Methanesulfonyl Azide: Molecular Structure and Photolysis in Solid

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Methanesulfonyl Azide: Molecular Structure and Photolysis in Solid Noble Gas Matrices Guohai Deng, Dingqing Li, Zhuang Wu, Hongmin Li, Eduard Bernhardt, and Xiaoqing Zeng J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b05533 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 7, 2016

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The Journal of Physical Chemistry

Methanesulfonyl Azide: Molecular Structure and Photolysis in Solid Noble Gas Matrices Guohai Deng,† Dingqing Li,† Zhuang Wu,† Hongmin Li,† Eduard Bernhardt,‡ and Xiaoqing Zeng†,* †College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 215123 Suzhou, China E-mail: [email protected]; Phone: +86 512 65883583

‡FB C-Anorganische Chemie, Bergische Universität Wuppertal, Gaussstrasse 20, 42119 Wuppertal, Germany

ACS Paragon Plus Environment 1


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ABSTRACT: The parent sulfonyl azide CH3SO2N3 has been characterized in a neat form by IR (gas, matrix-isolation) and Raman (solid) spectroscopy, and its structure has been established by X-ray crystallography. In both gas phase and solid state, the azide exhibits single conformation with the azido ligand being synperiplanar to one of the two S=O groups. In the crystal molecules of CH3SO2N3 are interconnected through three-dimensional O···H-C-H···O hydrogen bonds. Upon an ArF laser (193 nm) photolysis, the azide in solid noble gas matrices splits off N2 and yields the sulfonyl nitrene CH3SO2N in the triplet ground state. Subsequent photolysis with UV light (365 nm) causes the transformation from the nitrene to the pseudo-Curtius rearrangement product CH3NSO2. The identification of the photolysis intermediates by matrix-isolation IR spectroscopy is supported by quantum chemical calculations with DFT methods.


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INTRODUCTION As the parent compound of the synthetically useful sulfonyl azides,1 methanesulfonyl azide CH3SO2N3 has been broadly used in the synthesis of biologically active enaminones, amidines, and amidates.2-4 The first synthesis of CH3SO2N3 dates back to the 1960s by Horner and Christmann,5 who successfully isolated the pure substance as a colorless oil through the reaction of methanesulfonyl chloride with sodium azide in ethanol. Unlike the parent metastable carbonyl azide HC(O)N3,6 methanesulfonyl azide is stable at room temperature and it has a melting point of 18 ºC with a vapor pressure of 1.3 mbar at 41-42 ºC, and it starts decomposition at about 120 ºC.5 Despite being known for half century, methanesulfonyl azide remains barely characterized, as far as we know only its IR and UV absorption spectra in solution have been reported.7,8 This is in sharp contrast to the closely related perfluorinated sulfonyl azides FSO2N39 and CF3SO2N39,10 and sulfonyl diazide O2S(N3)2,11 for which the vibrational spectroscopy (IR and Raman), conformation, and X-ray crystal structures have been already studied from both aspects of experiment and theory. Like the closely related carbonyl azide RC(O)N312 and phosphoryl azide R2P(O)N3,13 sulfonyl azide RSO2N3, in principle, can split off molecular nitrogen and furnish the corresponding α-oxo nitrene RSO2N, which may further undergo pseudo-Curtius rearrangement into N-sulfonyl imine RNSO2. In fact, the decomposition of sulfonyl azide has attracted intensive interest in the past few decades, including the studies on the photolysis of sulfonyl azides by means of conventional product analysis,8 time-resolved spectroscopy (IR and UV/Vis)7,14,15 in solution, matrix-isolation spectroscopy (IR and EPR) in cryogenic matrices16-18 and low-temperature single crystals.19 The formation of the key nitrene intermediate RSO2N has been either inferred by the isolation of the nitrene trapping product or the direct observation by spectroscopy (IR, UV/Vis, and EPR), and a



