Photoinduced Sulfur–Nitrogen Bond Rotation and Thermal Nitrogen

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Photoinduced Sulfur-Nitrogen Bond Rotation and Thermal Nitrogen Inversion in Heterocumulene OSNSO Zhuang Wu, Ruijuan Feng, Jian Xu, Yan Lu, Bo Lu, Tao Yang, Gernot Frenking, Tarek Trabelsi, Joseph S. Francisco, and Xiaoqing Zeng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12622 • Publication Date (Web): 06 Jan 2018 Downloaded from http://pubs.acs.org on January 6, 2018

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Photoinduced SulfurSulfur-Nitrogen Bond Rotation and Thermal Ni Nitrogen Inversion in Heterocumulene OSNSO OSNSO Zhuang Wu,† Ruijuan Feng,† Jian Xu,† Yan Lu,† Bo Lu,† Tao Yang,‡ Gernot Frenking,*,‡,& Tarek Trabelsi,§ Joseph S. Francisco*,§ and Xiaoqing Zeng*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 215123 Suzhou, China Fachbereich Chemie, Philipps-Universität Marburg, Marburg D-35032, Germany & Institute of Advanced Synthesis, Nanjing Tech University, 211816 Nanjing, China § Department of Chemistry, Purdue University, West Lafayette, 47907 Indiana, USA ‡

Supporting Information Placeholder ABSTRACT: An exotic ternary S, N, O heterocumulene OSNSO in syn-syn (A) and syn-anti (B) conformations has been generated in the gas phase through flash vacuum pyrolysis of CF3S(O)NSO at 700 K. Upon visible light irradiation (570±20 or 532 nm), both A and B, isolated in cryogenic matrices (N2, Ne, Ar, and Kr, < 30 K), convert to a higher-energy anti-anti conformer (C). The reverse conformational transformation occurs either through S=N bond rotation (C to A and B) under the visible light irradiation (400±20 nm) at 2.8 K or through thermal nitrogen inversion (C to A) in the temperature range of 20–30 K, for which an exceptionally low activation barrier of 1.18±0.07 kcal mol–1 has been experimentally determined.

Ternary S, N, O species are of growing interest due to their broad importance in chemistry and biology, ranging from gunpowder reaction, the Lassaigne’s test, the Gmelin reaction, atmospheric chemistry, and cell biology.1 For instance, the simplest NSO is a pseudohalogen radical that has been generated in the gas phase and spectroscopically characterized.2 The labile acid HNSO has been well studied in the gas phase,3 and its trapping adduct by Lewis acid B(C6F5)3 in the solid state has been recently isolated and structurally characterized.4 Upon UV light irradiation, NSO converts to a potent interstellar species SNO.2,5 Structurally, NSO is distinct from the quasilinear triatomic pseudohalogens like NNN and NCO. The presence of lone-pair electrons at the central sulfur atom results in bent structures in NSO (125°, CCSD(T)/aug-cc-pV5Z)5a and HNSO (120.4°, microwave spectroscopy).3a The acid molecule prefers a syn conformation around the S=N bond, and the less stable anti-conformer was identified among the photolysis (> 300 nm) products of synHNSO in solid Ar-matrix, whereas, no reverse conversion was observed.6 According to the latest quantum chemical calculation at the CBS-Q level,7 the syn ↔ anti conformational conversion in HNSO solely involves a planar nitrogen inversion with a calculated barrier of 13.2 kcal mol–1. This barrier is significantly lower than the typical nitrogen inversion barriers (20–30 kcal mol–1) found for simple N-alkyl and N-aryl imines.8 In fact, a low barrier of 9.2 kcal mol–1 for the nitrogen inversion in an N-germyl imine was determined with low-temperature 1H NMR spectroscopy (– 108 – –70 °C).9

