Spectroscopic Characterization of Nicotinoyl and Isonicotinoyl Nitrenes and the Photointerconversion of 4-Pyridylnitrene with Diazacycloheptatetraene

Article abstract of DOI:10.1021/acs.jpca.9b01680

Recently, nicotinoyl nitrene (2) has been generated from the photodecomposition of nicotinoyl azide (1) and used as the key intermediate in probing nucleobase solvent accessibility inside cells. Following the 266 nm laser photolysis of nicotinoyl azide (1) and isonicotinoyl azide (5) in solid N₂ matrices at 15 K, nicotinoyl nitrene (2) and isonicotinoyl nitrene (6) have now been identified by matrix-isolation infrared (IR) spectroscopy. Both aroyl nitrenes 2 and 6 adopt closed-shell singlet ground states stabilized by significant N_nitrene···O interactions, which is consistent with the spectroscopic analysis and calculations at the CBS-QB3 level of theory. Upon subsequent visible light irradiations, 2 (400 ± 20 nm) and 6 (532 nm) undergo rearrangement to pyridyl isocyanates 3 and 7. Further dissociation of 3 and 7 under 193 nm laser irradiation results in CO elimination and formation of ketenimines 12 and 13 via the ring opening of elusive pyridyl nitrenes 4 and 8, respectively. In addition to the IR spectroscopic identification of 8 in the triplet ground state, its reversible photointerconversion with ring expansion to diazacycloheptatetraene 9 has been observed directly. The spectroscopic identification of the nitrene intermediates was aided by calculations at the B3LYP/6-311++G(3df,3pd) level, and the mechanism for their generation in stepwise decompositions of the azides is discussed in the light of CBS-QB3 calculations.

Full text of DOI:10.1021/acs.jpca.9b01680

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
Spectroscopic Characterization of Nicotinoyl and Isonicotinoyl Nitrenes and
the Photointerconversion of 4-Pyridylnitrene with Diazacycloheptatetraene
Qian Liu, Yuanyuan Qin, Yan Lu, Curt Wentrup, and Xiaoqing Zeng
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b01680 • Publication Date (Web): 12 Apr 2019
Downloaded from http://pubs.acs.org on April 13, 2019
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Spectroscopic Characterization of Nicotinoyl and Isonicotinoyl
Nitrenes and the Photointerconversion of 4-Pyridylnitrene with
Diazacycloheptatetraene
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Qian Liu,† Yuanyuan Qin,† Yan Lu,† Curt Wentrup,*,‡ Xiaoqing Zeng*,†
†College of Chemistry, Chemical Engineering and Materials Science, Soochow University,
215123 Suzhou, China
E-mail: xqzeng@suda.edu.cn
‡School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane,
Queensland 4072, Australia
E-mail: wentrup@uq.edu.au
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Abstract: Recently, nicotinoyl nitrene (2) has been generated from the photodecomposition of
nicotinoyl azide (1) and used as the key intermediate in probing nucleobase solvent accessibility
inside cells. Following the 266 nm laser photolysis of nicotinoyl azide (1) and isonicotinoyl azide
(5) in solid N2-matrices at 15 K, nicotinoyl nitrene (2) and isonicotinoyl nitrene (6) have now been
identified by matrix-isolation IR spectroscopy. Both aroyl nitrenes 2 and 6 adopt closed-shell
singlet ground states stabilized by significant Nnitrene···O interactions, consistent with the
spectroscopic analysis and calculations at the CBS-QB3 level of theory. Upon subsequent
visible-light irradiations, 2 (400 ± 20 nm) and 6 (532 nm) undergo rearrangement to pyridyl
isocyanates 3 and 7. Further dissociation of 3 and 7 under 193 nm laser irradiation results in
CO-elimination and formation of ketenimines 12 and 13 via the ring-opening of elusive pyridyl
nitrenes 4 and 8, respectively. In addition to the IR spectroscopic identification of 8 in the triplet
ground state, its reversible photointerconversion with ring-expansion to diazacycloheptatetraene 9
has been observed directly. The spectroscopic identification of the nitrene intermediates was aided
by calculations at the B3LYP/6-311++G(3df,3pd) level, and the mechanism for their generation in
stepwise decompositions of the azides is discussed in the light of CBS-QB3 calculations.
