Al-and Ga-containing open-Dawson polyoxometalates (POMs), K 10 [{Al 4 (μ-OH) 6 }{α,α-Si 2 W 18 O 66 }]·28.5H 2 O (Al 4 -open) and K 10 [{Ga 4 (μ-OH) 6 }(α,α-Si 2 W 18 O 66 )]·25H 2 O (Ga 4 -open) were synthesized by the reaction of trilacunary Keggin POM, [A-α-SiW 9 O 34 ] 10-, with Al(NO 3 ) 3 ·9H 2 O or Ga(NO 3 ) 3 ·nH 2 O, and unequivocally characterized by single-crystal X-ray analysis, 29 Si and 183 W NMR, and FT-IR spectroscopy as well as elemental analysis and TG/ DTA. Single-crystal X-ray analysis revealed that the {M 4 (μ-OH) 6 } 6+ (M = Al, Ga) clusters were included in an open pocket of the open-Recently, the open-Dawson POMs have been reported as an emerging class of POMs. [2] The conventional Dawson structural POMs are regarded as an assembly of two trilacunary Keggin POMs via six W-O-W bonds. In 2014, Mizuno et al. reported that the conventional Dawson POM with highly charged guest SiO 4 4-, TBA 8 [α-Si 2 W 18 O 62 ]·3H 2 O, was synthesized by dimerization of a trilacunary Keggin POM, [SiW 9 O 34 ] 10-, in an organic solvent. [3a] However, in aqueous media, the electrostatic repulsion between the halves, induced by the highly charged guest XO 4 4-(X = Si, Ge), is so strong that it inhibits the assembly of the usual Dawson structure. [3b] Therefore, the two trilacunary Keggin units with XO 4 4-(X = Si, Ge) are linked by only two W-O-W bonds, forming open-Dawson structural POMs. The open pocket of the open-Dawson POMs can accommodate multiple metals (1 to 6 metals), and several compounds have been reported so far that contain metal ions in the open pocket, e.g., V 5+ , [2f]542 crystallization water molecules were also observed in the crystal structures of Al 4 -and Ga 4 -open, forming a 3D network structure in the crystal ( Figure 5).
