Self-assembled synthesis, characterization and antimicrobial activity of zinc(II) salicylaldimine complexes Malgorzata T. Kaczmarek a , Renata Jastrza ˛b a , El _ zbieta Holderna-Ke ˛ dzia b , Wanda Radecka-Paryzek a, * a Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan ´, Poland b Institute of Medicinal Plants, Libelta 27, 61-707 Poznan ´, Poland article info Article history: Received 5 January 2009 Accepted 10 February 2009 Available online 20 February 2009 Keywords: Zinc(II) complexes Salicylaldimines Self-assembly Structure Speciation Antimicrobial activity abstract One-pot metal promoted reactions between salicylaldehyde and 4-methyl-1,3-phenylenediamine in the presence of metal salts acting as template agents yield zinc(II) salicylaldimine complexes containing N,N 0 -bis(salicylidene)-4-methyl-1,3-phenylenediamine (H 2 L) as a result of the [2 + 1] Schiff base conden- sation. The complexes of formula [Zn(HL)Cl(H 2 O) 2 ] Á C 2 H 5 OH and [Zn(H 2 L) 2 Cl(NO 3 )(H 2 O)] Á CH 3 OH were characterized as powder solids and in solution by spectroscopic methods (IR, 1 H and 13 C NMR, FAB- MS, ESI-MS, UV–Vis), thermogravimetric and elemental analysis, potentiometry, and tested for antimi- crobial activity against Staphylococcus aureus in a minimum inhibitory concentration (MIC) experiment. In these two powder solid species, the salicylaldimine, formed in a self-assembly process, acts in two dif- ferent coordination modes: as monodeprotonated bidentate chelator with an N,O donor set or as a neu- tral monodentate using exclusively oxygen as the donor atom without involving the nitrogen atoms in the coordination. However, crystals of these two complexes are isomorphous, with 1:2 metal:ligand stoi- chiometry, and display the latter, relatively rare coordination pattern. In solution, the presence of a 1:1 complex of monodeprotonated state is only detected. The complexes exhibit antimicrobial activity against S. aureus. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Transition metal salicylaldimines have been studied broadly due to their significance as biomimetic models for metalloproteins [1], effective catalysts for oxygenation processes and utilization in asymmetric catalysis [2–6], as building blocks for metal-containing liquid crystals and the functional self-assembled supramolecular architectures with interesting structures and properties [7–18]. Much effort has been devoted in recent years to the design and synthesis of salicylaldimines and their metal complexes displaying binding properties toward the deoxyribonucleic acid (DNA) with the aim to develop novel therapeutic agents with superoxide dis- mutase and catalase activity, which prevent the growth and repli- cation of cancerous cells [19–25]. The zinc complexes are of particular interest. They have been shown to be active as anti-tu- mor and anti-HIV agents [26,27]. Due to high Lewis acidity, nucle- ophile generation, and leaving group stabilization, zinc(II) is the most common metal ion in the active site of hydrolytic metalloen- zymes. Thus, zinc(II) complexes are very useful tools for studying the mode of catalysis of the phosphate diester cleavage in hydro- lytic processes, including DNA cleavage. Polymetallic complexes generally display higher hydrolytic activity due to the cooperative role played by the metals in the site-selective scission of DNA and RNA [28–31]. Following the idea of searching for new Schiff base complexes in view of their potential application in chemistry, bio- chemistry, medicine and technology [32–34], we report the syn- thesis, characterization and biological activity of zinc(II) salicylaldimine complexes. These complexes are formed in a self- assembly process which involves the templating action of the me- tal ion in promoting Schiff base condensation. 2. Experimental 2.1. Materials Zinc(II) chloride, gadolinium(III) nitrate, salicylaldehyde and 4- methyl-1,3-phenylenediamine were obtained from Aldrich Chemi- cal Company. 4-Methyl-1,3-phenylenediamine was purified by recrystallization from n-heptane. 2.2. Preparation of [Zn(HL)Cl(H 2 O) 2 ] Á C 2 H 2 OH To a mixture of zinc(II) chloride (48 mg, 0.35 mmol) in ethanol (10 mL) and salicylaldehyde (91 mg, 0.8 mmol) in ethanol (10 mL), 4-methyl-1,3-phenylenediamine (24 mg, 0.2 mmol) in ethanol (10 mL) was added dropwise with stirring. The reaction was car- ried out for 24 h at room temperature. The solution volume was 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.02.012 * Corresponding author. Tel.: +48 61 8291356; fax: +48 61 8291505. E-mail address: wrp@amu.edu.pl (W. Radecka-Paryzek). Inorganica Chimica Acta 362 (2009) 3127–3133 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica