p-Toluate-bridged dinuclear Cu(II) complexes in combination with tridentate chelating ligand: Crystal structure, density functional theory calculation, DNA/protein binding and catecholase activity

The dinuclear Cu(II) complexes [Cu2(L1)2(mb)]⋅ClO4 (1) and [Cu2(L2)2(mb)]⋅ClO4 (2) (HL1 = 2-[(2-diethylaminoethylimino)methyl]phenol; HL2 = 2-[1-(2-diethylaminoethylimino)propyl]phenol; mb = 4-methylbenzoate) were synthesized and characterized using X-ray crystal structure analysis and spectroscopic methods. Complexes 1 and 2 are dinuclear with distorted square pyramidal Cu (II) geometries, where Schiff base coordinates with tridentate (N,N,O) chelating mode and mb bridges two metal centres. Optimized structures and photophysical properties of ligands and complexes were calculated using density functional theory and time-dependent density functional theory methods using B3LYP functional with 6-31G (d,p) and LanL2MB basis sets. Interactions of the complexes with bovine serum albumin (BSA) and human serum albumin (HSA) were studied using UV–visible absorption and fluorescence spectroscopies and the calculated values of association constants (M−1) are 1.7 × 105 (1–BSA), 5.7 × 105 (2–BSA), 1.6 × 105 (1–HSA) and 6.9 × 105 (2–HSA). Interactions of the complexes with calf thymus DNA were also investigated and the binding affinities are 1.4 × 105 and 1.6 × 105 M−1 for 1 and 2, respectively. Both complexes catalytically oxidize 3,5-di-tert-butylcatechol to 3,5-di-tert-butylbenzoquinone in the presence of molecular oxygen.