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 TC3(IV→III)=30.7 K, the second at TC2(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 TC1(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 (Fm3¯m, No. 225) with two types of BF4− anions differing in a degree of disorder. In phase II (at 170 K) the structure remains cubic (Ia3¯, 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 BF4− anions perform fast reorientations (τR≈10−12 s), which are significantly slowed down below the phase transition at TC3. 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 BF4− anions reorientation as ∼6 kJ mol−1. Above the phase transition temperature half of BF4− anions perform a tumbling motion with Ea≈8 kJ mol−1.

A series of complementary methods, such as Adiabatic Calorimetry, Differential Scanning Calorimetry, Fourier Transform—Far and Middle Spectroscopy, proton and fluorine Nuclear Magnetic Resonance and structural methods reveal information about phase transitions in [Ru(NH3)6](BF4)3.Figure optionsDownload as PowerPoint slideHighlights
► Three novel phase transitions are found in [Ru(NH3)6](BF4)3.
► The thermodynamic parameters of the phase transitions are derived.
► The transitions are of order–disorder type.
► The complex belongs to Fm3¯m at 293 K and its symmetry changes to Ia3¯ at 170 K.
► [Ru(NH3)6](BF4)3 is a highly dynamically disordered crystal.