NMR spectroscopy study of the peroxovanadium(V) complexes of L- malic acid Licı ´nia L.G. Justino, M. Luı ´sa Ramos, M. Madalena Caldeira, Victor M.S. Gil * Department of Chemistry, University of Coimbra, 3000 Coimbra, Portugal Received 5 March 2003; accepted 18 June 2003 Dedicated in honor of Professor J.J.R. Frausto da Silva Abstract Continuing the solution speciation studies of peroxovanadium(V) complexes with a-hydroxycarboxylic acids, we now report on the complexes formed with L-malic acid, studied by multinuclear ( 1 H, 13 C, 51 V) 1D and 2D NMR spectroscopy. The system V(V)  /L- malic acid  /H 2 O 2 in aqueous solution was found to involve a large number of species: nine major peroxovanadium(V) complexes in the pH range 2  /7, the structures of which have been deduced, and eight weaker complexes in very low concentrations. The former are monoperoxo species with the stoichiometries (metal:acid:peroxide) 2:2:2 (two complexes), 2:2:1 (two complexes), 2:1:1 (two complexes) and 1:1:1 (three complexes). The 2:2:2 complexes have seven-coordinated metal centres which are doubly bridged by the hydroxyl oxygen atom of malic acid. In one case, all three functional groups of malic acid are bound to each of the two VO 3 metal centres, whereas in the other the C 4 carboxylic group of the ligand is dangling, two possibilities being advanced for the vanadium centres: either one V 2 O 3 4 centre or two VO 3 centres. The two complexes of stoichiometry 2:2:1 are the result of degradation (loss of one peroxide unit) of the 2:2:2 species in which L-malic acid is a bidentate ligand; they have seven- and five-coordinated metal centres, respectively, the peroxo and the oxo centres, and the malate groups are equally bidentate. The two complexes of stoichiometry 2:1:1 have presumably V 2 O 4 2 metal centres and all three functional groups of L-malic acid are involved in chelation; isomerism is interpreted as due to which of the two non-equivalent V(V) atoms is the peroxo centre. The three 1:1:1 complexes have either seven- or six-coordinated VO 3 metal centres; in one of the complexes, L-malic acid acts as a tridentate ligand, whereas in the other two, the ligand is bidentate. # 2003 Elsevier B.V. All rights reserved. Keywords: Peroxovanadium(V) complexes; L-Malic acid; Multinuclear NMR 1. Introduction Vanadium is widely distributed in living organisms and is known to be involved in an essential way in several biochemical processes (chlorophyll synthesis and the growth of some animals) [1]. Some organisms such as the fly agaric toadstool (Amanita muscaria ) and certain sea squirts (ascidians) are able to accumulate vanadium. In other species, this element is found at the active site of some enzymes. Two of such cases are the nitrogen-fixing bacterium Azotobacter which has a vanadium nitrogenase, and the lichens and various seaweeds in which vanadium is found in the haloperox- idases [1]. In addition, recent studies of vanadium interactions with living organisms have revealed impor- tant effects [2], such as enzyme inhibitory [3], mitogenic [2], antitumorigenic [4] and insulinomimetic activities [5  /7]. Among vanadium compounds, peroxo complexes have been for the past several years object of intense investigation, due to both their biological relevance (insulinomimetic and antitumor activities [7  /11]; func- tional models for the haloperoxidase enzymes [12  /14]), and their application in the oxidation of several substrates, such as benzene and other aromatics, alkenes, allylic alcohols, sulfides, halides and primary and secondary alcohols [15]. In particular, peroxovana- * Corresponding authors. Tel.:  /351-239-852080; fax:  /351-239- 827703. E-mail address: vgil@csjp.ua.pt (L.L.G. Justino). Inorganica Chimica Acta 356 (2003) 179  /186 www.elsevier.com/locate/ica 0020-1693/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0020-1693(03)00466-3