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Introduction
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Nitrenes (R–N) are highly reactive intermediates that have been fascinating chemists for
decades.1-4 As versatile reagents, nitrenes can be easily generated through either thermal,
photolytic, or catalytic decomposition of their corresponding azide precursors (R–N3). They have
been broadly used in diverse chemical transformations such as C-H amination,5,6 intramolecular
insertion for the synthesis of biologically active N-heterocycles such as azirinobenzoxazole and
benzoxazines,7 and transition-mental catalyzed synthesis of nitrogen-containing compounds.8-10
Transient nitrenes have also been used in functionalization of carbon nanomaterials11,12 and
cross-coupling of polymers.13
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Acyl nitrenes (RC(O)–N), the key intermediates in the stepwise Curtius-rearrangement of acyl
azides (RC(O)–N3),14 are also useful intermediates in organic synthesis and biology, for examples,
benzoyl nitrene (PhC(O)N), a parent aroylnitrene, has been used for photocatalyzed C–H
amidation of electron rich heteroarenes.15 In biological systems, aroylnitrenes can be quickly
protonated by solvent to form strong electrophilic and carcinogenic aroyl nitrenium ions.16 Aside
from the applications of acyl nitrenes, their spectroscopy, molecular structures, ground state
multiplicities, and rearrangement reactions have also been the focus of numerous computational
and experimental studies. For instance, benzoyl nitrene, generated by photolysis of either benzoyl
azide or a sulfilimine-based nitrene precursor, has been directly observed by time resolved
infrared spectroscopy in solution17,18 and matrix-isolation IR and UV-vis spectroscopy.19,20 Both
experimental results and extensive quantum chemical calculations have confirmed a closed-shell
singlet ground-state. Several other acyl nitrenes such as MeOC(O)N,21,22 FC(O)N,23,24
CF3C(O)N,25,26 H2NC(O)N,27 2-naphthoyl nitrene,28,29 pyrroyl nitrenes,30 and 3-furoylnitrene31
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have also been spectroscopically characterized. In contrast, knowledge about the nicotinoyl
nitrene (2, Scheme 1) and isonicotinoyl nitrene (6) is scarce, although the Curtius-rearrangement
of their azide precursors 1 and 5 to the corresponding isocyanates 3 and 7 have been investigated
in solution32 and on Pt-surfaces.33
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Scheme 1. The Curtius-rearrangement of nicotinoyl azides 1 and 5.
Very recently, Spitale et al.34,35 reported the application of nicotinoyl azide (1) in probing the
solvent accessibility of purine nucleobases in RNA and identifying rapid structural changes
resulting from ligand binding in a metabolite-responsive RNA inside cells (Scheme 2). The
method is known as Light Activated Structural Examination of RNA (LASER). Mechanistic
studies on the reactions of the azide with purines in CCl4 with time-resolved IR spectroscopy
suggests that the photolytically (310 nm) generated transient nicotinoyl nitrene (2) plays the key
role in LASER by reacting with purines at the nonhydrogen-bonding C8 position, and the
resulting bulky modification can be easily read out by its terminating effect on reverse
transcription. As for the most straightforward evidence for the formation of 2 in the singlet ground
state, a weak and broad band centered around 1734 cm–1 and a very short (picoseconds) lifetime
were observed in CCl4 at room temperature by time-resolved IR spectroscopy.
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Scheme 2. Proposed mechanism of nicotinoyl azide (1) activation and C8 adducts formation with
purines.34,35
Given the potential applications of nicotinoyl azide and the important role of nicotinoyl nitrene in
biology, herein we report a study on the photochemistry of nicotinoyl azide (1) and isonicotinoyl
azide (5) by combining matrix-isolation and quantum chemical calculations. In addition to the
characterization of the two heteroaroyl nitrenes 2 and 6, their rearrangement to isocyanates 3 and 7
and secondary photodecomposition to ketenimines via the intermediacy of pyridylnitrenes 4 and 8
have been determined (Scheme 3). Furthermore, the previously proposed36 reversible
interconversion of pyridylnitrene 8 with diazacycloheptatetraene (9) has also been observed
directly. These two species have already been detected by matrix-isolation EPR37 and IR
spectroscopy,38 respectively.
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Scheme 3. Decomposition pathways of 3 and 7.
