Mixed ligand cobalt(ii) picolinate complexes: synthesis, characterization, DNA binding and photocleavage

Abstract

Complexes of the type [Co(pic)₂(NN)], where pic = picolinate, NN = dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq) (4) and 4b,5,7,7a-tetrahydro-4b,7a-epiminomethanoimino-6H-imidazo[4,5-f][1,10]-phenanthroline-6,13-dione (bipyridyl-glycoluril) (bpg) (6) have been synthesized and characterized by elemental analysis, IR, UV-vis, NMR and ESI–MS spectroscopy and thermogravimetic analysis (TGA). Their physicochemical properties are compared with previously synthesized complexes, where NN = (H₂O)₂ (1), 2,2′-bipyridine (bpy) (2), 1,10-phenanthroline (phen) (3) and dipyrido[3,2-a:2′,3′-c]phenazine (dppz) (5). The crystal structures of the complexes 4–6 were solved by single-crystal X-ray diffraction. The complexes 4 and 5 crystallize from a mixture of chloroform and methanol in monoclinic and orthorhombic crystal systems, respectively, whereas complex 6 crystallizes from dimethyl sulfoxide (DMSO) in a tetragonal crystal system. The coordination sphere consists of two oxygen atoms and two nitrogen atoms from the two picolinates and two nitrogen atoms from the dpq, dppz or bpg ligand, respectively. Co(II)/Co(III) oxidation potentials have been determined by cyclic voltammetry. The DNA binding of complexes 1–5 has been investigated using thermal melting, fluorescence quenching and viscosity measurements, which indicate the partial intercalation of complex 5 with an apparent binding constant (k_app) of 8.3 × 10⁵ M⁻¹. DNA cleavage studies of complexes 1–5 have been investigated using gel electrophoresis in the presence of H₂O₂ as an oxidizing agent and also by photoirradiation at 365 nm. The mechanistic investigations suggest that singlet oxygen (¹O₂) is the major species involved in the DNA cleavage by these complexes. The structures of complexes 2–6 were optimized with density functional theory (DFT) method (B3LYP/6-31G(d,p)). The low vertical ionization potential values indicate photoredox pathways for the DNA cleavage activity by complexes 4 and 5, which is corroborated by DNA cleavage experiments.