Synthesis and Characterization of the SO₂N₃^-, (SO₂)₂N₃^-, and SO₃N₃^- Anions^†

SO2 solutions of azide anions are bright yellow, and their Raman spectra indicate the presence of covalently bound azide. Removal of the solvent at −64 °C from CsN3 or N(CH3)4N3 solutions produces yellow (SO2)2N3- salts. Above −64 °C, these salts lose 1 mol of SO2, resulting in white SO2N3- salts that are marginally stable at room temperature and thermally decompose to the corresponding azides and SO2. These anions were characterized by vibrational and 14N NMR spectroscopy and theoretical calculations. Slow loss of the solvent by diffusion through the walls of a sealed Teflon tube containing a sample of CsSO2N3 in SO2 resulted in white and yellowish single crystals that were identified by X-ray diffraction as CsSO2N3·CsSO3N3 with a = 9.542(2) Å, b = 6.2189(14) Å, c = 10.342(2) Å, and β = 114.958(4)° in the monoclinic space group P21/m, Z = 2, and Cs2S2O5·Cs2S2O7·SO2, respectively. Pure CsSO3N3 was also prepared and characterized by vibrational spectroscopy. The S−N bond in SO2N3- is much weaker than that in SO3N3-, resulting in decreased thermal stability, an increase in the S−N bond distance by 0.23 Å, and an increased tendency to undergo rotational disorder. This marked difference is due to SO3 being a much stronger Lewis acid (pF- value of 7.83) than SO2 (pF- value of 3.99), thus forming a stronger S−N bond with the Lewis base N3-. The geometry of the free gaseous SO2N3- anion was calculated at the RHF, MP2, B3LYP, and CCSD(T) levels. The results show that only the correlated methods correctly reproduce the experimentally observed orientation of the SO2 group.