The highly explosive molecules As(N(3))(3) and Sb(N(3))(3) were obtained in pure form by the reactions of the corresponding fluorides with (CH(3))(3)SiN(3) in SO(2) and purification by sublimation. The crystal structures and (14)N NMR, infrared, and Raman spectra were determined, and the results compared to ab initio second-order perturbation theory calculations. Whereas Sb(N(3))(3) possesses a propeller-shaped, pyramidal structure with perfect C(3) symmetry, the As(N(3))(3) molecule is significantly distorted from C(3) symmetry due to crystal packing effects.
The previous claim for the first generation of the pentazolate anion in solution was carefully reexamined; no evidence for the formation of cyclo-(N5-) was found under the reported conditions.
[C(6)F(5)Xe][AsF(6)] was prepared by metathesis from [C(6)F(5)Xe][(C(6)F(5))(2)BF(2)]. The thermal stability of the melt (=125 degrees C) is surprisingly high. The decomposition products reveal the ability of the cation to effect electrophilic pentafluorophenylation. [C(6)F(5)Xe][AsF(6)] crystallizes in the triclinic system, space group P&onemacr;, with four molecules in the unit cell. Of these, two are symmetry independent with Xe-C distances of 2.079(6) and 2.082(5) Å, Xe-F distances (cation-anion contacts) of 2.714(5) and 2.672(5) Å, and C-Xe-F angles of 170.5(3) and 174.2(3) degrees, respectively. The relation between cations and anions is best described as an asymmetric hypervalent (3c-4e) bond. Temperature dependent (19)F NMR measurements reveal the occurrence of separated ions in solution, with [C(6)F(5)Xe](+) coordinated by a basic solvent molecule. Minimum energy geometries and charge distributions were calculated for [C(6)F(5)Xe](+), [C(6)H(5)Xe](+), [C(6)F(5)](+), [C(6)H(5)](+), [CF(3)Xe](+), [CH(3)Xe](+), [C(6)F(5)Ng](+) (Ng = Kr, Ar, Ne, He), and [C(6)F(5)Xe][AsF(6)] at the ab initio RHF/LANL2DZ level. According to these calculations, C-Ng cations with short C-Ng distances are stable when the natural charge of the noble gas carries the main part of the positive net-charge and the ipso-C atom is not positive. In [C(6)F(5)Xe](+), for example, 89% of the positive charge is concentrated on Xe.
The azido group is highly energetic and adds about 70 kcalmol À1 to the energy content of a molecule. It is, therefore, not surprising that polyazides are highly endothermic compounds, and that their energy content increases with an increasing number of azido ligands. Compared to the relatively stable azide anion, which possesses two double bonds, the bonds in covalent azides are polarized towards a single and a triple bond, which greatly facilitates N 2 elimination and enhances their shock sensitivity.Consequently, the synthesis and characterization of covalent binary azides containing multiple azido ligands can present great experimental challenges, and binary tellurium azides are no exception to this general rule.Whereas numerous, partially azide-substituted tellurium compounds have been reported, [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Removal of the volatile products (CH 3 CN, (CH 3 ) 3 SiF, and excess (CH 3 ) 3 SiN 3 ) in vacuo results in the isolation of Te(N 3 ) 4 as a bright-yellow solid. In the absence of CsF, no fluorideazide exchange reaction was observed even after several days at room temperature. The catalytic function of the fluoride ion in these reactions probably involves the generation of intermediate free azide ions from the reaction of (CH 3 ) 3 SiN 3 and F À ions, and these free azide ions might be the actual reagent. The need for fluoride-ion catalysis, found in our study, is in contrast to the results from a previous 19 F NMR study [9] in which TeF 6 was reported to undergo facile fluorideazide exchange with (CH 3 ) 3 SiN 3 in CD 3 CN solution to produce all the members of the TeF n (N 3 ) 6Àn (n = 1-5) series. In their study, these authors also observed the azide-ionmediated reduction of Te VI to Te IV as a side reaction. As expected for a highly endothermic, binary covalent polyazide, Te(N 3 ) 4 is very sensitive and can explode violently. [12] Furthermore, the presence of covalent azido ligands [11][12][13][14][15][16] is confirmed by the observed 14 N NMR shifts of d = À139.8 ppm (N b , Dn 1/2 = 63 Hz) and À238 ppm (N g , Dn 1/2 = 680 Hz) in DMSO solution at 25 8C. In addition to quadrupolar broadening, rapid ligand exchange on the NMR timescale by the Berry pseudorotation mechanism [17] might contribute to the observation of only one set of azide signals and the relative broadness of the N b resonance.The observed Raman spectrum of solid Te(N 3 ) 4 is shown in Figure 1, and the observed infrared and Raman frequencies and intensities are listed in the Experimental Section. Assignments of the observed spectra were made by comparison with those calculated at the B3LYPSBKJC + (d) level of theory. [18] These calculations resulted in two minimum-energy structures of C 2 symmetry (Figure 2 a and b). Both structures are derived from pseudo trigonal bipyramids, in which one of the equatorial positions is occupied by the sterically active free valence-electron pair of the tellurium center. The two structures differ in energy by only 1.8 kcal mol À1 . Their main structural difference is ...
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