Crystal Structure, Hydrogen Bonding, and Br NQR of Low-Temperature Phase of 4-Aminopyridinium Tetrabromoantimonate(III)

The crystal structure of the low-temperature phase (LTP) of the title compound was determined at 220 K (monoclinic, P21 /c). The 4-aminopyridinium cations (4-NH2C5H4NH+) were found to be ordered in LTP, while being severely disordered in the room-temperature phase (monoclinic, C2/c). The tetrabromoantimonate anions (SbBr4^-) were incorporated into the infinite polyanion chains of irregular SbBr6 octahedra with two-edges sharing. The trans-Br–Sb–Br moiety in the SbBr4^anion was approximately symmetric differing from the asymmetric Br–Sb···Br moiety found in LTP of pyridinium tetrabromoantimonate(III). The N–H moieties in both of the pyridine ring and the amino (–NH2) group participate in the formation of N–H···Br hydrogen bonds. It was shown that the 81Br NQR spectrum of LTP is closely related to the anion structure and the hydrogen bonds. The distinctive anion structures, as well as the different types of phase transitions, exhibited by the PyH and 4-APyH salts, seemed to be attributable to the difference in the hydrogen bond scheme between them.