Structure and properties of [2-NH2C5H4NH][SbCl4] and [2-NH2C5H4NH][SbBr4]

The crystal structures of [2-NH2C5H4NH][SbCl4] (2-APyHSbCl(4)) and [2-NH2C5H4NH][SbBr4] (2-APyHSbBr(4)) are determined at 100 K. Both compounds crystallize in the monoclinic space group: P2(1)/c. The structure is composed of SbX4- (X = Cl, Br) ions which form infinite chains through the crystal via halogen linkages. The structural phase transformations are detected by the differential scanning calorimetry and dilatometric techniques: at 402 K, close to continuous, and at 412 K, clearly discontinuous, in 2-APyHSbCl(4) and 2-APyHSbBr(4), respectively. Dielectric relaxation studies in the frequency range between 1 kHz and 25 MHz indicate reorientations of the 2-aminopyridinium (2-APyH) cations in both compounds in the low temperature phases. The proton NMR second moment, M-2, and spin-lattice relaxation time, T-1, for 2-APyHSbCl(4) and 2-APyHSbBr(4) measured between 78 and 430 K reveal the in-plane reorientation of the 2-APyH cations. The possible mechanism of the phase transitions in the title crystals is discussed on the basis of the results presented.