Experimental Section
Sample synthesis
Nicotinoyl azide (1) was synthesized and purified according to the literature.39,40 Briefly, a
N,N-dimethylformamide (DMF) solution (0.4 mL) of diphenyl phosphorazidate (DPPA, 1.1 mL, 5
mmol) was added dropwise over 20 minutes to a DMF (4.0 mL) solution of nicotinic acid (0.615 g,
5 mmol) and triethylamine (0.7 mL) at 0 ºC. The vessel was warmed to 35 ºC and stirred for 4 h.
The reaction mixture was quenched immediately by pouring into a mixture of ether and ice. The
requisite product was extracted with ether (3  10 mL), and the organic layers were washed with a
10% solution of sodium bicarbonate and water, and then dried over Na2SO4. The solvent was
evaporated under reduced pressure, and the residue was purified by column chromatography using
hexane/diethyl ether (1:1) to give the crude nicotinoyl azide. The desired product was obtained
passing through three successive cold U-traps (–15, –60, and –196 ºC). The azide (ca. 0.149 g, 1
mmol) was isolated from the first trap as a pale yellow solid. The quality of the sample was
checked by NMR spectroscopy. 1H NMR (600 MHz, CDCl3): δ = 9.21 (m, 1H), 8.82 (m, 1H), 8.29
(m, 1H), 7.43 (m, 1H) ppm and 13C NMR (150 MHz, CDCl3): δ = 171.4, 154.6, 150.7, 137.0,
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126.8, 123.7 ppm. IR (3099, 2137, 1688, 1586, 1479, 1421, 1331, 1260, 1235, 1180, 1124, 1042,
1029, 992, 867, 832, 722, 712, 699, 622, 565 cm–1).
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Isonicotinoyl azide (5) was synthesized and purified in a similar manner. H NMR (600 MHz,
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DMSO): δ = 8.84 (m, 2H), 7.82 (m, 2H) and 13C NMR (150 MHz, DMSO): δ = 171.5, 151.1,
137.2, 122.0 ppm. IR (3054, 2136, 1692, 1595, 1564, 1491, 1407, 1325, 1241, 1210, 1169, 1128,
1089, 1064, 1006, 867, 841, 748, 686, 664 cm–1).
Matrix IR Spectroscopy
Matrix IR spectra were recorded on a Fourier transform infrared (FT-IR) spectrometer (Bruker
70V) in a reflectance mode using a transfer optic. A KBr beam splitter and liquid nitrogen cooled
mercury cadmium telluride (MCT) detector were used in the mid-IR region (4000–500 cm–1). For
each spectrum, 200 scans at a resolution of 0.5 cm–1 were co-added. The gaseous sample was
mixed by passing a flow of N2 gas through a U-trap (nicotinoyl azide: 5 ºC, isonicotinoyl azide: –5
ºC) containing ca. 10 mg of the azide. Then the mixture (sample/matrix gas ≈ 1 : 1000 estimated)
was passed through an aluminium oxide furnace (o.d. 2.0 mm, i.d. 1.0 mm), which can be heated
over a length of ca. 25 mm by a tantalum wire (o.d. 0.4 mm, resistance 0.4 Ω), and immediately
deposited (2 mmol/h) onto the Rh-plated copper block matrix support (15 K for N2) in a high
vacuum (10–6 Pa). Photolysis was performed using an ArF excimer laser (Gamlaser EX5/250,
193 nm), a Nd3+:YAG laser (266 nm, 532 nm, MPL-F-266, 10 mW), UV lamp (365 nm, 24 W),
and a high-power flashlight (Boyu T648, 20 W) with adjustable sources of purple light (400 ± 20
nm).
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Computational Details
Structures and IR frequencies of stationary points were calculated using the DFT-B3LYP41
method with the 6-311++G(3df,3pd) basis set. Local minima were confirmed by vibrational
frequency analysis, and transition states were further confirmed by intrinsic reaction coordinate
(IRC) calculations.42,43 Time-dependent (TD) DFT (B3LYP/6-311++G(3df,3pd)) calculations44,45
were performed for the prediction of UV-vis transitions. The energies of singlet and triplet
nitrenes were calculated accurately by applying several sophisticated theoretical methods, such as
CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd),46-48 CBS-QB3,49,50 and a modified version
of CASPT2 (referred to as “RS2C” in Molpro),51 which accounts for dynamic correlation, using
the CASSCF wave functions as references in the RS2C calculation. The active space includes 12
electrons and 11 active orbitals, i.e. CASPT2(12,11). All CASPT2 calculations were based on the
CASSCF(12,11)/cc-pVTZ geometries. The DFT and CBS-QB3 calculations were performed using
the Gaussian 09 software package.52 The CCSD(T) and CASPT2//CASSCF calculations were
performed with the MOLPRO 2012 program.53
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Results and Discussion
Photodecomposition of nicotinoyl azide (1)
A typical matrix IR spectrum obtained upon 266 nm laser photolysis of nicotinoyl azide (1) is
shown in Figure 1. After 15 s of irradiation, depletion of the azide occurs by 80 % (Figure 1a).
