| A study of the substitution chemistry of various trivacant heteropolytungstates with transition metal cations in order to prepare trisubstituted complexes as soluble metal oxide analogues is described.; It is found that B-type trivacant heteropolytungstates such as Na(,8)HPW(,9)O(,34) and Na(,12)P(,2)W(,15)O(,56) react with M('2+) (M = Zn, Co, Cu) forming the disubstituted dimeric heteropolytungstates, P(,2)W(,18)M(,4)(H(,2)O)(,2)O(,68)('10-) and P(,4)W(,30)M(,4)(H(,2)O)(,2)O(,112)('16-), respectively. ('183)W NMR including the use of tungsten atom connectivities obtained from ('2)J(,W-O-W) coupling constants observed in either the 1-dimensional spectrum or 2-dimensional INADEQUATE spectrum, as in the case of P(,4)W(,30)Zn(,4)(H(,2)O)(,2)O(,112)('16-), confirm the structure of these C(,2h) symmetry dimer complexes. The results also correct earlier misformulations of P(,4)W(,30)M(,4)(H(,2)O)(,2)O(,112)('16-) and P(,2)W(,15)O(,56)('12-). The homologous series of derivatives obtained indicate that these complexes are members of a larger class of disubstituted dimeric heteropolyanions.; On the other hand, reaction of A-type Na(,9)HSiW(,9)O(,34) with Nb(,6)O(,19)('8-) in hydrogen peroxide forms a new tri-niobium substituted heteropolytungstate isolated as the niobium oxo bridged dimer, {lcub}(n-C(,4)H(,9))(,4)N{rcub}(,6)H(,2)Si(,2)W(,18)Nb(,6)O(,77). ('183)W NMR and the first application of Fast Atom Bombardment Mass Spectroscopy to polyoxometalates confirm the proposed structure. This complex reacts with a variety of reagents paralleling similar reactions of hydrous niobium oxide.; In the presence of hydroxide, Si(,2)W(,18)Nb(,6)O(,77)('8-) cleaves to form the monomeric tri-niobium substituted complex, SiW(,9)Nb(,3)O(,40)('7-). A similar reaction with 2-aminoethanol results in the dimeric 2-aminoethanol bridged complex, Si(,2)W(,18)Nb(,6){lcub}O(CH(,2))(,2)NH(,2){rcub}(,6)O(,74)('8-). Reaction of Si(,2)W(,18)Nb(,6)O(,77)('8-) with hydrogen peroxide yields the tri-peroxo-niobium heteropolytungstate, SiW(,9)(NbO(,2))(,3)O(,37)('7-). The close chemical analogy between Si(,2)W(,18)Nb(,6)O(,77)('8-) and hydrous niobium oxide demonstrates that this anion behaves as a solubilized piece of niobium oxide, {lcub}Nb(,2)O(,5){rcub}(,3), which is sandwiched between two SiW(,9)O(,31)('4-) units. The results also substantiate the metal oxide analogy of trisubstituted heteropolyanions.; Finally, SiW(,9)Nb(,3)O(,40)('7-) is shown to serve as a metal oxide support for various metallic and organometallic species. In particular, C(,5)(CH(,3))(,5)Rh('2+) bonds to surface oxygens forming the complex, C(,5)(CH(,3))(,5)RhSiW(,9)Nb(,3)O(,40)('5-). Subtle structural changes of the heteropolytungstate that occur on support of the organometallic species are detected by corresponding changes of the ('2)J(,W-O-W) coupling constants. |