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triplet ground state has been established. However, the identification of the pseudo-Curtius rearrangement product RNSO2 was mostly found to be inconclusive due to the simultaneous formation of complex nitrene-trapping products or spectral overlapping with those of the azide precursor and nitrene in solution. Nevertheless, the occurrence of pseudo-Curtius rearrangement in perfluorinated sulfonyl nitrenes FSO2N16 and CF3SO2N18 was ascertained by the identification of N-sulfonyl imines RNSO2 (R = F, CF3) by IR spectroscopy with isotope labeling experiments in solid noble gas matrices. The decomposition of the parent molecule CH3SO2N3 has been frequently explored.7,8,19-21 Upon flash vacuum pyrolysis, the azide decomposes into CH3, SO2, CH2NH, O2SN, and N2, during which the sulfonyl nitrene CH3SO2N was assumed to be responsible for the production of CH3 and O2SN through the C–S bond cleavage.21 Photolytically, crystalline CH3SO2N3 at –160 ºC undergoes a stepwise decomposition with the initial formation of the nitrene CH3SO2N, as directly detected by the EPR spectroscopy (|D/hc| = 1.569 cm–1,|E/hc| = 0 cm–1).19 The photolysis of CH3SO2N3 in hydrocarbons and alcohol mainly yields complex formal nitrene trapping products, whereas, no pseudo-Curtius rearrangement product could be isolated.8 Very recently, the photochemistry of CH3SO2N3 was studied by combining ultrafast UV-pumpIR-probe spectroscopy and quantum chemical calculations.7 The results demonstrate the initially generated singlet nitrene CH3SO2N proceeds extremely fast intersystem crossing (ISC) to the persistent triplet state with formation time of 34±3 ps, so that not the singlet but the triplet was observed with an IR signal at 1134 cm–1. However, the expected pseudo-Curtius rearrangement product CH3NSO2, predicted to have similar IR vibration frequency (1157 cm–1) with the triplet nitrene (1155 cm–1), could not be identified either.


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As a continue of our interest in the structure and decomposition of α-oxo azides,16,21-23 herein we report a full spectroscopic and structural characterization of CH3SO2N3 in a neat form, and also its photochemistry in solid noble gas matrices. EXPERIMENTAL SECTION Sample Preparation. Methanesulfonyl azide, CH3SO2N3, was synthesized by the reaction of methanesulfonyl chloride CH3SO2Cl with NaN3 in acetone according to the literature.8 For the purification, the volatile crude product was passed through three successive cold U-traps (–29, – 65, and –196 ºC), and the pure azide was retained in the first trap as colorless crystals. The azide has a vapor pressure of about 0.5 mbar at room temperature, enabling the transfer of small amounts of substance in vacuo by vaporization and condensation. 1-15N sodium azide (98 atom % 15N, EURISO-TOP GmbH) was used for the preparation of 15N labeled sample. The quality of the sample was checked by gas phase IR (Bruker Tensor 27) and Raman spectroscopy (Bruker Equinox 55 FRA 106/S).

Single Crystal Structure Determination (a) Crystal Growth and Mounting. Crystals of CH3SO2N3 were grown in a glass tube (o.d. 0.6 cm, length 15 cm) equipped with a PTFE valve. Briefly, a small amount of the freshly purified sample (ca. 10 mg) was condensed into the upper part of the glass tube at the liquid nitrogen temperature.The tube was then placed into a cold dewar (–60 °C, ethanol bath) while leaving the upper part containing the sample in the air at room temperature. The azide melts and slowly condenses onto the inner surface of the



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lower part of the tube during which the temperature of the dewar increases to –20 °C overnight, and crystals were observed in the bottom of the tube. Due to the potential shock sensitivity of the azide, the cut of the tube and transfer of the crystals needs to be done with care, i.e., the glass tube containing crystals was kept cold in a dry-ice bath (ca. –78 °C) and connected to the vacuum line. Then the tube was slowly filled with argon gas to one atmosphere and carefully cut with an ampoule key, followed by a quick transfer of the crystals into a trough pre-cooled by a flow of cold nitrogen. A suitable crystal of CH3SO2N3 was selected at ca. –20°C under the microscope and mounted as previously described.9

(b) Collection and Reduction of X-ray Diffraction Data. Crystals were centered on an Oxford Diffraction Gemini E Ultra diffractometer, equipped with a 2K × 2K EOS CCD area detector, a four-circle kappa goniometer, an Oxford Instruments Cryojet, and sealed-tube Enhanced (Mo) and the Enhanced Ultra (Cu) sources. For the data collection the Cu source emitting monochromated Cu-Kα radiation (λ = 1.54184 Å) was used. The diffractometer was controlled by the CrysAlisPro Graphical User Interface (GUI) software.24 Diffraction data collection strategy for CH3SO2N3 was optimized with respect to complete coverage and consisted of 10 ω scans with a width of 1°, respectively. The data collection for CH3SO2N3 was carried out at −153 °C, in a 1024 x 1024 pixel mode using 2 x 2 pixel binning. Processing of the raw data, scaling of diffraction data and the application of an empirical absorption correction was completed by using the CrysAlisPro program.24 (c) Solution and Refinement of the Structure. The solutions were obtained by direct methods which located the positions of all atoms. The final refinement was obtained by introducing


