Cyclic oligomer of oxideclusters through a siloxane bond. Synthesis and structure of reaction products of α2-mono-lacunary Dawson polyoxometalate with tetrachlorosilane and tetraethoxysilane

Abstract

The title compound with the formula (Me₂NH₂)₁₃H₅[{α₂-P₂W₁₇O₆₁(Si₂O)}₃(μ-O)₃]·39H₂O Me₂NH₂-1 was obtained in 12.8% (0.258 g scale) yield by a 1 : 2 molar ratio reaction of α₂-mono-lacunary Dawson polyoxometalate (POM) [α₂-P₂W₁₇O₆₁]¹⁰⁻ with SiCl₄ in water and unequivocally characterized by complete elemental analysis, TG/DTA, FTIR, (solution and solid-state) ³¹P NMR, solid-state ²⁹Si NMR and X-ray crystallography. [Note: the moieties of the polyoxoanions in Me₂NH₂-1 and Me₂NH₂-2 are abbreviated simply as 1 and 2, respectively.] X-Ray crystallography revealed that the molecular structure of polyoxoanion 1 was a cyclic trimer consisting of three α₂-mono-lacunary Dawson POM subunits constituted through intra- and intermolecular siloxane bonds. The compound resulted in an approximate C_s symmetry, but not the idealized C_3v symmetry, because one Dawson unit was unequivalently incorporated into the cyclic framework. This compound is the first example of a siloxane-bonding cyclic trimer of POMs and its formation suggests that the oxygen atoms in the lacunary site of POM are reactive just like the surface oxygen atoms of silica. The formation of the cyclic trimer was also clarified by two control experiments: (1) the reaction of [α₂-P₂W₁₇O₆₁]¹⁰⁻ with Si(OEt)₄ under acidic conditions (pH 1), and (2) oligomerization under 6 M aqueous HCl acidic conditions (pH∼0) of the reactive monomer units (RMUs: tentatively assigned as “[α₂-P₂W₁₇O₆₁{O(SiOH)₂}]⁶⁻”), derived from the cleavage of Ph–Si bonds by the thermal degradation of [α₂-P₂W₁₇O₆₁{O(SiPh)₂}]⁶⁻. On the other hand, the reactions of α₂-mono-lacunary Dawson POMs with GeX₄ (X = OEt, Cl) gave the monomeric species of mono-germanium-substituted Dawson POM (Me₂NH₂)₇[α₂-P₂W₁₇O₆₁(GeOH)]·5H₂O Me₂NH₂-2, which was characterized by CHN analysis, TG/DTA, FTIR, X-ray crystallography, and (solid-state and solution) ³¹P NMR spectroscopy. X-ray crystallography revealed that compound Me₂NH₂-2 was a monomeric species, but the Ge site per formula unit was not determined due to disorder of α-Dawson structure.