Synthesis and Structural Analysis of Novel Phosphonium Hexatungstate Complexes

Sodium tungstate reacted with tetramethyl- and tetrabutyl-phosphonium bromide in presence of hydrochloric acid to afford two new phosphonium hexatungstate compounds ((CH₃)₄P)₂W₆O₁₉, 1, and ((CH₃CH₂CH₂CH₂)₄P)₂W₆O₁₉, 2, respectively. Under similar conditions, sodium tungstate reacted with methyltriphenyl-, allyltriphenyl-, butyltriphenyl- and benzyltriphenyl-phosphonium bromides to yield four new phosphonium hexatungstate compounds (CH₃Ph₃P)₂W₆O₁₉, 3, (CH₂CHCH₂Ph₃P)₂W₆O₁₉, 4, (CH₃CH₂CH₂CH₂Ph₃P)₂W₆O₁₉, 5, and (C₆H₅CH₂Ph₃P)₂W₆O₁₉, 6, respectively. All six compounds appeared to be stable in air, and were structurally characterized by a combination of FTIR, elemental analyses, and single-crystal X-ray diffraction analyses. The steric effect of the phosphonium cation was investigated and found to cause no significant change on the average bond distances of the hexatungstate anion. The crystal structure analyses of these compounds showed that hexatungstate anions were isolated and the distance between the anions increases with increase in the bulkiness of the surrounding phosphonium cations. Moreover, thermal stability and heat absorption of all six compounds were evaluated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).