The expected Curtius-rearrangement product 3-pyridyl isocyanate (3) forms indicated by the
appearance of a strong band at 2264.7 cm–1 (Figure 1b), the band position is in good agreement
with the observed broad band in the range of 2310–2185 cm–1 detected by time-resolved IR
spectroscopy.34 Interestingly, a distinct IR band at 1765.0 cm–1 appears immediately after the 266
nm laser irradiation of 1, and its intensity reaches a maximum in 15 s (Figure 1c). The assignment
of this band to closed-shell singlet nicotinoyl nitrene (2) is in agreement with both the previously
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observed frequency of 1734 cm–1 of 2 in CCl4 and the B3LYP/6-311++G(3df,3pd) calculated
strongest IR band for the nitrene 2 in the singlet ground state (syn conformer: 1752 cm–1, anti
conformer: 1753 cm–1, Table 1). Furthermore, it is very close to those of the most characteristic
C=N stretching vibration frequencies in other closed-shell singlet aroyl nitrenes such as PhC(O)N
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(1760 cm–1 in CD3CN),18,19 2-NpC(O)N (1737 cm–1 in CCl4),28,29 2-furoylnitrene (1752.3 cm–1 in
Ne-matrix), and 3-furoylnitrene (1764.2 cm–1 in Ne-matrix).31 In line with experimental
observations for triplet acyl nitrenes (e.g., MeOC(O)N: syn: 1644.8, anti: 1607.2 cm–1,21 FC(O)N:
1678.1 cm–1,23 and H2NC(O)N: 1643.7 cm–1 27), the calculated C=O stretching frequencies for the
two conformers of nicotinyl nitrene in the triplet state (syn: 1499, anti: 1497 cm–1) are much lower
than the observed bands (Table 1).
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Figure 1. N2-matrix IR spectra at 15 K. (a) IR spectra showing the photolytic behavior of
nicotinoyl azide (1) after irradiating 266 nm and subsequently 400 ± 20 nm. (b) IR spectra
showing the photolytic behavior of 3-pyridyl isocyanate (3) after irradiating 266 nm and
subsequently 400 ± 20 nm. (c) IR spectra showing the photolytic behavior of nicotinoyl nitrene (2)
after irradiating 266 nm and subsequently 400 ± 20 nm.
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Table 1. Experimentally observed and calculated vibrational frequencies (> 500 cm–1) of nicotinoyl nitrene (2) and isonicotinoyl nitrene (6).
