Photoinduced Sulfur-Nitrogen Bond Rotation and Thermal Nitrogen Inversion in Heterocumulene OSNSO

Article abstract of DOI:10.1021/jacs.7b12622

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 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.

Full text of DOI:10.1021/jacs.7b12622

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Communication
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
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Photoinduced Sulfur-Nitrogen Bond Rotation and Thermal Nitrogen
Inversion in Heterocumulene OSNSO
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Zhuang Wu,† Ruijuan Feng,† Jian Xu,† Yan Lu,† Bo Lu,† Tao Yang,‡ Gernot Frenking,*,‡,& Tarek Traꢀ
belsi,§ Joseph S. Francisco*,§ and Xiaoqing Zeng*,†
†College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 215123 Suzhou, China
‡Fachbereich Chemie, PhilippsꢀUniversität Marburg, Marburg Dꢀ35032, Germany
&Institute of Advanced Synthesis, Nanjing Tech University, 211816 Nanjing, China
§Department of Chemistry, Purdue University, West Lafayette, 47907 Indiana, USA
Supporting Information Placeholder
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 disꢀ
covered,12 and the two switching states in the simplest camphorꢀ
quinone imine have been more recently isolated in solid Arꢀmatrix
(22 K) for characterization with vibrational circular dichroism
(VCD) spectroscopy.13 Similar conformational changes have also
been observed for Nꢀsulfur 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, reꢀ
versible conformational interconversion involving planar nitrogen
inversion with S=N bond remains barely known.
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 higherꢀenergy 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 gunꢀ
powder reaction, the Lassaigne’s test, the Gmelin reaction, atmosꢀ
pheric 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
Continuing our interest in the chemistry of simple S=N bearing
species (e.g., O2SN,15 S2N2,16 and HNSO217), we report herein the
gasꢀphase 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 radiꢀ
cal 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 dissociaꢀ
tion energy of 25 kcal mol–1 (→ SO+ + SO2). By analogy, OSNSO
is expected to have three planar conformers due to the presence of
loneꢀpair electrons on each atom. The calculated molecular strucꢀ
tures and relative energies at the UB3LYP/augꢀccꢀpV(Q+d)ZꢀDK
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 zeroꢀpoint
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 isoꢀ
mer C. The activation barriers for the conformational conversion
of A–C through either S=N bond rotation (TS1 and TS3) or nitroꢀ
gen inversion (TS2) are significantly lower than the sulfurꢀ
nitrogen 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 openꢀchain
OSNSO conformers in the gas phase.
Structurally, NSO is distinct from the quasilinear triatomic pseuꢀ
dohalogens like NNN and NCO. The presence of loneꢀpair elecꢀ
trons at the central sulfur atom results in bent structures in NSO
(125°, CCSD(T)/augꢀccꢀpV5Z)5a and HNSO (120.4°, microwave
spectroscopy).3a The acid molecule prefers a syn conformation
around the S=N bond, and the less stable antiꢀconformer was
identified among the photolysis (> 300 nm) products of synꢀ
HNSO in solid Arꢀmatrix, whereas, no reverse conversion was
observed.6 According to the latest quantum chemical calculation
at the CBSꢀQ level,7 the syn ↔ anti conformational conversion in
HNSO solely involves a planar nitrogen inversion with a calculatꢀ
ed 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 Nꢀalkyl and Nꢀaryl imines.8 In fact, a low barrier
of 9.2 kcal mol–1 for the nitrogen inversion in an Nꢀgermyl imine
1
was determined with lowꢀtemperature H NMR spectroscopy (–
108 – –70 °C).9
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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).
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Figure 1. Calculated potential energy profile with zeroꢀpointꢀ
energy corrections for OSNSO isomers (A–D) at the
UB3LYP/augꢀccꢀpV(Q+d)ZꢀDK level. The relative energies (in
bold) and selected bond lengths (Å) are depicted
.
Gasꢀphase 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, Neꢀmatrix)2 and OCNSO (1140.8 cm–1, Arꢀ
matrix).20
Figure 3. A) IR difference spectrum showing the g→h, f conforꢀ
mational conversion in OSNSO upon yellow light irradiation
(570±20 nm) in N2ꢀmatrix 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 showꢀ
ing 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 shorterꢀ
wavelength green light (532 nm), the corresponding IR spectrum
