Structural determination by X-Ray diffraction and 183W NMR spectroscopy of mono substituted hexatungstates [(n-C4H9)4N]3MW5O19 (M = Nb, V)

The tetrabutylammonium salt of mononiobotungstate (n-C₄H₉)₄N]₃NbW₅O₁₉ (1) and the tetrabutylammonium salt of monovanadotungstate (n-C₄H₉)₄N]₃VW₅O₁₉ (2) are isotypes; both crystallize in the monoclinic system, space group C2/c (N° 15) with Z=8. The cell parameters for 1 are a=30.4038(8) ?, b=18.5948(8) ?, c=27.3330(3) ?, β=112.4555(6)°, V=14281.1(7) ?³ and the final reliability factors are R=0.043 and R _w=0.047 for 5801 reflections. The cell parameters for 2 are a=30.096(8) ?, b=18.373(3) ?, c= 27.201(6) ?, β=112.402(14)°, V=13906(5) ?³ and the final reliability factors are R=0.048 and R _w=0.054 for 6122 reflections. Both anions, NbW₅O_19] ³⁻ and VW₅O_19] ³⁻ exhibit the Lindqvist structure of the parent hexatungstate anion. The six metal positions are disordered and for each metal site the occupation factor is close to 1/6 M (M=Nb, V) and 5/6 W. Furthermore the two compounds were characterized by IR in the solid state, and ¹⁸³W solution NMR. The ¹⁸³W spectrum of NbW₅O_19] ³⁻ presents two resonances with relative intensities 4:1 in agreement with the C_4v symmetry of the anion.