3
4
Nicotinoyl nitrene (2)
Isonicotinoyl nitrene (6)
5
6
7
8
Calculateda
syn-triplet
3100 (6)
3086 (2)
3081 (<1)
3061 (10)
1574 (80)
1555 (18)
Observedb
anti-singlet N2-matrix
Calculateda
triplet
Observedb
N2-matrix
Assignmentc
anti-triplet
3103 (5)
3090 (4)
3064 (<1)
3060 (14)
1575 (85)
1553 (19)
syn-singlet
singlet
3102 (5)
3086 (2)
3072 (1)
3061 (10)
1752 (186)
1582 (37)
1555 (17)
1464 (9)
1408 (45)
1326 (7)
1255 (2)
1221 (20)
1180 (14)
1143 (10)
1101 (11)
1026 (<1)
1010 (11)
990 (<1)
963 (<1)
932 (<1)
807 (13)
764 (3)
3104 (4)
3090 (2)
3064 (2)
3060 (12)
3107 (2)
3097 (2)
3065 (4)
3062 (22)
1575 (<1)
1550 (13)
1494 (132)
1470 (13)
1398 (28)
1316 (8)
1234 (14)
1204 (19)
1184 (90)
1075 (1)
1055 (5)
1042 (19)
985 (<1)
978 (2)
3101 (<1)
3098 (2)
3068 (3)
3065 (20)
1756 (131)
1579 (16)
1551(15)
1474 (1)
1401 (34)
1321 (8)
1244 (6)
1213 (35)
1201 (4)
1141 (14)
1070 (<1)
1056 (4)
982 (<1)
979 (3)
νs(CH)
νas(CH)
νas(CH)
νas(CH)
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3062.5 vw
1761.2 s
1597.7 m
1753 (178)
1581 (42)
1554 (20)
1459 (16)
1414 (25)
1327 (6)
1251 (2)
1217 (17)
1187 (16)
1143 (8)
1094 (15)
1026 (<1)
1009 (14)
990 (<1)
963 (<1)
930 (<1)
810 (13)
764 (1)
1765.0 vs
1595.1 m
1578.1 m
1481.9/1475.5 m
1432.5/1425.5 s
1347.4/1334.5 vw
1287.3 vw
1260.1/1250.4 m
1200.5 w
1153.5 w
1116.2 vw
1036.4 vw
1024.3 m
ν(C=Nnitrene
ν(C=C)
ν(CC)
)
1499 (139) 1497 (131)
1454 (30)
1411 (12)
1322 (7)
1246 (5)
1191 (70)
1185 (44)
1112 (10)
1041 (24)
1020 (3)
1005 (26)
990 (<1)
968 (<1)
939 (<1)
807 (8)
1463 (5)
1404 (50)
1323 (9)
1251 (6)
1204 (86)
1177 (8)
1098 (13)
1052 (27)
1019 (<1)
1009 (15)
994 (<1)
970 (<1)
934 (<1)
814 (8)
1491.2 w
1413.9 s
1338.2 w
1261.5 w
1249.0 w
1217.3 w
1153.6 m
δ(CH)
δ(CH)
δ(CH)
δ(ring)
ν(C-O)
δ(CH)
δ(CH)
δ(CH)
Ring breathing
δ(CC)
γ(CH)
γ(CH)
γ(CH)
γ(CH)
δ(CC)
γ(CH)
990.4 w
966.6 vw
970 (<1)
872 (<1)
828 (16)
739 (5)
965 (<1)
865 (<1)
823 (24)
751 (5)
816.9 s
776.8 vw
703.2 s
621.3 w
615.5 m
537.2 vw
829.0 m
762.9 w
736.3 vw
752 (4)
752 (3)
700 (41)
642 (23)
614 (7)
700 (40)
648 (23)
617 (6)
699 (35)
616 (4)
615 (14)
539 (9)
699 (35)
616 (4)
615 (13)
535 (11)
738 (2)
735 (2)
658 (<1)
640 (54)
555 (43)
658 (<1)
613 (40)
525 (18)
δ(CNring
γ(CC(O)Nnitrene
δ(C(O)Nnitrene
)
613.9 s
522.0 m
)
558 (30)
561 (26)
)
aCalculated harmonic IR frequencies (cm–1, scaled by a factor of 0.9679)54 and intensities (km mol–1, in parentheses) at the B3LYP/6-311++G(3df,3pd) level.
bObserved band positions of the most intense matrix sites in N2-matrix at 15 K. The relative band intensities: vs = very strong, s = strong, m = medium strong, w =
weak, vw = very weak. cTentative assignment based on the calculated vibrational displacement vectors for the singlet state.
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According to the TD-B3LYP/6-311++G(3df,3pd) calculations, nicotinoyl nitrene (2) in the closed-shell singlet
state exhibits vertical transitions at 504 (syn) and 506 nm (anti) (Table S3 in the Supporting Information). Hence,
the matrix containing the 266 nm laser photolysis products of 1 was subsequently subjected to the irradiation of a
purple-light emitting diode (LED, 400 ± 20 nm), leading to efficient depletion of nitrene 2 (Figure 1c) and the
formation of isocyanate 3 (Figure 1b), whereas the bands of the azide remained almost intact. The selective
depletion of 2 allows identification of most of the IR bands (Figure 2) for the fundamental vibration modes in the
available spectral range 4000–500 cm–1 (Table 1). It is noteworthy that some IR bands of the nitrene split into two
components (Figure 2, inset), likely indicating the presence of two conformers. Similar generation of two
conformers has also been observed for other acyl nitrenes such as MeOC(O)N21 and Me(H)NC(O)N.55
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Figure 2. (A) IR difference spectrum (10 times expanded along the ΔA axis) showing the photo-induced (400 ±
20 nm) rearrangement from nicotinoyl nitrene (2) to 3-pyridyl isocyanate (3) in solid N2-matrix. (B-C)
B3LYP/6-311++G(3df,3pd) calculated IR spectra (scaled by a factor of 0.9679)54 for the two conformers of 2 (B:
syn; C: anti) in the singlet state.
