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)₂O₃₈] ^14- have been prepared and characterized by single-crystal X-ray diffraction and ⁶Li MAS NMR. The structure of Li₇K[Nb ₆O¹⁹]·15H₂O (a = 9.859(10) A°, b - 17.500(17) A°, c = 17.384(17) A°, β= 91.609(16)^o, monoclinic, P2₁/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 ⁶Li MAS NMR data are consistent with five tetrahedral Li⁺ sites (δ = 0.20 ppm) and two octahedral Li⁺ sites (δ = -1.38 ppm). The reaction of Li₇K[Nb₆O₁₉]·15H ₂O and Si(OC₂H₅)₄ yields Li ₁₃K[SiNb₁₂(OH)₂O₃₈H ₇H₂O (a = 20.341(4) Å, β = 10.735(2) Å, β = 21.438(4) Å, β = 105.41(3)^o, monoclinic, P2 ₁/c, R1 = 6.49% based on 6161 reflections), an α-Keggin ion ([SiNb₁₂(OH)₂O₃₈]^14-) linked by Li⁺ cations and a single K⁺ cation. Interpretation of structural data suggests that Li⁺ has weaker ion pairing to [Nb ₆O₁₉]^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 [Nb ₆O₁₉]^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)₂O ₃₈]^14- clusters from a [Nb₆O₁₉] ^8- precursor; the analogous reaction with Na₇[HNb ₆O₁₉] produces a Keggin chain material.