Calorimetric, spectroscopic and structural investigations of phase polymorphism in [Ru(NH3)6](BF4)3. Part I

Four crystalline phases of the coordination compound [Ru(NH3)6](BF4)3 are identified by adiabatic calorimetry. Three phase transitions, one at T C3(IV→III)=30.7 K, the second at T C2(III→II)=91.7 K (both accompanied by comparable entropy changes 3.0 and 3.1 J K−1 mol−1, respectively) and the third at T C1(II→I)=241.6 K (accompanied by an entropy change of 8.1 J K−1 mol−1) were discovered. X-ray single crystal diffraction (at 293 K) demonstrates that phase I is a highly dynamic disordered cubic phase (View the MathML sourceFm3¯m, No. 225) with two types of View the MathML sourceBF4− anions differing in a degree of disorder. In phase II (at 170 K) the structure remains cubic (View the MathML sourceIa3¯, No. 206), with two different types of cations and four different types of anions. Splitting of certain IR bands connected with NH3 ligands at the observed phase transitions suggests a lowering of the symmetry of the [Ru(NH3)6]3+ complex cation. Both NH3 ligands and View the MathML sourceBF4− anions perform fast reorientations (τ R≈10−12 s), which are significantly slowed down below the phase transition at T C3. 1H NMR studies led to estimate the values of the activation energy of NH3 ligands reorientation in the phases II and I as equal to ∼8 kJ mol−1. In phase I the whole hexammineruthenium(III) cations reorientation as a tumbling process can be noticed. The activation energy value of this motion is ∼24 kJ mol−1. 19F NMR studies give the values of the activation energy of View the MathML sourceBF4− anions reorientation as ∼6 kJ mol−1. Above the phase transition temperature half of View the MathML sourceBF4− anions perform a tumbling motion with Ea≈8 kJ mol−1.