Secondary bonding and coordination of mercury in 5-nitrobenzimidazolium trihalogenomercurates(II) [C7H6N3O2]HgX3·H2O (X = Cl, Br, I): Synthesis, characterization by FT IR, NQR and single crystal X-ray diffractions

Three new mercury(II) halide complex salts, [C7H6N3O2] HgX3·H2O (1: X = Cl, 2: X = Br, 3: X = I) synthesized by reacting stoichiometric quantities of appropriate reactants in aqueous medium, have been investigated by FT IR, NQR spectroscopies, and single crystal X-ray diffractions. The crystal structures of 1 and 2 are isostructural with orthorhombic system (Pnma); a = 15.943(5), b = 6.357(6), c = 12.733(7) Å, Z = 4 at 296(2) K for 1 and a = 16.104(4), b = 6.539(4), c = 13.059(4) Å, Z = 4 at 296(2) K for 2, respectively. In the crystal Structures, a planar cation of C7H6N3O2+, a planar trigonal anion of HgX3– [X = Cl, Br] and water molecule are arranged through the hydrogen bonds of X⋯H–O, O⋯H–N, and X⋯H–N into 2D sheets which are parallel to ac plane. The sheets are connected by double chains of Hg⋯X interionic bonds, which run along the b directions. In contrast, 3 crystallizes in monoclinic (P21/n) space group with a = 7.321(6), b = 16.205(5), c = 13.113(4) Å, β = 96.94(4)°, and Z = 4 at 298(2) K. The crystal structure of 3 consists of a pair of 5-nitrobenzimidazolium cations associated through hydrogen bonds with water molecules and a Hg2I62- dimer in which Hg atoms are tetrahedrally coordinated. In accordance with the crystal structures, three NQR lines due to 81Br and 127I(m = ± 1/2 ↔  ± 3/2) were observed at room temperature for 2 and 3, respectively, while two 35Cl NQR lines being assignable to the terminal Cl atoms were observed for 1. The splitting into two 81Br NQR lines of each line below around 160 K shows an occurrence of a phase transition followed by a slight structural change into a structure with lower crystal symmetry. A corresponding disappearance of 35Cl NQR lines below about 110 K may indicate an occurrence of a phase transition in 1. The effects of secondary bonding of interionic Hg⋯X and hydrogen bonds on the coordination of Hg atoms as well as the crystal structures have been discussed.