Vanadium Borate and Borophosphate Cluster Anions

The general procedure for the preparation of vanadium borates consists in heating a concentrated H2O solution of boric acid and vanadium oxide in an autoclave at 170 °C for several days [143]. Two different vanadium borate clusters 105 and 106 are obtained, one with two polyborate chains coordinated to a contorted vanadium oxide ring (105) and another one with a macrocyclic Bi8036(0H)6 ring (106). The latter ring is composed of six B306(0H) units and has a chair-like conformation (Fig. 27) [143]. 

The macrocyclic borate ring is sandwiched by two smaller (V0)60i2 rings of triangular form via six axial B-(/X3-0)-V bonds, of which three are directed to one side of the macrocycle and three to the other one. On average, ten of the 

Vanadium borophosphates 107-109 that contain cluster anions with different ring sizes can be synthesized by hydrothermal reactions of vanadium(III) oxide, boric acid and an appropriate phosphate salt [145,146]. The ring size of the cluster anion with the composition [(VO)2BP20iol 4-6 is determined by the size of the cation that is, however, included only in the interior of the cages with n = 5 and 6 (Fig. 28). 

Fig 28. Vanadium borophosphates with the composition [(VO)2BP20io] n form cage-like cluster anions, which may be empty (107, n = 4) or centered by a cation (109, n = 6). For clarity only half of the vanadium and phosphorus atoms of the cluster anions are shown 

107 with a 4-ring is formed in the absence of any small cation and is not centered. It should be noticed that the formation of these compounds is somewhat fortuitous, since a series of other vanadium borophosphates of similar composition, but with polymeric chain-like structures, are known [151, 152]. 

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