Conformational interconversion via planar nitrogen inversion or bond rotation in imines (C=N)10 and azobenzene derivatives (N=N)11 has been extensively studied due to potential applications as molecular machines driven by heat or light. For examples, the thermal nitrogen inversion and directional photoinduced C=N bond rotation in camphorquinone imines have been recently discovered,12 and the two switching states in the simplest camphorquinone imine have been more recently isolated in solid Ar-matrix (22 K) for characterization with vibrational circular dichroism (VCD) spectroscopy.13 Similar conformational changes have also been observed for N-sulfur binding imines R2C=N–S(O)nR (n = 1 or 2), in which activation barriers of ca. 20 kcal mol–1 for the thermal planar nitrogen inversion were obtained.14 However, reversible conformational interconversion involving planar nitrogen inversion with S=N bond remains barely known. Continuing our interest in the chemistry of simple S=N bearing species (e.g., O2SN,15 S2N2,16 and HNSO217), we report herein the gas-phase generation and characterization of a highly delocalized heterocumulene radical OSNSO in three conformations, for which interconversion involving explicit S=N bond rotation and nitrogen inversion has been observed in cryogenic matrices. Formally, OSNSO can be regarded as an adduct of the NSO radical and the diatomic molecule SO. It is isoelectronic with the planar sulfur oxide cation OSOSO+,18 which was detected in the gas phase by mass spectrometry with an estimated bond dissociation energy of 25 kcal mol–1 (→ SO+ + SO2). By analogy, OSNSO is expected to have three planar conformers due to the presence of lone-pair electrons on each atom. The calculated molecular structures and relative energies at the UB3LYP/aug-cc-pV(Q+d)Z-DK level for the three conformers (A–C) and an additional cyclic isomer (D) are shown in Figure 1. The energetically lowest lying syn-syn conformer A, which is, after correction for zero-point energy contribution, 0.8 kcal mol–1 more stable than the syn-anti form B and 5.8 kcal mol–1 lower in energy than the anti-anti isomer C. The activation barriers for the conformational conversion of A–C through either S=N bond rotation (TS1 and TS3) or nitrogen inversion (TS2) are significantly lower than the sulfurnitrogen bond dissociation energy (A → OSN + SO, 43.3 kcal mol–1). Additionally, a highest lying cyclic isomer D was also located, which represents an intermediate in the decomposition of conformer B to SN + SO2. The associated large barrier (TS5) of 49.4 kcal mol–1 indicates thermal persistence of the open-chain OSNSO conformers in the gas phase.

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reflects the depletion of the IR bands (g) at 1185.1, 1148.8, 831.5, and 621.4 cm–1, IR bands (f) at 1171.6 and 1154.3 cm–1 and new IR bands (h) at 1182.8, 1164.5, 773.6, and 672.5 cm–1 appear. It should be noted that only the first two sets of IR bands (f and g) are present in the IR spectrum of the FVP products of CF3S(O)NSO (Figure 2B).

Figure 1. Calculated potential energy profile with zero-pointenergy corrections for OSNSO isomers (A–D) at the UB3LYP/aug-cc-pV(Q+d)Z-DK level. The relative energies (in bold) and selected bond lengths (Å) are depicted.

Gas-phase generation of OSNSO was carried out through flash vacuum pyrolysis (FVP) of sulfoxide CF3S(O)NSO19 at 700 K. In the IR spectrum of the matrix isolated pyrolysis products (Figure 2B), the IR bands of SO2 (b, 1351.0 and 1152.7 cm–1), CF3 (c, 1248.7, 1083.6, and 700.9 cm–1), OCF2 (d, 1940.1, 1248.7, 1912.6, 965.1, and 775.6 cm–1), C2F6 (e, 1242.7 and 1111.1 cm–1) were identified.20 In addition, a few new bands (f and g) in the range of 1200–1150 cm–1 appear, which can be associated with NSO stretching vibrations by comparing with the IR spectra of OSN (1199.3 cm–1, Ne-matrix)2 and OCNSO (1140.8 cm–1, Armatrix).20

