Synthesis and characterization of a new series of unsymmetrical macrocyclic binuclear vanadyl(IV) complexes: Electrochemical, antimicrobial, DNA binding and cleavage studies L. Leelavathy a , S. Anbu a , M. Kandaswamy a, * , N. Karthikeyan b , N. Mohan b a Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai 600 025, India b CAS in Botany, University of Madras, Guindy Campus, Chennai 600 025, India article info Article history: Received 20 November 2008 Accepted 31 December 2008 Available online 21 February 2009 Keywords: Electrochemistry DNA binding and cleavage properties Macrocyclic oxovanadium(IV) complexes abstract A new series of unsymmetrical macrocyclic binuclear bis-phenoxo bridged oxidovanadium(IV) com- plexes have been synthesized and characterized by elemental and spectral techniques. A cyclic voltam- metric investigation of these binuclear V IV O complexes evidenced that two successive quasi-reversible one electron transfer reduction waves (E 1 pc = 0.54 to 0.67 V, E 2 pc = 0.80 to 0.85 V) are obtained. In the positive potential region (+0.50 to +1.00 V) two quasi-reversible oxidation couples are observed for all the complexes. The first one electron oxidation is observed around (E 1 pc ) +0.63 to +0.78 V and the second around (E 2 pc ) +0.82 to +0.97 V. The ESR spectra of all the binuclear V IV O complexes showed a single broad-band resonance at ca. g = 1.97–2.11 with the half field signal at 1500G (M = ±2), which sug- gest magnetic interactions between two V IV O ions through the phenolate bridge. DNA binding experi- ments show that the aromatic diimine containing macrocyclic V IV O complexes display better DNA interactions than the aliphatic diimine containing V IV O analogues. These complexes significantly promote the oxidative cleavage of supercoiled plasmid DNA under physiological conditions in the presence of H 2 O 2 . All the complexes show noticeable growth inhibition of some plant pathogenic fungal species and human pathogenic bacterial species. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Transition-metal complexes with their varied coordination envi- ronments and versatile redox and spectral properties offer im- mense scope for designing species that are suitable to bind and cleave the DNA. Metal ions, and thus vanadium ions, can play a role in biology as counterions for protein, DNA, RNA and in various bio- logical organelles. The structural role is often manifested by the maintenance of various biological structures, whereas a functional role brings key reactivity to a reaction center for a protein [1]. Addi- tionally, vanadium is a biocompatible metal ion used earlier in insu- lin mimetics and antitumor agents [2]. Bleomycin vanadyl(IV) and [VO(phen)(H 2 O) 2 ] 2+ show ‘‘chemical nuclease” activity in the pres- ence of H 2 O 2 [3]. The vanadyl–H 2 O 2 interaction is of interest in the context of the insulin enhancing properties and chemo-preventive carcinogenesis [4]. The cationic form of vanadium complexes with the oxidation state (IV) have been shown to function as modulators of cellular redox potentials, regulate cellular phosphorylation events and exert pleiotropic effects in multiple biological systems [5–8]. Besides the ability of the vanadium metal to assume various oxidation states, its coordination chemistry also plays a key role in its interactions with various biomolecules [4]. In macrocyclic sys- tems, the redox and magnetic exchange properties of the metal cen- ters are affected by the ring size, coordination sites and electronic effect of the ligand system. Hence, the synthesis and studies of model systems are important and give more insight about the coop- erative phenomena, electron transfer and magnetic interaction be- tween the metal centers [9]. Symmetrical binuclear phenoxo bridged macrocyclic complexes have been synthesized and their electrochemical and magnetic properties studied in detail [10]. However, studies on unsymmetrical binuclear macrocyclic com- plexes are sparse [11,12]. Several efficient DNA cleaving agents have been developed over the course of time, however most of the studies deal with mononuclear VO(IV) complexes [13]. Re- cently, we have reported that macrocyclic binuclear Cu(II) com- plexes shows remarkable DNA binding and cleavage activity [14,15]. It is remarkable to notice that the macrocyclic binuclear transition metal complexes show greater cleaving efficiency or DNA interaction than their mononuclear analogues [16,17]. The efficient DNA binding and cleavage ability of the binuclear com- plexes may be due to a synergy between the metal centers and the presence of the 4-methyl phenol group in the macrocyclic ring [15]. This work focuses on the synthesis of binuclear V IV O 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.12.062 * Corresponding author. Tel./fax: +91 44 22300488. E-mail address: mkands@yahoo.com (M. Kandaswamy). Polyhedron 28 (2009) 903–910 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly