Designing antiferromagnetically coupled mono-, di- and tri-bridged copper(ii)-based catecholase models by varying the ‘Auxiliary Parts’ of the ligand and anionic co-ligand

Abstract

Three Schiff base ligands were synthesized via condensation between 2,6-diformyl-4-R-phenol [where R = tert-butyl (HL¹), iso-propyl (HL²), and chloro (HL³)] and N,N-dimethylethylene-1,2-diamine to prepare a series of copper(II) complexes. [Cu₂(L¹)(OH)(ClO₄)](ClO₄) (1) and [Cu₂(L¹)(SCN)₃] (2) were synthesized from HL¹, whereas [Cu₂(L²)(SCN)₂](ClO₄) (3) and [Cu₂(L³)(N₃)(ClO₄)](ClO₄) (4) were obtained from the ligands HL² and HL³, respectively. All the complexes were characterized by the usual physico-chemical techniques (FT-IR, UV-vis, single-crystal XRD, ESI-MS, ¹H NMR and EPR) and their variable temperature magnetic study was also carried out. Single-crystal XRD analysis proved that a phenoxyl bridge was present in all four complexes. Complexes 1 and 4 contained additional bridges as hydroxo and μ_1,1-azido ligands, respectively, together with perchlorate species to generate new binuclear Cu^II complexes containing a triple-mixed bridge. Complex 2 formed a thiocyanate-bridged polymeric chain. Interestingly, a polymeric chain was also formed via the participation of perchlorate ions in the case of complex 3 although thiocyanate, a better bridging anion, was present as a pending co-ligand. The influence of the ‘auxiliary part’ of the ligands, i.e., the ‘R’ group towards the structural diversity of complexes 1–4 was evaluated with the help of a DFT study. A magnetic study revealed the fact that the copper centres were antiferromagnetically coupled in all four complexes. The catecholase activities of all four complexes were determined in a DMSO medium and complex 4 was found to be inactive. The order of the catecholase activity of the remaining three complexes was 1 < 2 ≈ 3. The trend of the catecholase activity was explained with the help of an ESI-MS study.