Figure 2. A) IR spectrum of CF3S(O)NSO in N2-matrix at 15.0 K. B) IR spectrum of the flash vacuum pyrolysis products of CF3S(O)NSO in N2-matrix at 15.0 K. To aid the assignment, IR frequency calculations for the most likely candidate species OSNSO (Table 1), forming from the C–S bond fragmentation in CF3S(O)NSO, were performed at the UB3LYP/aug-cc-pV(Q+d)Z-DK level. In line with the experimental observation, the two strongest IR bands for OSNSO located at about 1200 cm–1 belong to the stretching vibrations of the two terminal SO moieties, whereas, the remaining IR fundamentals have significantly lower intensities. Given the calculated electronic transitions around 500 nm for the OSNSO conformers (Table S1), the N2-matrix was irradiated first with yellow light (570±20 nm), the resulting IR difference spectrum (Figure 3A)

Figure 3. A) IR difference spectrum showing the g→h, f conformational conversion in OSNSO upon yellow light irradiation (570±20 nm) in N2-matrix at 15.0 K. B) IR difference spectrum showing the g, f→h conformational conversion in OSNSO upon green light irradiation (532 nm). C) IR difference spectrum showing the h→f, g conformational conversion in OSNSO upon purple light irradiation (400±20 nm). The bands of perturbed CF3S(O)NSO (a) and CF3 (c) in matrix are also labeled. When the irradiation was subsequently changed to a shorterwavelength green light (532 nm), the corresponding IR spectrum (Figure 3B) demonstrates the depletion of f and g and the exclusive formation of h. Further purple light irradiation (400±20 nm) leads to the reverse conversion from h to f and g (Figure 3C). The IR bands of CF3S(O)NSO (a) and CF3 (c) are also perturbed due to changes of the matrix sites upon irradiation. The gas-phase generation of f and g and the reversible photo-induced interconversion with h are fully reproducible when Ar, Ne, or Kr was used as the carrier gas in the pyrolysis (Figure S1-S3). All the IR bands of the three species in difference matrices vary slightly (Table 1), except the very weak one for h at 672.5 cm–1 (N2-matrix), which shifts dramatically to 690.2 cm–1 in Ar-matrix. These shifts imply interactions of the carriers with the surrounding molecules in solid matrix cages. In contrast, the IR bands of f, g, and h in Ne-matrix in the range of 1200–1150 cm–1 can not be resolved due to spectral overlap (Figure S2). In agreement with the predicted intense absorptions at around 300 nm (Table S1), OSNSO can be destroyed by 266 nm laser irradiation, SN and SO2 formed (Figure S4). The conclusion is that the signals that are assigned to f, g and h identify the species as the three conformations A, B and C of OSNSO, respectively.

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Journal of the American Chemical Society Table 1. Calculated and observed IR spectra of OSNSO conformers. syn-syn OSNSO (A)

syn-anti OSNSO (B) b

calculated

observed

B3LYPa

N2-matrix

1181 (125) 1173 (164)

anti-anti OSNSO (C) b

observed

Ar-matrix

B3LYPa

N2-matrix

Ar-matrix

B3LYPa

N2-matrix

Ar-matrix

1171.6 (65)

1170.5

1187 (171)

1185.1 (32)

1180.5

1190 (26)

1182.8 (14)

1184.6

1154.3 (100)

1157.7

1167 (186)

1148.8 (100)

1154.5

1179 (342)

1164.5 (100)

1170.4

853 (13)

831.5 (5)

822.3

827 (18)

773.6 (7)

773.0

639 (5)

640.4 (3)

637.8

639 (11)

621.4 (3)

621.3

703 (36)

672.5 (3)

690.2

606 (18)

610.3 (5)

607.6

525 (8)

769 (4)

calculated

observedb

calculated

367 (0)

a

Calculated harmonic IR frequencies (> 500 cm–1, unscaled) and intensities (km mol–1, in parentheses) with the aug-cc-pV(Q+d)Z-DK basis set; Full list of the calculated data is given in Table S2. bObserved band positions for the most intense sites in matrices at 15.0 K, and relative integrated band intensities in parentheses.

