Spectroscopic, structural and thermal characterization of crystalline $[Cr(OC(NH_2)_2)_6]X_3$, (where X = $ClO_4, BF_4$ and Cl) complexes

Three coordination compounds with urea (CO(NH2)2) ligands, namely [Cr(urea)6](ClO4)3, [Cr(urea)6](BF4)3, and [Cr(urea)6]Cl3 were investigated. In the temperature range of 130–320 K only the first two aforementioned complexes undergo one solid phase transition at: = 298.4 K and = 255.4 K (on heating), respectively. X-ray single crystal diffraction at 293 K demonstrates that [Cr(urea)6](BF4)3 crystallises in the trigonal crystal system (R-3c space group) and is isostructural with the other two title compounds. Both the BF4- anions and CO(NH2)2 ligands are in the high temperature phase dynamically disordered. [Cr(urea)6](ClO4)3, [Cr(urea)6](BF4)3 and [Cr(urea)6]Cl3 are thermally stable up to 500, 470 and 440 K, respectively, in argon atmosphere. Above these temperatures [Cr(urea)6](BF4)3 and [Cr(urea)6]Cl3 decompose in two main steps. In the first step six urea molecules are liberated and in the second step metallic chromium is created. Whereas [Cr(urea)6](ClO4)3 decomposes explosively just at ca. 550 K.
European Chemical Bulletin, Vol 3, No 5 (2014): European Chemical Bulletin