RESUMO
Bi2(NCN)3, the first binary pnictogen carbodiimide, and its ammonia derivative Bi2(NCN)3·NH3 have been prepared via nonaqueous liquid-state low-temperature ammonolysis. The crystal structure of Bi2(NCN)3·NH3 in space group Cc solved via single-crystal X-ray diffraction corresponds to a two-dimensional-like motif with layers of NCN2- alternating with honeycomb-like layers of edge-sharing distorted BiN6 octahedra, half of which are also coordinated by molecular ammonia occupying the octahedral holes. By contrast, Bi2(NCN)3 adopts a higher-symmetric C2/c structure with a single Bi position and stronger distortion but empty octahedral voids. In both cases, Bi3+ and its 6s2 lone pair are well mirrored by antibonding Bi-N interactions below the Fermi level. Density functional theory calculations reveal an exothermic reaction for the intercalation of NH3 into Bi2(NCN)3, consistent with the preferential formation of Bi2(NCN)3·NH3 in the presence of ammonia. A Bärnighausen tree shows both compounds to be hettotypic derivatives of the R3Ì c M2(NCN)3 corundum structure that express highly distorted hexagonal-close-packed layers of NCN2- in order to accommodate the aspherical Bi3+ cations. Although Bi2(NCN)3 does not resemble the isovalent Bi2Se3 in forming two-dimensional layers and a topological insulator, theory suggests a driving force for the spontaneous formation of Bi2Se3/Bi2(NCN)3 sandwiches and a conducting surface state arising within the uppermost Bi2(NCN)3 layer.
RESUMO
Crystals of M(NH3)2[N(CN)2]2 with M = Ni and Co were obtained from the reaction of stoichiometric amounts of Na[N(CN)2] with NiCl2·6H2O or CoCl2·6H2O in aqueous, ammoniacal solutions. X-ray single-crystal structure analyses show that M(NH3)2[N(CN)2]2 with M = Ni and Co crystallize isotypically to each other and adopt the monoclinic space group P21/ c (no. 14). The lattice parameters of Ni(NH3)2[N(CN)2]2 are a = 5.8498(9) Å, b = 10.6739(12) Å, and c = 6.8089(17) Å, ß = 98.037(3)° and Z = 2, while those of Co(NH3)2[N(CN)2]2 are a = 5.8303(11) Å, b = 10.746(2) Å, c = 6.7773(13) Å, and ß = 96.422(3)°. In addition, the crystal structure of the nickel compound was refined from neutron powder diffraction, augmented by DFT calculations as regards atomic displacement parameters. The IR spectra of the title compounds exhibit modes typical for the dicyanamide anion and ammonia. The UV/vis spectrum of Ni(NH3)2[N(CN)2]2 shows that the dicyanamide moiety is a medium-field ligand. Additional superconducting quantum interference device (SQUID) magnetic susceptibility measurements of Ni(NH3)2[N(CN)2]2 and Co(NH3)2[N(CN)2]2 confirm not only significant high-spin moments of χm T = 1.24 cm3·K·mol-1 (µeff = 3.15 µB) and 2.89 cm3·K·mol-1 (µeff = 4.81 µB), respectively, at 290 K and 0.1 T but also an absence of magnetic ordering.
RESUMO
1-(Chloromethyl)-3-nitrobenzene, C7H6NClO2, and 1-(bromomethyl)-3-nitrobenzene, C7H6NBrO2, were chosen as test compounds for benchmarking anisotropic displacement parameters (ADPs) calculated from first principles in the harmonic approximation. Crystals of these compounds are isomorphous, and theory predicted similar ADPs for both. In-house diffraction experiments with Mo Kα radiation were in apparent contradiction to this theoretical result, with experimentally observed ADPs significantly larger for the bromo derivative. In contrast, the experimental and theoretical ADPs for the lighter congener matched reasonably well. As all usual quality indicators for both sets of experimental data were satisfactory, complementary diffraction experiments were performed at a synchrotron beamline with shorter wavelength. Refinements based on these intensity data gave very similar ADPs for both compounds and were thus in agreement with the earlier in-house results for the chloro derivative and the predictions of theory. We speculate that strong absorption by the heavy halogen may be the reason for the observed discrepancy.