Dense compounds of C,H, N,and O atoms: Nitramine derivatives of diimino- and dioxodecahydro-1 h,5h-diimidazo-[4,5-b:4′,5′-e]pyrazine

Guanidine condensed with 1,4-diformyl-2,3,5,6-tetrahydroxypiperazine 1 to give 2,6-diiminodecahydro-1H,5H-diimidazo[4,5-b:4′,5′-e]pyrazine 3 isolated as the tetrahydrochloride salt. nitric acid (100%) at −40°C converted the bisguanidine 3 to 2,6-dinitrimino-4,8-dinitrodecahydro-1H,5H-diimidazo[4,5-b:4′,5′-e]- pyrazine 8 isolated as a dihydrate, whereas nitration by nitronium tetrafluoroborate at 0° to 25°C afforded 2,6-diimino-4,8-dinitrodecahydro-1H,5H-diimidazo[4,5-b:4′,5′-e]pyrazine 9 isolate as the monohydrated bistetrafluoroborate salt, and treatmetn with nitric acid (100%) and acetic anhydride or phosphorus pentoxide converted the bisguanidine 3 to 2,6-dioxo-1,3,4,5,7,8-hexanitrodecahydro-1H,5H-diimidazo[4,5-b:4′,5′-e]pyrazine 4 , also obtained from the tetra N-nitro compound 8 · 2 H₂O and from the dinitramine 9 · 2 BHF₄ · H₂O after similar treatment. The cis-syn-cis- configuration of the tricyclic bisurea 4 and bisguanidine 9 was confirmed by X-ray crys-tallographic analysis. Nitrosation by nitrous acid or by dinitrogen tetroxide converted the bisguanidine 3 to a hydrated 2,6-dinitrosimino-4,8-dinitrosodecahydro-1H,5H-diimidazo[4,5-b:4′,5′-]pyrazine 10 · 2.5 H₂O, whereas treatment with nitrosonium tetrafluo-roborate afforded the bistetrafluoroborate salt of 4,8-dinitroso derivative 11 · 2 BHF 4 . The nitrosamines 10 and 11 were converted to the tetranitro compound 8 · 2 H₂O on treatment with nitric acid (100%) at −40°C. Treatmnt with fluoroboric acid etherate in acetonitrile converted nitroimino groups in compound 8 · 2 H₂O and nitrosimino groups in compound 10 · 2.5 H₂O to imino groups in compounds 9 · 2 BHF₂ · H₂O and 11 · 2 HBF₄ respectively.     

Realization of Z2 Topological Metal in Single-Crystalline Nickel Deficient NiV2Se4

Temperature-dependent electronic and magnetic properties are reported for nickel-deficient NiV₂Se₄. Single-crystal X-ray diffraction shows it to crystallize in the monoclinic Cr₃S₄ structure type with space group $I2/m$ and vacancies on the Ni site, resulting in the composition Ni_0.85V₂Se₄ in agreement with our electron-probe microanalysis. Structural distortions are not observed down to 1.5 K. Nevertheless, the electrical resistivity shows metallic behavior with a broad anomaly around 150–200 K that is also observed in the heat capacity data. This anomaly indicates a change of state of the material below 150 K. It is believed that this anomaly could be due to spin fluctuations or charge-density-wave fluctuations, where the lack of long-range order is caused by vacancies at the Ni site of Ni_0.85V₂Se₄. The non-linear temperature dependence of the resistivity as well as an enhanced value of the Sommerfeld coefficient $\gamma =104.0(1)$ mJ mol⁻¹ K⁻² suggest strong electron–electron correlations in this material. First-principles calculations performed for NiV₂Se₄, which are also applicable to Ni_0.85V₂Se₄, classify this material as a topological metal with $Z_2=(1;110)$ and coexisting electron and hole pockets at the Fermi level. The phonon spectrum lacks any soft phonon mode, consistent with the absence of periodic lattice distortion in the present experiments.