Stability and structure in [alpha]- and [beta]-keggin heteropolytungstates, [Xn+W12O40](8-n)-, X = p-block cation

Abstract

[Beta]-[SiW12O40]4- (C3v symmetry) is sufficiently higher in energy than its [alpha]-isomer analogue that effectively complete conversion to [alpha]-[SiW12O40]4- (Td) is observed. By contrast, [beta]- and [alpha]-[AlW12O40]5- ([beta]- and [alpha]-1; C3v and Td, respectively) are sufficiently close in energy that both isomers are readily seen in 27Al NMR spectra of equilibrated ([alpha]−[beta]) mixtures. Recently published DFT calculations ascribe the stability of [beta]-1 to an electronic effect of the large, electron-donating [AlO4]5- (Td) moiety encapsulated within the polarizable, fixed-diameter [beta]-W12O36 (C3v) shell. Hence, no unique structural distortion of [beta]-1 is needed or invoked to explain its unprecedented stability. The results of these DFT calculations are confirmed by detailed comparison of the X-ray crystal structure of [beta]- 1 ([beta]-Cs4.5K0.5[AlIIIW12O40]‚7.5H2O; orthorhombic, space group Pmc21, a = 16.0441(10) Å, b = 13.2270(8) Å, c = 20.5919(13) Å, Z = 4 (T ) 100(2)K)) with previously reported structures of [alpha]-1, [alpha]- and [beta]-[SiW12O40]4-, and [beta]1-[SiMoW11O40]4-.

Citation

Neiwert, Wade A.; Cowan, Jennifer J.; Hardcastle, Kenneth I.; Hill, Craig L.; Weinstock, Ira A. 2002. Stability and structure in [alpha]- and [beta]-keggin heteropolytungstates, [Xn+W12O40](8-n)-, X = p-block cation. Inorganic chemistry. Vol. 41, no. 26 (2002): pages 6950-6952.