Experimental and Theoretical Characterization of Cationic, Neutral, and Anionic Binary Arsenic and Antimony Azide Species

Abstract

Cationic, neutral, and anionic arsenic and antimony halides formed binary arsenic and antimony azide species M(N₃)₄⁺, M(N₃)₄^-, and M(N₃)₆^- (M = As, Sb) upon reaction with trimethylsilyl azide or sodium azide. The compounds were obtained as pure substances or salts, and their identity was established by vibrational spectroscopy and multinuclear NMR spectroscopy and partially by elemental analysis. Attempts to synthesize pentaazides, M(N₃)₅ (M = As, Sb), failed due to spontaneous decomposition of the compounds. Density functional theory (B3LYP) was applied to calculate structural and vibrational data. Vibrational assignments of the normal modes for the isolated azide compounds were made on the basis of their vibrational spectra in comparison with computational results. The molecular structures and vibrational spectra of the arsenic and antimony pentaazides have been investigated theoretically. These calculations (B3LYP) show minima structures (NIMAG = 0) for all reported compounds. It is shown that the M(N₃)₄⁺ (M = As, Sb) cations exhibit ideal S₄ symmetry and the M(N₃)₆^- anions (M = As, Sb) ideal S₆ symmetry. The structure of the hexaazidoarsenate(V) has been determined by X-ray diffraction as its pyridinium salt. [py-H][As(N₃)₆] crystallizes in the triclinic space group P1̄ with a = 6.8484(7), b = 7.3957(8), and c = 8.0903(8) Å, α = 91.017(2), β = 113.235(2), and γ = 91.732(2)°, V = 376.29(7) ų, and Z = 1. The structure of the As(N₃)₆^- anion exhibits only S₂ symmetry but shows approximately S₆ symmetry. The calculated and experimentally observed structure as well as the calculated and observed IR and Raman frequencies for all azide species (except M(N₃)₅) are in reasonable agreement.