It is known that covalent isocyanates can undergo CO-elimination and furnish nitrenes upon laser
irradiation.21,22,30,31 In an attempt to generate 3-pyridylnitrene (4) for IR spectroscopic detection, the matrix
containing 3-pyridyl isocyanate (3) was further irradiated with a 193 nm laser. The resulting IR difference
spectrum demonstrates the depletion of 3 and dominant formation of CO (2140.0 cm–1) and other new species
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(Figure 3). Similar to the photochemsitry of 3-pyridyl azide in Ar matrix,56 cyanovinylnitrile ylide 11 (1942.8,
1578.0, and 760.7 cm–1) and ketenimine 12 (2037.9 cm–1) can be identified, whereby the latter is formed from 11
through a 1,7-H shift.57 Notably, the presence of a second conformer of 11 can be inferred by the appearance of
the IR band at 1962.9 cm–1 for the most prominent C=N+=C– asymmetric stretching vibration, and the band is in
excellent agreement with the previously reported result (1961 cm–1 in Ar matrix).56 Consistent with previous IR
spectroscopy studies, no distinguishable IR bands for 3-pyridylnitrene (4) could be found in the spectrum
probably due to low intensities, although its existence as a triplet species has already been established by
matrix-isolation EPR spectroscopy.58,59
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Figure 3. IR difference spectrum showing the 193 nm laser photolysis of 3-pyridyl isocyanate (3) in solid
N2-matrix. Band of unknown species is marked with asterisk.
Photodecomposition of isonicotinoyl azide (5)
Laser photolysis (266 nm) of isonicotinyl azide (5, Figure 4a) in N2-matrix gave rise to a series of new peaks
including several signals around 2260 cm–1 (Figure 4b) that are characteristic for the asymmetric NCO stretching
vibration to the rearrangement prodcut 4-pyridyl isocyanate (7). The band position is very close to that
aforementioned for 3-pyridyl isocyanate (3, 2264.7 cm–1, N2 matrix) and previously reported for 2-pyridyl
isocyanate (2245 cm–1, Ar matrix).60 Notably, a new species bearing a very weak but distinguishable band at
1761.2 cm–1 (Figure 4c) formed among the photolysis products of 5. Comparing with the IR band for singlet
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nicotinoyl nitrene (2) at 1765.0 cm–1 (Figure 1c), this band can be reasonably assigned to the C=N stretching
vibration of singlet isonicotinoyl nitrene (6).
6
7
8
9
Given the theoretically predicted vertical transition at 511 nm (f = 0.0005, Table S4 in the Supporting Information)
for the closed-shell singlet isonicotinoyl nitrene (6), the matrix was further irradiated with green light (532 nm).
The corresponding IR spectra show depletion of the nitrene 6 (Figure 4c) and the simultaneous formation of
4-pyridyl isocyanate (7) (Figure 4b), traces of the azide precursor 5 were also depleted under the green light
irradiation.
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Figure 4. N2-matrix IR spectra at 15 K. (a) IR spectra showing the photolytic behavior of isonicotinoyl azide (5)
after irradiating 266 nm and subsequently 532 nm. (b) IR spectra showing the photolytic behavior of 4-pyridyl
isocyanate (7) after irradiating 266 nm and subsequently 532 nm. (c) IR spectra showing the photolytic behavior
of isonicotinoyl nitrene (6) after irradiating 266 nm and subsequently 532 nm.
For clarity, the IR difference spectra showing the decomposition of the azide 5 upon a 266 nm laser irradiation
and the subsequent rearrangement of the nitrene 6 to isocyanate 7 under further 532 nm irradiation are depicted in
Figure 5. Moreover, to aid the IR spectral assignment calculations on isonicotinoyl nitrene (6) in the singlet and
triplet were carried out, and the results are summarized in Table 1 and show good agreement with experimental
observation (Figure 5).
