Effect on catecholase activity and interaction with biomolecules of metal complexes containing differently tuned 5-substituted ancillary tetrazolato ligands

Four new metal complexes (one mononuclear and three dinuclear) viz. [NiL(5-phenyltetrazolato)] (1a), [Ni₂L₂{5-(2-cyanophenyl)-tetrazolato}₂] (1b), [Cu₂L₂{5-phenyltetrazolato}₂]·C₆H₅CN (2a) and [Cu₂L₂{5-(2-cyanophenyl)-tetrazolato}₂] (2b) [HL = 3-(2-dimethylamino-ethylimino)-1-phenyl-butan-1-one] have been synthesized by in-situ generation of tetrazolato ligands and characterized by X-ray crystallography, different spectroscopic techniques and elemental analyses. Introduction of one nitrile group in the 5-substituted phenyl ring makes the tetrazolato ligand comparatively weaker donor leading the complex to be more vulnerable towards dissociation facilitating subsequent substrate binding for catecholase oxidation study. Copper complex 2b has been found to be more active with respect to analogous nickel complex 1b as it provides the metal centered oxidation route rather than ligand centered oxidation in nickel complex. The comparative catalytic activities of complex 1b and 2b are further explored by DFT calculations which also support the experimental results. UV–Vis and fluorescence-based spectroscopic techniques revealed that two of the metal-tetrazolato complexes (1a and 2b) interact with double stranded DNA via intercalation and also with serum albumin protein BSA. A comparative assessment indicated that the Ni(II)-tetrazolato complex 1a displayed superior DNA binding. The interaction of these compounds with bovine serum albumin (BSA) suggested that the Cu(II)-tetrazolato complex 2b quenched the intrinsic fluorescence of BSA in a static quenching process and shows stronger binding constant.