127I NQR, 1H NMR and crystal structure of a novel compound, guanidinium heptaiodomercurate(II) [C(NH2)3]3Hg2I7

The crystal structure of [C(NH2)3]3Hg2I7 crystallizes in the monoclinic space group C2/c with a = 10.301(2), b = 11.919(2), c = 22.143(4) Å, β = 100.50(3)°, and Z = 4. The asymmetric unit of the structure consists of an Hg2I73− anion and three guanidinium cations [C(NH2)3]+. The Hg2I73− anion is composed of two distorted HgI4 tetrahedra which share an apex of I(4) atom and are related to each other by the C2 axis. The lengths of the four Hg–I bonds of the tetrahedron are different, the longest being the bridging Hg–I(4) bond. In accordance with the crystal structure, four 127I NQR resonance lines have been observed and are assigned to m = ±1/2 ↔ ±3/2 transitions. With decreasing temperature one line assigned to the bridging I(4) atom faded out around 250 K. The temperature dependence of 1H NMR T1 has been analyzed by postulating the C3 reorientations of two crystallographically independent cations with the BPP and the Arrhenius type relations. The hydrogen bond interactions N–H⋯I are important in the architecture of the crystal structure, which manifest themselves on the differences in the Hg–I bond lengths, in the NQR frequencies, and in the activation energies of the cation reorientations.