NEW SYNTHESIS, CRYSTAL-STRUCTURE, AND VIBRATIONAL-SPECTRA OF TETRAMETHYLAMMONIUM AZIDE AND REACTIONS OF THE FLUORIDE ANION WITH HN3 AND OF THE AZIDE ANION WITH HF

Tetramethylammonium azide, N(CH3)4+N3-, was obtained in high purity and quantitative yield by the reaction of N(CH3)4+F- with Si(CH3)3N3 in CH3CN solution. This compound is isostructural with N(CH3)4+HF2- and crystallizes in the orthorhombic system: space group, Pmn2(1) (No. 31); a = 6.879 (5) angstrom; b = 5.479 (4) angstrom; c = 8.858 (7) angstrom; Z = 2; R(F) = 0.0388. Its N3- anion is symmetric and linear and the N(CH3)4+ cation is somewhat distorted from tetrahedral symmetry due to crystal packing effects. The infrared and Raman spectra of N(CH3)4+N3- were also recorded. The symmetric stretching mode of N3- exhibits the pronounced frequency decrease expected for increasing ionicity with increasing cation size. A study of the HN3-MF systems [M = Na, K, Rb, Cs, N(CH3)4] revealed some unexpected chemistry. Even at -80-degrees-C, HN3 displaces F- from MF with formation of equimolar amounts of M+N3- and HF. The latter reacts with MF to give M+HF2-. On the other hand, HF quantitatively displaces N3- from MN3 with formation of HN3 and M+HF2-. This apparent discrepancy can be explained by the vast difference in basicity between F- and HF2-. Attempts to isolate stable F-H-N3- or N3-H-N3- anions from HN3 and either MF or MN3, respectively, were unsuccessful.