Deoxygenation of Polynuclear Metal-Oxo Anions: Synthesis, Structure, and Reactivity of the Condensed Polyoxoanion [C₄H₉]₄(NbW₅O₁₈) ₂O

The deoxygenation of the mixed-metal polyoxoanion [(C₄H₉)₄N]₃NbW₅O ₁₉ with benzoyl chloride in dichloromethane forms quantitatively the condensed polyoxanion [(C₄H₉)₄N]₄(NbW₅O ₁₈)₂O, in which two polyoxoanion fragments are linked together by a Nb-O-Nb oxo bridge. The product is characterized by a strong IR band at 692 cm^-1 assigned to a Nb-O-Nb stretch and a broad single ⁹³Nb NMR resonance at 975 ppm. Partial hydrolysis of [(C₄H₉)₄N]₄(NbW₅O ₁₈)₂O to NbW₅O₁₉O^3- in wet acetonitrile was observed by IR and ¹⁷O NMR spectroscopy. The reaction of [(C₄H₉)₄N]₄(NbW₅O ₁₈)₂O with a variety of alcohols and phenol forms alkoxide-derivatized polyoxoanions [(C₄H₉)₄N]₂Nb(OR)W₅O ₁₈ (R = methyl, ethyl, isopropyl, cholesteryl, phenyl). The similarity of the IR spectra of these deriviatives suggests that functionalization occurs at the terminal NbO oxygen. A crystallographic study of [(C₄H₉)₄N]₄(NbW₅O ₁₈)₂O revealed a crystallographically imposed linear Nb-O-Nb oxo bridge (Nb-O_bridge = 1.887(3) Å) and a structure in which the terminal tungsten-oxo bonds on the adjoining polyoxoanion fragments are eclipsed. Crystal data: orthorhombic, Cmca; Z = 4, a = 15.817(2) Å, b = 17.870(2) Å, c = 35.058(2) Å; V = 9928.0(10) ų; R = 5.52%.