Synthesis, characterization and thermal behaviour of solid phases in the quasi-ternary system Mg(SCN)(2)-H2O-THF

摘要

Mg(SCN)(2)center dot 4H(2)O can be converted into previously unknown compounds Mg(SCN)(2)center dot(4 - x) H2O center dot xTHF with x = 0, 2 and 4 by multiple recrystallization in tetrahydrofuran (THF). The phases were characterized by infrared spectroscopy (IR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and their crystal structures were solved from X-ray powder diffraction (XRPD) data. In the crystal structures isolated Mg(NCS)(2)(H2O)(4-x)(THF)(x) units form layered motifs. The thermal behavior of Mg(SCN)(2)center dot 4H(2)O and Mg(SCN)(2)center dot 4THF was investigated by temperature dependent in situ XRPD, where Mg(SCN)(2)center dot 4THF was found to acquire a room temperature (alpha-form) and high temperature modification (beta-form). The phase transformation is associated with an order-disorder transition of the THF molecules and with a reversion of the stacking order of the layered motifs. Further heating eventually leads to the formation of Mg(SCN)(2)center dot 2THF. There thiocyanate related sulfur atoms fill the voids in the coordination sphere of magnesium, which leads to the formation of one dimensional electroneutral (infinity)[Mg(NCS)(2/2)(SCN)(2/2)(THF)(2)] chains. All investigated Mg(SCN)(2)center dot(4 - x) H2O center dot xTHF phases exhibit a remarkable anisotropic thermal expansion, and Mg(SCN)(2)center dot 4H(2)O and Mg(SCN)(2)center dot 2THF were found to show both positive and negative thermal expansion coefficients.