Syntheses of phenoxo-bridged Zn(II) and metallamacrocyclic Hg(II) complexes of organochalcogen (Se, Te) substituted Schiff-bases: structure and DNA-binding studies of Zn(II) complexes

Abstract

The coordination of organochalcogen (especially Se and Te) substituted Schiff-bases L¹H, L²H, L³H, and L⁴H toward Zn(II) and Hg(II) has been studied. Reactions of these ligands with ZnCl₂ in 1 : 1 molar ratio gave binuclear complexes [{2-[PhX(CH₂) _n N = C(Ph)]-6-[PhCO]-4-MeC₆H₂O}₂Zn₂Cl₂] (where X = Se, n = 2 (1); X = Se, n = 3 (2); X = Te, n = 2 (3); and X = Te, n = 3 (4)) with partial hydrolytic cleavage of proligands. In these complexes, two partially hydrolyzed ligand fragments coordinate tridentate (NOO) with two Zn's. Reaction of HgBr₂ with L¹H and L²H in 1 : 1 molar ratio gave monometallic complexes [C₆H₂(4-Me)(OH)[2,6-{C(Ph) = N(CH₂) _n Se(Ph)}₂HgBr₂]] (n = 2 (5) or 3 (6)) and under similar conditions with L³H and L⁴H gave bimetallic complexes [C₆H₂(4-Me)(OH)[2,6-{C(Ph) = N(CH₂) _n Te(Ph)}₂Hg₂Br₄]] (n = 2 (7) or 3 (8)) in which the ligands coordinate with metal through selenium or tellurium, leaving the imino nitrogen and phenolic oxygen uncoordinated. The proligands L¹H, L²H give 14- or 16-membered metallamacrocycles through Se–Hg–Se linkages and L³H, L⁴H give 16- or 18-membered metallamacrocycles through Te–Hg–Br–Hg–Te linkages. All the complexes were characterized by elemental analyses, ESIMS, FTIR, multinuclear NMR, UV-Vis, and conductance measurements. The redox properties of the complexes were investigated by cyclic voltammetry (CV). Complexes 1–4 exhibited ligand-centered irreversible oxidation processes. Complexes 5 and 6 showed metal-centered quasi-reversible single electron transfer, whereas dinuclear complexes 7 and 8 displayed two quasi-reversible, one-electron transfer steps. A single-crystal X-ray structure determination of 1 showed that the coordination unit is centrosymmetric with Zn(II) in square-pyramidal coordination geometry and the two square pyramids sharing an edge. The Zn ··· Zn separation is 3.232 Å. The DNA-binding properties of 1 and 3 with calf thymus DNA were explored by a spectrophotometric method and CV.

Keywords:

• Organoselenium/tellurium
• Schiff-base
• Zn(II) and Hg(II) complexes
• X-ray structure
• Cyclic voltammetry
• DNA-binding

Acknowledgments

We are grateful to DRDO, New Delhi for the financial assistance and for the award of fellowship to AKA. We thank Prof. B.L. Khandelwal, DIMAT, Raipur and Dr J.D. Smith, University of Sussex, UK for useful discussion, and Prof. Sandeep Verma, IIT Kanpur for providing mass spectra. We also thank the reviewers for their constructive suggestions.