The first two strongest IR bands at about 1200 cm–1 in the two C2v symmetric conformers A and C mainly correspond to the in-phase (A1) and out-of-phase (B2) combination of the two S=O stretching vibrations, which were also observed in the diamagnetic molecule C2v-OSNSNSO (1185 and 1150 cm–1, IR spectroscopy).21 The two weaker ones (Table 1) belong to the asymmetric (B2) and symmetric (A1) SNS stretching vibrations, respectively, they are close to those found in C2v-OSNSNSO (684 and 656 cm–1),21 SSNSO– (815 and 704 cm–1, Raman spectroscopy),22 and SSNSS– (893 and 711 cm–1, IR spectroscopy).23 In the Cs symmetric synanti conformer B, the two bands at 1185.1 and 1148.8 cm–1 correspond to the stretching vibrations of the two terminal S=O moieties in the syn and anti configurations, respectively. The transformation from h to f and g also occurs when the matrix is exposed to the irradiation from the IR spectrometer even at 2.8 K. However, selective h → f conversion occurs slowly at about 25 K while keeping the matrix in the dark with also strict exclusion of the irradiation from the IR spectrometer (Figure 4).

(Figure 1). Thermal nitrogen inversion in h also happens in Ar and Kr matrices at about 20 K. However, no reproducible kinetics could be obtained in these matrices due to overlap of the IR bands. The bonding situation of the heterocumulene OSNSO was studied with the natural bond orbital (NBO)24 analysis. The spin density in each conformer (A–C) is delocalized over the entire molecule (Figure 5). Similar to other sulfinyl radicals CF3SO,25 CH3SO,26 and t-BuSO,27 the positive spin density of the unpaired electron is almost equally delocalized on the oxygen and sulfur atoms, whereas, the central nitrogen atoms carry the negative spin density. In line with the electronegativity, the nitrogen atom (~ –1.0e) and the terminal oxygen atoms carry negative partial charges (~ – 0.8e) while the sulfur atoms are positively charged (~ +1.3e). Consistent with the distribution of the spin density, the calculated Wiberg bond indices (WBI) suggest that all bonds have some multiple bond characters; hence, the π bonding in the molecules is highly delocalized.

Figure 5. Spin density (isovalue = 0.01) of the OSNSO isomers (A–C) at the UM06-2X/def2-TZVPP//UB3LYP/aug-ccpV(Q+d)Z-DK level. The green and red colors show positive and negative spin densities, respectively. The calculated Wiberg bond indices (in italics) are also depicted.

Figure 4. IR spectra in the range of 1195–1140 cm–1 showing the h→f conversion in N2-matrix by standing in the dark over the course of 32 minutes at 27.5 K. To obtain reliable kinetics for the thermal conversion, only five scans were used for recording each IR spectrum in the temperature range of 26.5–29.0 K (Figures S5-S10). For instance, a rate constant of (5.38±0.13)×10–5 s–1 was obtained in solid N2 at 27.5 K, corresponding to a half-life (t1/2) of 3.6 hours. Accordingly, an activation barrier of 1.18±0.07 kcal mol–1 in N2-matrix is established, which is in good agreement with the theoretically calculated 1.3 kcal mol–1 for the planar nitrogen inversion from h to f

In summary, a highly delocalized heterocumulene radical OSNSO in three conformations (syn-syn, syn-anti, and anti-anti) has been generated and characterized in cryogenic matrices. The multistep conformational interconversion in OSNSO via photoinduced S-N bond rotation and thermal planar nitrogen inversion has been observed, and an activation of 1.18±0.07 kcal mol–1 was obtained for the latter process (anti-anti to syn-syn conversion) from the experimentally observed kinetics in solid N2-matrix.

ASSOCIATED CONTENT Supporting Information Experimental details, calculation methods, matrix IR spectra, and calculated results of OSNSO isomers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected]; [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT XQZ acknowledges the National Natural Science Foundation of China (21422304 and 21673147) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). GF gratefully acknowledges financial support from by Nanjing Tech University. TY is grateful to the Alexander von Humboldt foundation for a postdoctoral fellowship.

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