Effect of temperature and ligand protonation on the electronic ground state in Cu(ii) polymers having unusual secondary interactions: a magnetic and catechol oxidase study

Abstract

Two new copper(II) polymeric complexes, {[Cu(HPymat)(H₂O)](NO₃)}_n (1) and [Cu₂(Pymat)₂(H₂O)₃]_n (2), have been synthesized using the Schiff base ligand H₂Pymat [H₂Pymat = (E)-2-(1-(pyridin-2-yl)-methyleneamino)terephthalic acid]. Complex 1 is a cationic 1D polymer, whereas complex 2 is a two dimensional polymer. Both complexes were crystallographically, spectroscopically and magnetically characterized. Theoretical studies were performed and the catecholase activity of the complexes was also examined. Complex 1 is a ferromagnetically coupled complex with J = 2.8 cm⁻¹ and 2 shows antiferromagnetic coupling with J = −1.6 cm⁻¹. Both complexes show notable features in the EPR study. They show rhombic spectra at 77 K in the solid state, but by varying the temperature or solvents the nature of the spectra can be changed or inverted. This behaviour indicates a change of the ground state from d_x²−y² to d_z² orbitals. Theoretical calculations of 1 focus on the evaluation and characterization of interesting anion–π–anion assemblies that are formed in the solid state. In 2 we have analysed the unconventional chelate ring⋯chelate ring π-stacking interactions that govern its solid state architecture. Both complexes act as functional models and show catechol oxidase activity with a k_cat value of the order of 10³ h⁻¹.