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anisotropic thermal parameters and the recommended weightings for all of the atoms except hydrogen. Hydrogen atoms were fixed on idealized positions and refined with isotropic thermal parameters based on the bonded atom. All calculations were performed using the SHELXTLplus package for the structure determination and solution refinement and for the molecular graphics.25 Matrix IR Spectroscopy. Matrix IR spectra were recorded on a FT-IR spectrometer (Bruker 70V) in a reflectance mode using a transfer optic. A KBr beam splitter and liquid nitrogen cooled MCT detector were used in the mid-IR region (4000–650 cm–1). For each spectrum, 200 scans at a resolution of 0.5 cm–1 were co-added. The gaseous sample diluted with noble gas (Ar and Ne, 1 : 1000) was deposited (2 mmol/h) onto the Rh-plated copper block matrix support (2.8 K) in a high vacuum (∼10–6 Pa). Photolysis experiments were performed with ArF excimer laser (193 nm, Gamlaser EX5/250, 6 mJ, 3Hz), Nd3+:YAG laser (266 nm, MPL-F-266, 10 mW), and highpressure mercury arc lamp (TQ 150, Heraeus) by conducting the light through water-cooled quartz lens. Computational Details. Geometry optimizations were performed using DFT methods (B3LYP,26 MPW1PW91,27 BP86,28 M06-2X29) combined with the 6-311++G(3df,3pd) basis set. The complete basis set method CBS-QB330 was used for energy calculation. Time-dependent (TD) DFT (B3LYP/6-311++G(3df,3pd)) calculations31,32 were performed for the prediction of UV-vis transitions. Local minima were confirmed by vibrational frequency analysis. All the calculations were performed using the Gaussian 09 software package.33



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RESULTS AND DISCUSSION Vibrational Spectroscopy of CH3SO2N3 The IR (gas-phase, Ar-matrix) and Raman (solid) spectra of CH3SO2N3 are shown in Figure 1. To aid the assignment, DFT calculations on its vibrational spectra were carried out and the results are summarized in Table 1. Generally, the B3LYP method performs better for the prediction of the IR spectrum than the M06-2X.

Figure 1. Upper trace: IR spectrum of CH3SO2N3 isolated in a Ne-matrix at 2.8 K (absorbance A, resolution: 0.5 cm–1). Middle trace: IR spectrum of gaseous CH3SO2N3 at 300 K (transmission T, resolution: 2 cm–1). Lower trace: Raman spectrum of solid CH3SO2N3 at 77 K (Raman intensity I, resolution: 2 cm–1). Bands associated with H2O are marked with asterisks.


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In the gas-phase IR spectrum, the two sets of bands at 2141/1194 and 1394/1180 cm–1 are assigned to the characteristic assymetric/symmetric stretching vibrations of the N3 and SO2 groups, respectively. They are consistent with the frequencies 2135/1196 and 1373/1172 cm–1 observed in CCl4 solution.7 In Ar-matrix at 2.8 K, the band positions slightly change to 2138.6/1197.9 and 1391.5/1180.0 cm–1. The assignments are supported by both the calculations (Table 1) can also the 15N isotope labeling experiments, in which distinct 14/15N isotope shifts of 27.7 and 7.3 cm–1 were observed for the two N3 stretching vibrations. Whereas, the two bands associated with the SO2 stretches exhibit now distinguishable shift upon 15N labeling. Generally, the frequencies observed for CH3SO2N3 are red-shifted in comparison to those of gaseous FSO2N3 (2161/1181, 1473/1230 cm–1) and CF3SO2N3 (2152/1168, 1440/1220 cm–1),9 in consistent with the order of electronegativity F > CF3 > CH3. Interestingly, in the Raman spectrum of solid CH3SO2N3, the two SO2 stretches occur at 1357 and 1151 cm–1, both are significantly lower than those observed in the gas phase (1394 and 1180 cm–1), solution (1373 and 1172 cm–1), and matrix (1391.5 and 1180.0 cm–1), implying the presence of intermolecular interactions in the solid state (vide infra).



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Table 1. Experimentally observed and calculated vibrational frequencies (cm–1) of CH3SO2N3 Observeda IR(gas phase)b

2141 vs

calculated (IIR)[IRa]f IR(Ar matrix)c

2138.6 vs

d

IR (CCl4)

2135 vs

1423.5 w 1373 s

approx. mode descriptiong

Raman (solid)e

B3LYP

M06-2X

3037 m, sh

3175 (

Methanesulfonyl Azide: Molecular Structure and Photolysis in Solid

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