Coordination Behavior of 3-Ethoxycarbonyltetronic Acid towards Cu(II) and Co(II) Metal Ions

Hindawi Publishing Corporation Bioinorganic Chemistry and Applications Volume 2008, Article ID 547915, 6 pages doi:10.1155/2008/547915 Research Article Coordination Behavior of 3-Ethoxycarbonyltetronic Acid towards Cu(II) and Co(II) Metal Ions Giorgos Athanasellis,1 Georgia Zahariou,2 Stefanos Kikionis,1 Olga Igglessi-Markopoulou,1 and John Markopoulos3 1 Laboratory of Organic Chemistry, School of Chemical Engineering, National Technical University of Athens, Zografou Campus, 15773 Athens, Greece 2 Institute of Materials Science, NCSR ‘Demokritos’, 15310 Aghia Paraskevi Attikis, Greece 3 Laboratory of Inorganic Chemistry, Department of Chemistry, University of Athens, Panepistimiopolis, 15771 Athens, Greece Correspondence should be addressed to John Markopoulos, Received 10 October 2008; Accepted 5 November 2008 Recommended by Elena Milaeva Tetronic acids, 4-hydroxy-5H-furan-2-ones, constitute a class of heterocyclic compounds with potent biological and pharmacological activity. The β, β -tricarbonyl moiety plays an integral role in biological systems and forms a variety of metal complexes. In this report, we present the complexation reactions of 3-ethoxycarbonyl tetronic acids with acetates and chlorides of Cu(II) and Co(II). These complexes have been studied by means of EPR spectroscopy and magnetic susceptibility measurements. From the obtained results, a preliminary complexation mode of the ligand is proposed. Copyright © 2008 Giorgos Athanasellis et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION The chemistry of tetronic acids is a field of continuing interest. The appreciable number of tetronic acids found in nature [1, 2] and their very promising biological activities [3–5] prompted many research groups to attempt new methods for the synthesis of this class of heterocyclic compounds. Tetronic acids and their derivatives are present in a large number of natural products which exhibit a variety of biological and pharmacological properties. This class of heterocyclic compounds includes agglomerins A-D, ylidene tetronic acids [6], and the ATP-ase gastric inhibitors A88696C and A88696F [7], as well as aspertetronins and gregatins isolated from fungi which exhibit antibacterial and antifungal activities [8, 9]. The recent literature gives us a few examples of tetronic acids, both those isolated from nature and those synthesized in the laboratory. Such compounds are the CCK-B receptor antagonist tetronothiodin [10], the marine furanosesterpene natural product (18S)-variabilin [11], and the antibiotic abyssomicin C [12–14]. The coordination chemistry of tetronic acids has been investigated by many research groups in the past. Studies of complexes of oximidobenzotetronic acid complexed with Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and U(VI) by conductometric and by pH-metric titrations revealed that the metals form 1:2 (metal:ligand) complexes with the exception of Fe(II) and Co(II) complexes which form 1:3 ratios [15]. Processes for the synthesis of a Pt(II) complex with a 3-acetyl tetronic acid [16] and Pd(II) complexes involving tetronic acid derivatives [17] have been reported. Furthermore, a series of 3-acyl tetronic acids and their Cu(II) complexes, which possess a tricarbonylmethane structure, were prepared and tested for antimicrobial activity [18]. In addition, 3-(1-iminoalkyl) tetronic acids and their Cu(II) complexes were prepared and tested for inhibitory activity towards chlorophyll development of plants [18]. Finally, Xray crystallographic studies of the complexes of a dinuclear nitrogen bridged tetronic acid with Cu(II) and Ni(II) [19] showed that Cu(II) coordinates by means of two nitrogen and two oxygen atoms of the ligand and one water molecule on the top of a tetragonal pyramid. In contrast, the Ni(II) complex, having an extra water molecule, forms a nearly regular octahedron structure. In the course of our research program on the synthesis of five membered heterocyclic compounds, we have developed 2 Bioinorganic Chemistry and Applications O 2.1.2. [Cu(ETA)(OAc)] (1) (Ac = acetyl) HO OEt 4 3 5 2 O O HETA Scheme 1: 3-Ethoxycarbonyltetronic acid. a new advantageous methodology for the synthetic approach of functionalized tetronic [20] and thiotetronic acids [21]. The common feature of these heterocycles is the β,β tricarbonyl system which provides them with sites available for metal complexation. Based upon the observation that tetramic acid analogues with metal ions show increased biological activities [22, 23], we have investigated the complexation of tetramic acids with several metal ions [24–29]. In this paper, we examined the complexation of 3ethoxycarbonyltetronic acid (HETA) (Scheme 1) with Cu(II) and Co(II) ions. We report herein our results based on the data collected after EPR spectroscopy and magnetic susceptibility measurements, and using these data, we propose structures for these complexes. 2. EXPERIMENTAL A methanolic solution (12 mL) of the ligand (2.5 mmol) was added to a refluxing methanol solution (30 mL) of Cu(OAc)2 ·H2 O (2.5 mmol). The resulting solution was refluxed for 2 hours. The solution was evaporated to a small volume and the deposited precipitate was collected by filtration, washed with cold methanol, diethylether and dried in vacuo over P2 O5 . Powder (0.53 g, 72%), μeff 2.08 μB ; (Anal. Found: C, 36.82; H, 3.67. Calc. for C9 H10 O7 Cu: C, 36.70; H, 3.41); vmax /cm−1 (C=O and C=O) 1724s, 1634s, 1554s, 1497s, 1402m, (Cu-O) 525w, 477w. 2.1.3. [Cu(ETA)(OAc)·H2 O]2 (2) (Ac = acetyl) A methanolic solution (12 mL) of the ligand (1.9 mmol) was added to a refluxing methanol solution (15 mL) of Cu(OAc)2 ·H2 O (0.95 mmol). The resulting solution was refluxed for 2 hours. The solution was evaporated up to a small volume of the solvent and the deposited precipitate was collected by filtration, washed with cold methanol, diethylether, and dried in vacuo over P2 O5 . Powder (0.25 g, 90%), μeff 1.63 μB ; (Anal. Found: C, 34.97; H, 3.67. Calc. for C18 H24 O16 Cu2 : C, 34.67; H, 3.85); vmax /cm−1 (OH) 3542br, 3444br, (C=O and C=C) 1726s, 1635s, 1557s, 1499s, 1441m, 1403m, (Cu-O) 523w, 475w. 2.1. Materials and methods Reagent grade chemicals and solvents (Fluka, Aldrich, Acros) were used without further purification unless otherwise noted. Infrared spectra were recorded in KBr in the range 4000–400 cm−1 on a Nicolet Magna 560R FT-IR spectrophotometer. C, H, and N analyses were performed in the Organic Chemistry Laboratory (NTUA) using a EuroVector EA 3000 elemental analyzer. 1 H and 13 C NMR spectra were recorded on a Varian Gemini-2000 300 MHz spectrometer. The magnetic susceptibility measurements were made using a Gouy balance at room temperature using mercury tetrathiocyanatocobaltate (II), Hg[Co(NCS)4 ] as calibrant. EPR (...truncated)