Hydrothermal syntheses, crystal structures and thermal decompositions of two novel heteropolytungstates with anion-chain structure formed by monosubstituted Keggin units

Two novel heteropolytungstates, [Cu(en)2][Hen]2[HPW11CuO39] · 2H2O (1), [H2en]4[SiNaW11O39]Cl · 2H2O (2) have been prepared under mild hydrothermal conditions and structurally characterized by elemental analysis, TG, IR, and single-crystal X-ray diffraction. Compounds 1 and 2 exhibit a straight chain structure in which monosubstituted Keggin anions are connected through their terminal oxygen atoms. Compound 2 is a rare example of a disorder-free monovacant Keggin. The deformation of the Keggin structure caused by the replacement of one tungsten atom is indicated. A large number of hydrogen bonds exist between ethylenediamine molecules, water molecules and anion-chains, which built a three-dimensional architecture. Their thermal decomposition mechanism is inferred and includes a carbonization–oxidation reaction of organic contents.