Structure and Thermal Transformations of Methylammonium Tungstate

Methylammonium paradodecatungstate, (CH3NH3)10[H2W12O42] ⋅ nH2O, was prepared by the reaction of WO3 or H2WO4 in aqueous methylamine, followed by drying. The [H2W12O42]10− anion consists of two HW3O13 groups containing three edge-sharing WO6 octahedra and two W3O14 groups having two edge-sharing connections. On drying the produced material at 70 °C, some lattice water was released, but this did not affect the [H2W12O42]10− structure, although there was a decrease in the unit cell volume. By adding water, the loss of lattice water and structural changes were reversed. The existence of paradodecatungstate anions was confirmed by single-crystal and powder X-ray diffraction, Fourier transform infrared, Raman, 183W nuclear magnetic resonance, and ultraviolet-visible spectroscopy. When the methylammonium paradodecatungstate was heated to more than 150 °C, CH3NH2 and H2O were released, and amorphous WO3 was observed as an intermediate product and monoclinic WO3 as the final product. These results reveal the structure of methylammonium tungstate, which was first reported in 1909 and now is used as an important negative staining reagent for virus observation, is methylammonium paradodecatungstate.This study was supported by JSPS KAKENHI, Grant Number JP19H00843 (Grant-in-Aid for Scientific Research (A)), JST A-STEP Grant Number JPMJTM20RF, International Network on Polyoxometalate Science at Hiroshima University, and JSPS Core-to-Core program.Supporting information for this article is available on the WWW under https://doi.org/10.1002/zaac.20210021