(Figure 3B) demonstrates the depletion of f and g and the excluꢀ
sive 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 gasꢀphase
generation of f and g and the reversible photoꢀinduced interconꢀ
version with h are fully reproducible when Ar, Ne, or Kr was used
as the carrier gas in the pyrolysis (Figure S1ꢀS3). 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 (N2ꢀmatrix), which
shifts dramatically to 690.2 cm–1 in Arꢀmatrix. 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 Neꢀmatrix
in the range of 1200–1150 cm–1 can not be resolved due to specꢀ
tral overlap (Figure S2). In agreement with the predicted intense
absorptions at around 300 nm (Table S1), OSNSO can be deꢀ
stroyed 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.
Figure 2. A) IR spectrum of CF3S(O)NSO in N2ꢀmatrix at 15.0 K.
B) IR spectrum of the flash vacuum pyrolysis products of
CF3S(O)NSO in N2ꢀmatrix 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/augꢀccꢀpV(Q+d)ZꢀDK level. In line with the experiꢀ
mental observation, the two strongest IR bands for OSNSO locatꢀ
ed at about 1200 cm–1 belong to the stretching vibrations of the
two terminal SO moieties, whereas, the remaining IR fundamenꢀ
tals have significantly lower intensities. Given the calculated elecꢀ
tronic transitions around 500 nm for the OSNSO conformers (Taꢀ
ble S1), the N2ꢀmatrix was irradiated first with yellow light
(570±20 nm), the resulting IR difference spectrum (Figure 3A)
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Table 1. Calculated and observed IR spectra of OSNSO conformers.
syn-syn OSNSO (A) syn-anti OSNSO (B)
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4
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6
7
8
anti-anti OSNSO (C)
calculated
B3LYPa
1181 (125)
1173 (164)
769 (4)
observedb
calculated
Arꢀmatrix B3LYPa
observedb
calculated
B3LYPa
1190 (26)
1179 (342)
827 (18)
703 (36)
367 (0)
observedb
N2ꢀmatrix
N2ꢀmatrix
Arꢀmatrix
1180.5
1154.5
822.3
N2ꢀmatrix
1182.8 (14)
1164.5 (100)
773.6 (7)
Arꢀmatrix
1184.6
1170.4
773.0
1171.6 (65)
1154.3 (100)
1170.5
1157.7
1187 (171) 1185.1 (32)
1167 (186) 1148.8 (100)
853 (13)
639 (11)
525 (8)
831.5 (5)
621.4 (3)
9
639 (5)
640.4 (3)
610.3 (5)
637.8
607.6
621.3
672.5 (3)
690.2
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606 (18)
aCalculated harmonic IR frequencies (> 500 cm–1, unscaled) and intensities (km mol–1, in parentheses) with the augꢀccꢀpV(Q+d)ZꢀDK
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 inꢀphase
(A1) and outꢀofꢀphase (B2) combination of the two S=O stretching
vibrations, which were also observed in the diamagnetic molecule
C2vꢀOSNSNSO (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 C2vꢀOSNSNSO (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 syn-
anti conformer B, the two bands at 1185.1 and 1148.8 cm–1 correꢀ
spond to the stretching vibrations of the two terminal S=O moieꢀ
ties in the syn and anti configurations, respectively.
(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 tꢀBuSO,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 densiꢀ
ty. 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.
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 5. Spin density (isovalue = 0.01) of the OSNSO isomers
(A–C)
at
the
UM06ꢀ2X/def2ꢀTZVPP//UB3LYP/augꢀccꢀ
pV(Q+d)ZꢀDK level. The green and red colors show positive and
negative spin densities, respectively. The calculated Wiberg bond
indices (in italics) are also depicted.
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 SꢀN
bond rotation and thermal planar nitrogen inversion has been obꢀ
served, 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 experꢀ
imentally observed kinetics in solid N2ꢀmatrix.
Figure 4. IR spectra in the range of 1195–1140 cm–1 showing the
h→f conversion in N2ꢀmatrix 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 temperaꢀ
ture range of 26.5–29.0 K (Figures S5ꢀS10). 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 halfꢀlife (t1/2) of 3.6 hours. Accordingly, an
activation barrier of 1.18±0.07 kcal mol–1 in N2ꢀmatrix is estabꢀ
lished, which is in good agreement with the theoretically calculatꢀ
ed 1.3 kcal mol–1 for the planar nitrogen inversion from h to f
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
*xqzeng@suda.edu.cn; francisc@purdue.edu;
frenking@chemie.uniꢀmarburg.de.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
XQZ acknowledges the National Natural Science Foundation of
China (21422304 and 21673147) and the Priority Academic Proꢀ
gram 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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