Halides and Esters Derived from Orthovanadic Acid

3 Halides and Esters Derived from Orthovanadic Acid 

Formal substitution of three OH groups in hypothetical orthovanadic acid H3V04[0= V(0H)3] for halide leads to the trihalides VOX3 (X = F, Cl, Br). The corresponding nitrate V0(N03)3 is aiso known. The oxovanadium trihalides can add an additional 

As pointed out in the previous section, phosphate derivatives related to H3VO4 are of interest when it comes to the phosphate-vanadate antagonism. Similarly, vanadate esters are analogues of phosphate esters (Section 5.2.1). Esters of orthovanadic acid have been known since IQIS.I They are obtained by, e. g., condensation of vanadates and alcohols (ROH) in aqueous media [Equation (2.11)], alcoholysis of VOCI3 [Equation (2.12)] or the reaction of vanadium pentoxide and alcohols in the presence of water scavengers [Equation (2.13)]. The triesters are sensitive to moisture. The primary product of hydrolysis is the hexavanadic acid derivative V50i3(0R)4, giving rise to a yellow colouration as the colourless triesters come into contact with moist air. 

The formation constants for the monoesters [x = 1 in Equation (2.11)] are small, typically around 0.1 m for alkyl alcohols, and an order of magnimde more for phenols.I ] Esters derived from divanadate, such as [HV205(0R)] , have also been noted. In analogy with the vanadates, the esters are subject to protonation-deprotonation equilibria. 

Structures of oxovanadium tris(alkoxides) (triesters of H3VO4 ). In solution, there are concentration-dependent equilibria between monomers and dimers. In the solid state, monomers (bulky R substituents), dimers (R = cyclopentyl) or polymers exist (R = Me).l l 

---