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

Markus Joos,Thomas Schleid,Robert E Dinnebier,Maurice Conrad,Sebastian Bette,Joachim Maier,Rotraut Merkle

doi:10.1039/d1dt00469g

Abstract

Mg(SCN)2·4H2O can be converted into previously unknown compounds Mg(SCN)2·(4 - x) H2O·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)2·4H2O and Mg(SCN)2·4THF was investigated by temperature dependent in situ XRPD, where Mg(SCN)2·4THF was found to acquire a room temperature (α-form) and high temperature modification (β-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)2·2THF. There thiocyanate related sulfur atoms fill the voids in the coordination sphere of magnesium, which leads to the formation of one dimensional electroneutral ∞[Mg(NCS)2/2(SCN)2/2(THF)2] chains. All investigated Mg(SCN)2·(4 - x) H2O·xTHF phases exhibit a remarkable anisotropic thermal expansion, and Mg(SCN)2·4H2O and Mg(SCN)2·2THF were found to show both positive and negative thermal expansion coefficients.

Highlights

Magnesium halides and pseudo-halides exhibit a great variety of chemical and structural properties and are used in many different applications, which is why they have been extensively studied.[1,2,3,4,5] in the row of magnesium pseudo-halides, i.e. cyanides, cyanates, azides etc., thiocyanates are often overseen, and only little chemical and structural knowledge is available for magnesium thiocyanates and their derivatives.The first known crystalline magnesium thiocyanate phase was reported to be the tetrahydrate Mg(SCN)2·4H2O in 1930,6 which was later confirmed by the elucidation of its crystal structure.[7]
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 this study we report the synthesis, structure, spectroscopic properties and thermal behavior of the solid phases dithiocyanatetetraaquamagnesium(II) [Mg(SCN)2(H2O)4] ( referred to as magnesium dithiocyanate tetrahydrate, Mg (SCN)2·4H2O), bis(tetrahydrofuran)diaquadi-thiocyanatemagnesium(II), [Mg(SCN)2(H2O)2(THF)2], and dithiocyanatetetrakis-magnesium(II), [Mg(SCN)2(THF)4] (from hereon we will address these compounds as Mg(SCN)2·(4 − x) H2O·xTHF with x = 0, 2, 4).[41]

Summary

Introduction

Magnesium halides and pseudo-halides exhibit a great variety of chemical and structural properties and are used in many different applications, which is why they have been extensively studied.[1,2,3,4,5] in the row of magnesium pseudo-halides, i.e. cyanides, cyanates, azides etc., thiocyanates are often overseen, and only little chemical and structural knowledge is available for magnesium thiocyanates and their derivatives.The first known crystalline magnesium thiocyanate phase was reported to be the tetrahydrate Mg(SCN)2·4H2O in 1930,6 which was later confirmed by the elucidation of its crystal structure.[7]. Magnesium halides and pseudo-halides exhibit a great variety of chemical and structural properties and are used in many different applications, which is why they have been extensively studied.[1,2,3,4,5] in the row of magnesium pseudo-halides, i.e. cyanides, cyanates, azides etc., thiocyanates are often overseen, and only little chemical and structural knowledge is available for magnesium thiocyanates and their derivatives. Besides the tetrahydrate Mg(SCN)2·4H2O, crystal structure investigations of Mg(SCN)2-containing compounds have generally been neglected so far and only spectroscopic studies or thermal analyses are present.[10,12] Further structural studies of related Mg2+-SCN−-phases are only available for mixed cationic systems involving other cations like K+ or Cd2+.14,15.

Results

Discussion

Conclusion