Polyoxovanadates with organic ligands

Polyoxovanadates are inhibitors to various phosphate-metabolizing enzymes. The question arises of how the cluster is bound to the protein matrix. This paper describes oxovanadates with carboxylate and hydroxide ligands in the periphery of the cluster, which may be considered to model oxovanadate binding to carboxylic and alcoholic side-chain functions of the protein. Examples for complexes carrying alkoxo ligands dealt with in this article are dinuclear vanadate(V) esters, and hexa-, octa- and decanuclear, mixed-valence (V^V/V^IV) clusters, the latter related to decavanadate. The possible role of dimeric vanadate esters as transition state anologues in enzymatic phosphoester cleavage is addressed. Examples for carboxylate complexes are the mononuclear, seven-coordinate mixed anhydride between orthovanadic acid and pivalic acid, containing the carboxylate in the bidentate mode, tetra-, penta- and hexanuclear V^IV/V^V clusters with bridging carboxylate, and trinuclear V^IV and V^II/V^III clusters, bridged by carboxylates and trebly bridged by O^2–. Special attention is given to a comparison of the bowls containing a V₄(-O)₃(-OH)(O₂CR)₄ and V₄(-O₄(O₂CR)₄ core, respectively, which can accomodate a K⁺ or a NO ₃ ^– .⁵¹V NMR spectroscopy is shown to be a useful tool, in many cases, for the vanadium speciation of complex systems.