Kristallstruktur, Infrarot- und Ramanspektren sowie thermische Zersetzung von Magnesiumtetrahydrogendimesoperiodat, MgH4I2O10 · 6H2O

Abstract

Crystal structure, DRIFT, infrared and Raman spectra, and the results of thermal analyses of the hitherto wrongly as Mg(H4IO6)2 · 4H20 and Mg(IO4) · 8H2O described dimesoperiodate MgH4I2O10 · 6H2O and of the isostructural zinc compound are presented. The compounds crystallize in the monoclinic space group P21 (Z = 2) with a = 1071.0(2), b = 547.0(1), c = 1194.9(2) pm, and β = 112.58(3)° and a = 1073.3(3), b = 545.3(2), c = 1188.3(5) pm, and β = 112.52(3)°, respectively. The structure, which was refined from X-ray single crystal data of the magnesium compound (R1 = 2.72%, 3824 independet reflections), is built up from isolated distorted M(H2O)6 2+ octahedra and dimesoperiodate ions H4I2O10 2- connected by a network of hydrogen bonds formed by the H4I2O10 2- ions and six crystallographically different hydrate H2O molecules. The strength of the hydrogen bonds ranges from unusually weak bonds corresponding to uncoupled (isotopically dilute samples) OD stretching modes of > 2600cm-1 and very strong ones (νOD: < 2200 cm-1). The IO stretching modes of the transconfigurated H4I2O10 2- ions are assigned to terminal I-O groups (816 cm-1), I-OH groups (746 and 762cm-1) and bridging I-O groups (618 and 647cm-1). On heating, MgH4I2O10 · 6H2O undergoes dehydration in the range of 373 - 485 K (Differential Scanning Calorimetry) to two different polymorphs of magnesium metaperiodate (H4I2O10 2-→2IO4 - + 2H2O). Anhydrous Mg(IO4)2 is instable. Above 423 K (high-temperature Raman data), it decomposes to magnesium iodates.