Aluminum- and Gallium-Containing Open-Dawson Polyoxometalates

Al‐ and Ga‐containing open‐Dawson polyoxometalates (POMs), K10[{Al4(μ‐OH)6}{α,α‐Si2W18O66}]·28.5H2O (Al4‐open) and K10[{Ga4(μ‐OH)6}(α,α‐Si2W18O66)]·25H2O (Ga4‐open) were synthesized by the reaction of trilacunary Keggin POM, [A‐α‐SiW9O34]10–, with Al(NO3)3·9H2O or Ga(NO3)3·nH2O, and unequivocally characterized by single‐crystal X‐ray analysis, 29Si and 183W NMR, and FT‐IR spectroscopy as well as elemental analysis and TG/DTA. Single‐crystal X‐ray analysis revealed that the {M4(μ‐OH)6}6+ (M = Al, Ga) clusters were included in an open pocket of the open‐Dawson polyanion, [α,α‐Si2W18O66]16–, which was constituted by the fusion of two trilacunary Keggin POMs via two W–O–W bonds. These two open‐Dawson structural POMs showed clear difference of the bite angles depending on the size of ionic radii. In cases of both compounds, the solution 29Si and 183W NMR spectra in D2O showed only one signal and five signals, respectively. These spectra were consistent with the molecular structures of Al4‐ and Ga4‐open, suggesting that these polyoxoanions were obtained as single species and maintained their molecular structures in solution.