = 10.1459(5) Å, b = 15.1239(7) Å, c = 16.3279(5) Å, α = 110.231(4)°, β = 104.689(4)°,γ = 101.637(4)°. The structure exhibits binuclear nature, showing the distance Cu–Cu of 3.507 Å, and Jahn-Teller distortions with large elongation along the axial positions. The thermal stability of the ligands was found to be higher than that of the corresponding complexes up to 66–77 °C. EPR spectroscopy proved the dissociation of the complexes in the methanol solution into the monomeric forms. Complexation with Cu(II) cation shifted oxidation peaks of the ligands towards the lower potentials. The target compounds may be applied to create selective systems for metal ion separation, and may also possess potential antimicrobial, antioxidant, and antitumor properties.