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[Nb6O19]·15H2O (a = 9.859(10) Å, b = 17.500(17) Å, c =
17.384(17) Å, β = 91.609(16)°, 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]·15H2O and
Si(OC2H5)4 yields Li13K[SiNb12(OH)2O38]·17H2O (a = 20.341(4) Å, b = 10.735(2) Å, c = 21.438(4) Å, β = 105.41(3)°, monoclinic,
P21/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 [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)2O38]14- clusters from a [Nb6O19]8-
precursor; the analogous reaction with Na7[HNb6O19] produces a Keggin chain material.