Enormous lattice distortion through an isomorphous phase transition in an organic-inorganic hybrid based on haloantimonate(III)

Bis(diisobutylammonium) octabromodiantimonate(III), [(i-C4H9)(2)NH2](2)Sb2Br8, has been synthesized. The differential scanning calorimetric measurements indicate a reversible, first-order phase transition at 222/229 K (cooling/heating). The single crystal X-ray diffraction studies reveal that the phase transition is isomorphous and is accompanied by a huge distortion of the crystal lattice. By comparison of the crystal structures of [(i-C4H9)(2)NH2](2)Sb2Br8 and [(i-C4H9)(2)NH2](2)Sb2Cl8, an analogous mechanism of the phase transition of the former is proposed. The change of the electronic structure of the complex during the phase transition was analyzed by UV-vis spectroscopy. A low-frequency dielectric relaxation process appears over phase I (below the room temperature) and corresponds to the dynamics of dipolar diisobutylammonium cations. The detailed analysis of the molecular motions of the organic cations studied by means of proton magnetic resonance (H-1 NMR) in a wide temperature range indicates a leading role of the methyl groups in the relaxation mechanism. A variable-temperature investigation of the infrared spectra of [(i-C4H9)(2)NH2](2)Sb2Br8 confirms, in turn, the influence of the diisobutylammonium cation dynamics on the molecular mechanism of the structural transformation at 229 K.