Synthesis of the Long Sought After Compound Pentafluoronitrosulfane, SF5NO2
2005-12-30

Call off the search: Photolysis of a mixture of SF5Br and NO2 results in the synthesis of gram quantities of pentafluoronitrosulfane, SF5NO2 (see picture), whose preliminary structure was obtained by gas-phase electron diffraction studies and reveals the longest SVIN bond reported (1.903(7) Å). The chemistry of SF5-containing molecules1–3 is no longer just of interest to sulfur and fluorine chemists. Recently, this class of compounds has become a very important issue in the area of atmospheric chemistry. According to a recent report, more than 4000 tons of the super greenhouse gas SF5CF3 are present in the stratosphere.4–6 In addition, scientists have also proposed the use of molecules such as SF5CF3, SF6, fluoroalkanes, and so on to terraform Mars.7 Two methods were used to prepare SF5NO2, starting either from the novel amine, (SF5)3N (method A), or from SF5Br (method B).10 Owing to its elongated and weak NS bonds, (SF5)3N readily forms (SF5)2N11, 12 and SF5 radicals, which in turn react with NO2 (or with NO2Cl) at room temperature to generate SF5NO2. By method A, SF5NO2 was prepared for the first time by taking advantage of the weak NS bonds in (SF5)3N in its reaction with NO2.13 The 19F NMR spectrum of the SF5NO2 thus obtained displayed a typical AB4 pattern, which is characteristic of the SF5 group. Furthermore, the IR spectrum of SF5NO2 showed the diagnostic stretches and bends for both the SF5 and NO2 moieties. The SF5 group usually displays three strong vibrational peaks below 1000 cm−1; in SF5NO2, these appeared at 908, 801, and 594 cm−1. The two NO2 stretching bands in SF5NO2 were unambiguously assigned as as NO2=1654 cm−1 and s NO2=1303 cm−1, respectively. The 19F NMR spectrum of SF5NO2 revealed an AB4 pattern. The simulated NMR data are δ=46.79 ppm (Fax), δ=43.02 ppm (Feq), and coupling constant 2J(Fax–Feq)=144.3 Hz. In addition, the 14N NMR spectrum of SF5NO2 was recorded and is shown in Figure 1. The spectrum does not reveal perfect quintet splitting because of the quadrupolar effects of the 14N nucleus (14N NMR (versus external reference NO3− at δ=383 ppm): δ=283.8 ppm; 2J(F–N)=8 Hz). 14N NMR spectrum of SF5NO2. The IR spectrum of SF5NO2 is shown in Figure 2. In Table 1, the as NO2 and s NO2 vibrational frequencies of NO2 are compared to XNO2 (X=F, CF3, and SF5). As a result of the inductive effects of the strong electron-withdrawing substituents (F, CF3, and SF5), the as NO2 and s NO2 stretching frequencies of these three compounds are all shifted to higher frequencies relative to those for the NO2 molecule. IR spectrum of SF5NO2. Compound as NO2 [cm−1] s NO2 [cm−1] δ NO2 [cm−1] NO2 1613 1261 751 FNO216 1792 1310 822 SF5NO2[b] (this work) 1654 1303 801 CF3NO217, 18 1627 1310 751 A mass spectrum of SF5NO2 was obtained and shows the fragmentation pattern, m/z 127 (SF5+, 100 %), 108 (SF4+, 6.7 %), 89 (SF3+, 51.0 %), 81 (SFNO+, 1.5 %), 70 (SF2+, 11.5 %), 64 (SO2+, 12.1 %), 51 (SF+, 5.9 %), 46 (NO2+, 69.3 %). Although the molecular ion was not observed, the mass spectrum did reveal fragments at m/z 46 and 127 that indicate the presence of NO2+ and SF5+, respectively. In addition, the peak at m/z 81, which corresponds to SFNO+, provided evidence that the two previously mentioned fragments are originally bonded together. By gas density measurements, the relative molecular mass was determined as Mr=173.0±0.5. The equation derived from the vapor pressure curve is ln(p/po)=−3788/T + 13.33. The normal extrapolated boiling point for SF5NO2 was determined as 9 °C, its heat of vaporization is approximately 29.3 kJ mol−1, and it melts at −78±2 °C. Thermal studies of SF5NO2 indicate that it