Isomerism and nuclearity control in bis(lawsonato)zinc(II) complexes Francisco L.S. Bustamante a , Marcos M.P. Silva a , Wagner A. Alves b , Carlos B. Pinheiro c , Jackson A.L.C. Resende a , Mauricio Lanznaster a,⇑ a Instituto de Química, Universidade Federal Fluminense, Outeiro S. João Batista S/N, 24020-141 Niterói-RJ, Brazil b Departamento de Química, Universidade Federal Rural do Rio de Janeiro, Rodovia BR-465, Km 7, 23890-000 Seropédica-RJ, Brazil c Departamento de Física, Universidade Federal de Minas Gerais, Av. Antonio Carlos 6627, Pampulha, 31270-901 Belo Horizonte-MG, Brazil article info Article history: Received 19 February 2012 Accepted 25 April 2012 Available online 3 May 2012 Keywords: Zinc(II) complexes Lawsone Cyclic voltammetry Isomerism and nuclearity control Spectrocopy X-ray crystallography abstract This work aims to provide new insights concerning isomerism and nuclearity control in lawsone complexation reactions, as well as exploit physical and chemical properties related to different structural features. Three new bis(lawsonato)zinc(II) complexes, cis,cis,cis-[ZnL 2 (H 2 O) 2 ]2H 2 O(1), trans,trans,trans- [ZnL 2 (H 2 O) 2 ](2) and [Zn 2 L 4 (py) 2 ]2H 2 O(3), were synthesized and characterized by X-ray crystallography, thermogravimetric analysis, 1 H NMR, IR and UV–Vis spectroscopies and cyclic voltammetry. It has been demonstrated that, for the reaction between zinc(II) and lawsone, the coordination geometry and nucle- arity of the product can be directed by modifying the addition sequence of the reactants and the nature of the base (pyridine versus triethylamine). Complex 1 has a zinc(II) ion coordinated by two lawsonate anions and two water molecules in a cis configuration, in a distorted octahedral environment. On the other hand, the structure of 2 differs from 1 mainly by the trans conformation of the ligands. Complex 3 has a dinuclear structure with an inversion center lying between the two metal centers, where each one of the two zinc atoms is coordinated by two lawsonate anions in a cis configuration and by one pyr- idine molecule. When dissolved in dmso, however, all data indicate that complexes 1–3 are converted to one single species, [ZnL 2 (dmso) 2 ]. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The pharmacological properties of naturally occurring 1,2- and 1,4-naphthoquinone nuclei and their derivatives have been a subject of increasing interest [1]. Such molecules present a large variety of activities including bactericide [2–4], fungicide [5], anti-viral [6,7], antitumoral [8–10], trypanocidal [11,12] and antimalarial [13,14] properties, which are usually related to their capability of being reduced by one electron to form a semiqui- none radical [15]. In biological systems, reduction mediated by reductases generates reactive oxygen species (ROS) that cause DNA damage, oxidative stress and apoptosis among other harmful effects [16]. Modulation of the reactivity by changing the chemical environment of the naphthoquinones (NQs) has been attempted, in order to improve their pharmacological activities and reduce side effects [17–19]. It is known that the coordination of NQs to metal ions is a way of modifying their redox potential and stabilizing the semiquinone intermediate [20]. Increase on the cytotoxicity of 2-hydroxy-1,4-NQ (lawsone) derivatives upon coordination has being reported in studies of copper complexes with buparvaquone [21] and azo derivatives of lawsone [22–24]. Surprisingly, the coordination chemistry of lawsone (Fig. 1) has not been fairly exploited. Special attention was given to [Fe(lawsonato) 2 (H 2 O) 2 ], since this complex has been firstly described as a mimetic model for the photosynthetic bacterial reaction center [25–27], and later submitted to biological tests [28]. Recently, Rane and co-workers have evaluated spectroscopic and magnetic properties of [Cu(lawsonato) 2 (H 2 O) 2 ] [29]. Other spectroscopic and electrochemical studies on compounds of for- mula [M(lawsonato) 2 (py) 2 ], M = Co(II), Ni(II), Zn(II) [20] have been reported by Frontana and co-workers. In order to improve the knowledge about the coordination chemistry of lawsone and its derivatives, as well as the effects of coordination on the biological activities, we report here an investigation regarding synthesis and structural, electrochemical and spectroscopic characterization of the complexes cis,cis,cis-[ZnL 2 (H 2 O) 2 ]2H 2 O(1), trans,trans,trans- [ZnL 2 (H 2 O) 2 ](2) and [Zn 2 L 4 (py) 2 ]2H 2 O(3) (L = lawsonate). To the best of our knowledge, the geometric isomerism found for [ZnL 2 (H 2 O) 2 ] is an unprecedented occurrence concerning 2- hydroxy-1,4-NQ complexes. Therefore, this work aims to provide new insights concerning isomerism and nuclearity control in law- sonate complexation reactions, as well as to exploit physical and chemical properties related to different structural features. 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.04.027 ⇑ Corresponding author. Tel.: +55 2126292224. E-mail address: ml@id.uff.br (M. Lanznaster). Polyhedron 42 (2012) 43–49 Contents lists available at SciVerse ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly