Lithium Polyniobates. A Lindqvist-Supported Lithium−Water Adamantane Cluster and Conversion of Hexaniobate to a Discrete Keggin Complex

A Li 7 K salt of [Nb 6 O 19 ] 8- and a Li 13 K salt of [SiNb 12 (OH) 2 O 38 ] 14- have been prepared and characterized by single-crystal X-ray diffraction and 6 Li MAS NMR. The structure of Li 7 K[Nb 6 O 19 ]·15H 2 O ( a = 9.859(10) Å, b = 17.500(17) Å, c = 17.384(17) Å, β = 91.609(16)°, monoclinic, P 2 1 / c, R1 = 4.94% based on 5343 reflections) consists of a super-octahedron of six edge-sharing NbO 6 octahedra capped by a Li 4 O 10 adamantane-like structure. The 6 Li 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 Li 7 K[Nb 6 O 19 ]·15H 2 O and Si(OC 2 H 5 ) 4 yields Li 13 K[SiNb 12 (OH) 2 O 38 ]·17H 2 O ( a = 20.341(4) Å, b = 10.735(2) Å, c = 21.438(4) Å, β = 105.41(3)°, monoclinic, P 2 1 / c, R1 = 6.49% based on 6161 reflections), an α-Keggin ion ([SiNb 12 (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 [Nb 6 O 19 ] 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 [Nb 6 O 19 ] 8-, the increase in solubility of alkali salts of [Nb 6 O 19 ] 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 [SiNb 12 (OH) 2 O 38 ] 14- clusters from a [Nb 6 O 19 ] 8- precursor; the analogous reaction with Na 7 [HNb 6 O 19 ] produces a Keggin chain material.