Systematic synthesis, structural characterization, and reactivity studies of vanadium(V)-citrate anions [VO2(C6H6O7)](2)(2-), isolated from aqueous solutions in the presence of different cations

The aqueous chemistry of vanadium with physiologically relevant ligands con stitutes a subject of burgeoning research, extending from bacterial metallo enzymic functions to human-health physiology. Vanadium, in the form of VCl3 and V2O5, reacted expediently with citric acid, in a 1:2 molar ratio in wa ter at pH similar to 4, and, in the presence of various cations, afforded c rystalline materials bearing the general formula (Cat)(2)[V2O4(C6H6O7)(2)]( .)nH(2)O (A) (Cat(+) = Na+, NH4, n = 2; Me4N+, K+, n = 4). Exploration of t he reactivity of A toward H2O2 yielded the peroxo-containing complexes (Cat )(2)[V2O2(O-2)(2)(C6H6O7)(2)](.)2H(2)O (B) (Cat(+) = K+, NH4+). Both classe s of compounds were characterized analytically and spectroscopically. The X -ray structures of complexes A and B emphasize the exceptional stability of the dimeric rhombic unit V-2(V) O-2, which is retained upon H2O2 reaction, and the preserved mode of coordination of the citrate ligand as a doubly d eprotonated moiety. In these complexes, typical six and eight coordination numbers were observed for the Na+ and K+ counter-ions, respectively. The va riety of synthetic approaches leading to A, along with the stepwise and dir ect assembly and isolation of peroxo-compounds (B), denotes the significanc e of reaction pathways and intermediates in vanadium(III-V)-citrate synthet ic chemistry. Hence, a systematic investigation of reactivity modes in aque ous vanadium-citrate systems emerges as a crucial tool for the establishmen t of chemical interconnectivity among low MW complex species, potentially p articipating in the intricate biodistribution of that metal ion in biologic al fluids. (C) 2001 Elsevier Science B.V. All rights reserved.