Synthesis, Structural Characterization, and Thermal Decomposition Study of Mg(H2O)6B10H10·4H2O

Abstract

Compound 1 (Mg(H2O)6B10H10·4H2O) was synthesized and characterized using NMR, IR, XRD, and elemental analysis. Its thermal decomposition behavior was studied using Simultaneous Thermogravimetric Modulated Beam Mass Spectrometry (STMBMS), TGA, DSC, IR, and 11B NMR. The crystal structure of 1 reveals multiple dihydrogen and hydrogen bonding interactions that form a 3D extended structure. A reaction network characterizing the thermal decomposition of 1 and its secondary products over a temperature range from 20 to 1000 °C has been developed. Thermal decomposition of 1 is primarily controlled by two competing branches in the reaction network, where coordinated water evolves as either H2O (dehydration) or H2 (dehydrogenation). The extent of reaction to form H2 depends on the fraction of the coordinated water remaining in the sample when its temperature is between 160 and 225 °C. The evolution of coordinated water is reversible and controlled by dissociative sublimation. For the release of coordinated water between 160 and 215 °C, the vapor pressure of water is given by LogeP (Torr) = 30.4561 – 12425.2/T (K) and ΔHs = 103.3 ± 0.3 kJ/mol. The nature of the condensed phase secondary product remaining after all coordinated water is removed by either dehydration and/or dehydrogenation depends strongly on the extent of reaction to form Mg(OH)xB10H10–x. Results of STMBMS experiments where x varies from 0.2 to ∼4 are used to develop the reaction network that characterizes the thermal decomposition process. Heating of 1 at 205 °C resulted in the formation of water-soluble Mg(OH)x(H2O)2–xB10H10–x, while prolonged heating of 1 at 270 °C and heating up to 1000 °C led to decomposition.