Lithium polyniobates. A lindqvist-supported lithium-water adamantane cluster and conversion of hexaniobate to a discrete keggin complex

A Li7K salt of [Nb6O19](8-) and a Li13K salt of [SiNb12(OH)(2)O-38](14-) have been prepared and characterized by single-crystal X-ray diffraction and Li-6 MAS NMR. The structure of Li7K[Nb6O19]center dot 15H(2)O (a = 9.859(10) A, b = 17.500(17) A, c = 17.384(17) A, beta = 91.609(16)degrees, monoclinic, P2(1)/c, R1 = 4.94% based on 5343 reflections) consists of a super-octahedron of six edge-sharing NbO6 octahedra capped by a Li4O10 adamantane-like structure. The Li-6 MAS NMR data are consistent with five tetrahedral Li+ sites (delta = 0.20 ppm) and two octahedral Li+ sites (delta = -1.38 ppm). The reaction of Li7K[Nb6O19]center dot 15H(2)O and Si(OC2H5)(4) yields Li13K[SiNb12(OH)(2)O-38]center dot 17H(2)O (a = 20.341(4) A, b = 10.735(2) A, c = 21.438(4) A, beta = 105.41(3)degrees, monoclinic, P2(1)/c, R1 = 6.49% based on 6161 reflections), an alpha-Keggin ion ([SiNb12(OH)(2)O-38](14-)) linked by Li+ cations and a single K+ cation. Interpretation of structural data suggests that Li+ has weaker ion pairing to [Nb6O19](8-) in the solid state than do Na+, K+, Rb+, or Cs+ (i.e., ion pairing increases with an increase in the ionic radius of the alkali metal cation). Given the high charge density of [Nb6O19](8-), the increase in solubility of alkali salts of [Nb6O19](8-) in water from Li+ to Cs+ (Li+ < Na+ < K+ < Rb+ < Cs+) suggests that similar ion pairing in solution decreases the effective charge of the polyanion and increases stability as long as charge neutralization is not achieved. In addition, Li+ facilitates the formation of discrete [SiNb12(OH)(2)O-38](14-) clusters from a [Nb6O19](8-) precursor; the analogous reaction with Na-7[HNb6O19] produces a Keggin chain material.