The thioantimonate anion [Sb4S8]4- acting as a tetradentate ligand: Solvothermal synthesis, crystal structure and properties of {[In(C6H14N2)2]2Sb4S8}Cl2 exhibiting unusual uniaxial negative and biaxial positive thermal expansion

The new compound {[In(C6H4N2)(2)](2)Sb4S8}Cl-2 was prepared under solvothermal conditions reacting InCl3, Sb and S using 1,2-trans-diaminocyclohexane as solvent and structure directing molecule. The compound crystallizes in the monoclinic space group C2/c with a = 29.0259(12), b = 6.7896(2), c = 24.2023(12) angstrom, beta = 99.524(4)degrees, V = 4703.9(3) angstrom(3). The central structural motif is the thioantimonate(III) anion [Sb4S8](4-) acting as a tetradentate ligand thus joining two symmetry related In3+ centered complexes. This binding mode was never observed before for the [Sb4S8](4-) anion. The optical band gap was determined as 2.03 eV in agreement with the red color of the compound. The thermal decomposition was monitored with in-situ X-ray diffraction experiments. After the emission of the amine molecules an amorphous intermediate is formed followed by the crystallization of InSbS3 which is stable up to about 590 degrees C. On further heating, InSbS3 is destroyed and reflections of gamma-In2S3 appear being contaminated with some elemental Sb. Temperature dependent in-situ X-ray powder diffractometry performed between 30 and 220 degrees C reveals an unusual reversible negative and positive thermal expansion. The decrease of the a-axis in the temperature range is about 0.74 angstrom and the increase of the c-axis ca. 0.54 angstrom. Interestingly, the b-axis exhibits also a thermal expansion, i.e., a biaxial positive and an uniaxial negative thermal expansion coexist which is very unusual. The relative negative expansion coefficients for the a-axis of -194 x 10(-6)K(-1) (30-120 degrees C) and -82 x 10(-6)K(-1) (120-220 degrees C) are in the region of so-called colossal thermal expansion. (C) 2010 Elsevier Masson SAS. All rights reserved.