Tetranuclear Schiff base copper(II) complexes: Syntheses, crystal structure, DNA/protein binding and catecholase-like activity

Two tetranuclear Schiff base copper(II) complexes, namely [Cu 4 (L) 2 (LH) 2 (H 2 O) 2 ](ClO 4 ) 2 ·3H 2 O (1) and [Cu 4 (L) 2 (LH) 2 (H 2 O) 2 ](ClO 4 )·(tp) 0.5 ·3H 2 O (2) (where H 2 L = 2-[(2-hydroxy-ethylimino)-methyl]-6-methoxy-phenol, tp = terephthalate), were synthesized and characterized by X-ray single crystal diffraction. Both complexes 1 and 2 are comprised of structurally similar tetranuclear cationic [Cu 4 (L) 2 (LH) 2 (H 2 O) 2 ] ²⁺ species with a double open cubane core, in which two metal centers possess a square pyramidal environment and the other two exhibit a slightly distorted octahedral coordination geometry. The interaction of complexes 1 and 2 with calf thymus DNA (CT-DNA) was investigated using electronic absorption and fluorescence studies, and the results showed that the complexes interact with CT-DNA with the related intrinsic binding constants (K ib ) of 1.8 × 10 ⁶ and 1.1 × 10 ⁷ L mol ⁻¹ for 1 and 2, respectively. Their interactions with bovine serum albumin (BSA) and human serum albumin (HSA) were also investigated and spectroscopic techniques showed that both complexes interact with these proteins through a ground state association process. Using 3,5-di-tertbutylcatechol (3,5-DTBC) as a model substrate, both complexes show catecholase-like activity, being able to oxidase 3,5-DTBC to 3,5-di-tert-butylquinone (3,5-DTBQ) in the presence of oxygen.