Synthesis, crystal structure and DNA hydrolysis activity of ternary (N-salicylidene-L-methioninato) copper(II) complexes of heterocyclic bases

Copper(II) complexes of different nuclearities, viz. [Cu(salmet)(bpy)](1), $[Cu(salmet)(HIm)]_2(2)$ and $[Cu(salmet)(1-MeIm)]_Y (3)$, where salmet is dianionic Schiff base N-salicylidene-L-methioninato, are prepared, structurally characterized by X-ray crystallography and their hydrolytic DNA cleavage activity studied (bpy, 2,2'-bipyridine; HIm, imidazole; 1-MeIm, 1-methylimidazole). Complex 1 has a square pyramidal (4 + 1) $CuN_3O_2$ coordination geometry in which the ONO-donor Schiff base occupies the basal plane and the bpy is bonded at the axial-equatorial sites. Complex 2 is a binuclear species in which two {Cu(salmet)(HIm)} moieties are covalently linked through two phenoxo oxygen atoms forming a ${Cu_2(m-OR)_2}$ unit with a $CuN_2O_3$ square pyramidal (4 + 1) coordination geometry for the metal. The NH hydrogen atom of the imidazole and the unbound carboxylate oxygen atom are involved in hydrogen bonding interactions to form a supramolecular structure. Complex 3 is a 1D-chain polymeric species in which the {Cu(salmet)(1-MeIm)} units are covalently linked at the elongated axial site of the $CuN_2O_3$ coordination geometry. The pendant -SMe moiety of the Schiff base does not show any interaction with the metal ion in these structures. Complexes 1-3, having salmet displaying three different binding modes, are one-electron paramagnetic per copper. The complexes show significant DNA cleavage activity in dark giving an order: 1 > 3 > 2. The hydrolytic nature of the DNA cleavage is evidenced from the control experiments showing no apparent inhibition of cleavage under argon atmosphere and in the presence of hydroxyl radical inhibitor DMSO or singlet oxygen quencher azide ion