mainly decomposes to SOF4 and FNO at a rate of 3 % a day at room temperature, but when heated at 80 °C total decomposition took place within minutes. A gas-phase electron diffraction study of SF5NO2 was carried out. The preliminary data, as shown in Figure 3, reveal the longest reported SVIN bond at 1.903(7) Å, which is some 0.2 Å longer than a normal SVIN single bond. Quantum chemical calculations (HF/6-31 G* and B3LYP/6-311+G*) predict rather different values for the SVIN bond from 1.844 Å to 2.049 Å. Further details of this structure19 will be reported at a later date. The structure of SF5NO2 obtained from gas-phase electron diffraction studies. The goal of this research was to synthesize both SF5NO2 and SF5NO. Here, SF5NO2 was successfully prepared by two independent methods but SF5NO remains unknown. Of the two routes used to prepare SF5NO2, one employed the novel amine (SF5)3N as a starting material while the other started from SF5Br. Because (SF5)3N is extremely difficult to prepare, the second route was the preferred method for preparing gram quantities of SF5NO2. Along the way, a modified procedure for preparing SF5Br on a 500-gram scale was developed. The success of the photochemical preparative method for SF5NO2 is based on the use of blue light from a diazo lamp (λmax=420 nm), as the molecule NO2 photodissociates at wavelengths shorter than 395 nm as associated with the use of a mercury immersion lamp. The former source of irradiation excites the NO2 molecule, and the excited NO2 molecule is believed to further participate in the formation of SF5 radicals. The molecule SF5NO2 has also been studied as a 15N-labeled compound. The IR spectrum of SF515NO2 shows the expected mass effects of the 15N isotope. The 15N NMR spectrum of SF515NO2 shows a clear quintet splitting (15N NMR (vs external reference NO3− at δ=383 ppm): δ=283.4 ppm (quintet), 2J(F–N)=11.6 Hz), and its 19F NMR spectrum shows additional multiplicities in the equatorial fluorines (AB4X spin system). The structural data obtained from gas-phase electron diffraction studies indicate the longest SVI-N single bond reported. Further details concerning this study will appear in due course. Preparation of SF5NO2: Method A: The amine (SF5)3N (0.36 g, 0.90 mmol) was transferred under vacuum into a fluorinated ethylene propylene (FEP) tube equipped with a metal valve. Nitrogen dioxide (0.12 g, 2.60 mmol) was then condensed into the FEP tube at −196 °C, and the reaction vessel was allowed to gradually warm to room temperature. After 4 h, all of the (SF5)3N crystals had disappeared. The volatile products were subjected to a series of distillations through −105, −130, and −196 °C traps. The −130 °C trap stopped the crude SF5NO2. The percentage yield was not calculated owing to the difficult purification of the product. Method B: SF5Br (3.9 g, 18.8 mmol) and NO2 (0.9 g, 19.6 mmol) were transferred to a 4-L or 20-L pyrex reactor. A photolysis chamber with 12 diazo lamps (TL40W/03; each 40 W, 48 inches (ca. 122 cm) long) was used to photolyze this mixture. After 12 h irradiation, the resulting products were condensed into a 300-mL stainless-steel cylinder held at −196 °C. This cylinder was then warmed to dry-ice temperature (−78 °C), and all the materials that are volatile at this temperature were then transferred under vacuum into another cylinder cooled to −196 °C containing 400 grams (large excess) of CsF, which was used to easily remove Br2, SOF4, and SF4 and also to convert NO2 into FNO. Then, a trap-to-trap distillation through traps at −78 °C, −130 °C, and −196 °C was carried out to separate SF5NO2 from impurities such as FNO and SF6. The product SF5NO2 (0.09 g, 0.56 mmol; 3 % yield) was recovered in the trap at −130 °C.