Synthesis, Characterization, and Interconversion of the Niobotungstic Acid Nb2W4O19H3-, Its Anhydride, and Its Alkyl/Silyl Esters

Article abstract of DOI:10.1021/ja00254a023

The compounds Nb2W4O19R<(n-C4H9)4N>3 (R = CH3, CH2CH3, CH(CH3)2, C(CH3)3, Si(CH3)3, Si(CH3)2, and Si(C6H5)3) have been prepared according to the equation Nb2W4O19H^3- + ROH = Nb2W4O19R^3- + HOH and isolated in crystalline form.For R = CH3 and CH2CH3 only, the alkyloxy group occupies a bridging position between two niobiums in the Nb2W4O19R^3- framework.The bulkier alkoxy/silyloxy groups are bonded terminally to a single niobium center.Dissolution of Nb2W4O19H<(n-C4H9)4N>3 into dry CH2Cl2 results in the formation of the anhydride (Nb2W4O18)2O^6- as a tetra-n-butylammonium salt.Reaction of Nb2W4O19<(n-C4H9)4N>4 with RX (R = CH3, CH2CH3; X = OSO2R) yields Nb2W4O19R<(n-C4H9)4N>3 in which the Nb2W4O19R^3- anion exists as a mixture of five diastereomers where the R groups bind to five nonequivalent types of Nb2W4O19^4- doubly bridging oxygens.Analogous reactions of Nb2W4O19^4- with RX , X = O3SCF3; R = Si(CH3)3, X = Cl, NHSi(CH3)3> yields Nb2W4O19R^3- anions where the R groups bind exclusively to a terminal ONb oxygen in Nb2W4O19^4-.Protonation of the Nb2W4O19R^4- ion occurs predominantly at the unique bridging ONb2 oxygen.The structure and reactivity of the Nb2W4O19^4- ion and its derivatives are thus seen to reflect the relative labilities of different types of metal-oxygen bonds, the steric environments of different surface oxygens, and the surface charge distribution in the Nb2W4O19^4- framework.Specifically, niobium-oxygen bonds are far more labile than tungsten-oxygen bonds, bridging oxygens occupy a more sterically congested environment than terminal oxygens, bridging oxygens are more basic/nucleophilic than terminal oxygens, and ONb2 oxygens are marginally the most basic/nucleophilic bridging oxygens, and the ONb oxygens are by far the most nucleophilic terminal oxygens.