Synthesis, crystal structure and photo-induced DNA cleavage activity of ternary copper(II) complexes of NSO-donor Schiff bases and NN-donor heterocyclic ligands

New ternary copper(II) complexes [CuLnB](ClO4) (1–3), where HLn is the NSO donor Schiff base derived from the condensation of 2-mercaptoethylamine hydrochloride with salicylaldehyde (HL1) or 2-hydroxy-3-methoxybenzaldehyde (HL2) and B is NN-donor heterocyclic base like 2,2′-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2) or 2,9-dimethyl-1,10-phenanthroline (dmp, 3), are prepared, structurally characterized by X-ray crystallography and their DNA cleavage activity studied. The complexes show distorted square-pyramidal (4 + 1) CuN3OS coordination geometry in which the NSO-donor Schiff base is bonded at the basal plane and the NN-donor heterocyclic base displays axial-equatorial mode of bonding [Cu–S distance: ∼2.4 Å]. The one-electron paramagnetic (μ eff = ∼1.9 μ B) complexes display axial EPR spectra in DMF glass at 77 K giving g∥ = ∼2.2 (A∥ = 162 G) and g⊥ = ∼2.0, indicating {dx2-y2}1{dx2-y2}1 ground state. The complexes exhibit visible spectral d–d band in MeCN near 650 nm and two charge transfer bands near 400 nm. Complexes 1 and 2 display quasireversible cyclic voltammetric response in DMF–Tris buffer (1:4 v/v, pH 7.2) for the Cu(II)/Cu(I) couple at ca. −0.1 V vs. SCE. Complex 3 exhibits an irreversible reduction process forming [CuI(dmp)2]+. Binding of 1–3 to calf thymus DNA shows the relative order: 2 (phen) ⩾ 3 (dmp) > 1 (bpy). Complex 2 efficiently cleaves supercoiled pUC19 DNA in the presence of mercaptopropionic acid (MPA) forming hydroxyl radical or on irradiation with light of 312, 532 and 632.8 nm wavelength in a type-II process. Complexes 1 and 3 are cleavage inactive.