Synthesis, isolation, spectroscopic and structural characterization of a new pH complex structural variant from the aqueous vanadium(V)-peroxo-citrate ternary system M. Kaliva c , E. Kyriakakis c , C. Gabriel a,b , C.P. Raptopoulou d , A. Terzis d , J.-P. Tuchagues e , A. Salifoglou a,b, * a Laboratory of Inorganic Chemistry, Department of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece b Chemical Process Engineering Research Institute, Thermi, Thessaloniki 57001, Greece c Department of Chemistry, University of Crete, Heraklion 71409, Greece d Institute of Materials Science, NCSR ‘‘Demokritos’’, Aghia Paraskevi 15310, Attiki, Greece e Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex, France Received 14 June 2006; accepted 6 July 2006 Available online 21 July 2006 Abstract In a pH-specific fashion, V 2 O 5 , citric acid and H 2 O 2 reacted at pH 5.5–6.0 and afforded a red crystalline product at 4 °C. Elemental analysis pointed to the molecular formulation K 10 ½V V 2 O 2 ðO 2 Þ 2 ðC 6 H 5 O 7 Þ 2 ½V V 2 O 2 ðO 2 Þ 2 ðC 6 H 4 O 7 Þ 2  20H 2 O ð1  20H 2 OÞ. Complex 1 was further characterized by UV/Vis, FT-IR, NMR, cyclic voltammetry, and X-ray crystallography. The X-ray structure of 1 reveals two dinuclear vanadium-peroxo-citrate subunits, A and B, linked through a hydrogen bond. In both A and B, the citrate ligands have dif- ferent protonation states, ultimately affording a pentagonal bipyramidal geometry around each V(V) ion. The peroxide ligands bind V(V) in a side-on fashion. pH-Dependent, non-thermal and thermal transformations of 1 unravel its connection with key participants in the vanadium-peroxo-citrate ternary system and project its association with other non-peroxo binary complexes of variable vanadium oxi- dation state, geometry, citrate binding mode and state of protonation. Overall, the surprising twist in the aqueous synthetic chemistry of the investigated ternary system: (a) projects a new pH structural variant (species A) as a component of the speciation; (b) provides an in- depth look at that speciation under specific pH conditions; and (c) offers significant insight into the aqueous structural speciation of vanadium with peroxide and citrate, and its potential relevance to biological processes. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Vanadium-citrate-peroxo compounds; X-ray structure; Insulin mimetic vanadium; Structural speciation; Chemical reactivity 1. Introduction The participation of vanadium in diverse abiotic applications and biological systems has been amply estab- lished in the past decades and has spurred considerable research activities geared toward delineating its role and bio- logical action [1–6]. To the degree that vanadium’s involve- ment influences key metabolic processes and ultimately the physiology of organisms, vanadium metallobiochemistry has been brought to front action, featuring key metallo- enzyme systems [7–9] such as nitrogenases [10] and haloperoxidases [11–13]. Concurrently, vanadium as an inorganic cofactor has been shown to possess and promote bioactivities ranging from antitumorigenicity [14–16] to mitogenicity [17,18], inhibition of metabolic enzymes such as phosphoglucomutases and others [19,20]. However, a profound influence was found to emerge from the associa- tion of vanadium with the heterogeneous syndrome of Dia- betes mellitus through its insulin mimetic action [21–33]. 0020-1693/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.07.011 * Corresponding author. Address: Laboratory of Inorganic Chemistry, Department of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece. Tel.: +30 2310 996 179; fax: +30 2310 996 196. E-mail address: salif@auth.gr (A. Salifoglou). www.elsevier.com/locate/ica Inorganica Chimica Acta 359 (2006) 4535–4548