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Figure 5. (A) IR difference spectrum showing the decomposition of isonicotinoyl azide (5) (bands pointing
downward) in N2-matrix (15 K) upon a 266 nm laser irradiation (30 seconds). (B) IR difference spectrum (10
times expanded along the ΔA axis) showing the rearrangement of isonicotinoyl nitrene (6) to 4-pyridyl isocyanate
(7) upon subsequent green light irradiation (532 nm, 20 minutes). (C) B3LYP/6-311++G(3df,3pd) calculated IR
spectra (scaled by a factor of 0.9679)54 for the 6 in the singlet state. Unknown species are marked with (*) and (#).
In contrast to the full spectroscopic characterization of tetrafluoro- and tetrachloro-4-pyridylnitrenes formed in the
photolysis of the corresponding azides in low-temperature matrices with UV, EPR and IR spectroscopy,37 the
parent 4-pyridylnitrene (8) was only detected by EPR spectroscopy, whereas the corresponding IR spectroscopy
showed that the photolysis of 4-pyridyl azide mainly led to ring expansion to diazacycloheptatetraene (9).36-38 In
order to identify the nitrene intermediate 8 in our experiments, the decomposition of 4-pyridyl isocyanate (7),
generated through flash vacuum pyrolysis (FVP) of a mixture of 5 in N2 (ca. 1:1000), was subjected to prolonged
193 nm laser irradiation, and the IR difference spectrum showing the decomposition of 7 is depicted in Figure 6.
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Figure 6. IR difference spectrum showing the 193 nm laser photolysis of 4-pyridyl isocyanate (7) in solid
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N2-matrix. Band of unknown species is marked with an asterisk.
Decomposition of 7 mainly yields CO and some new species with several IR absorptions in the range of 2100–600
cm–1. Given the calculated vertical transition at 543 nm (f = 0.0002) for the expected nitrene 8 (Table S4 in the
Supporting Information), the matrix was further irradiated with green light (532 nm). The corresponding IR
difference spectrum (Figure 7B) demonstrates the sole depletion of the IR bands for nitrene 8 and the formation of
ring-opening product N-(isocyano-vinyl)ketenimine (13) indicated by the appearance of the IR band for the
characteristic ketenimine moiety (N=C=C) at 2028.3 cm–1, it is also close to previously reported broad absorption
in the range of 2028–2060 cm–1 in an Ar matrix.38 Consistent with the experimentally observed result in the
photolysis of 4-azidopyrine, a full set of IR bands at 1869.2, 1545.4, 1335.2, 1274.8, 1216.2, 1109.5, 1021.5,
893.8, 828.9, 797.9, 711.5, and 659.4 cm–1 for diazacycloheptatetraene (9) was also observed. Interestingly,
further UV irradiation (365 nm) of the matrix results in efficient depletion of all IR bands of
diazacycloheptatetraene (9) and concurrent increase of the IR absorptions for nitrene 8 and ketenimine 13. In
order to aid the IR spectral assignment of 4-pyridylnitrene (8), the IR spectra of nitrene 8 in both singlet and
triplet states were calculated at the B3LYP/6-311++G(3df,3pd) level, and the calculated result for the triplet
ground state nicely reproduced the experimental data (Table 2). Notably, 2-pyrazinylcarbene (10), another
isomeric intermediate involving in the photochemistry of diazacycloheptatetraene (9), was not detected.36,38
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Figure 7. (A) B3LYP/6-311++G(3df,3pd) calculated IR spectra (scaled by a factor of 0.9679)54 for the
4-pyridylnitrene (8) in the triplet state. (B) IR difference spectrum showing 4-pyridylnitrene (8) →
diazacycloheptatetraene (9) conversion upon green light irradiation (532 nm). (C) IR difference spectrum showing
diazacycloheptatetraene (9) → 4-pyridylnitrene (8) conversion upon subsequent UV light irradiation (365 nm).
Bands of unknown species are marked with asterisks.
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Table 2. Calculated and experimentally observed IR vibrational frequencies (> 500 cm–1) of 4-pyridyl nitrene (8).
7
8
9
Calculateda
singlet
Observedb
N2-matrix
Assignmentc
triplet
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νas(CH)
νs(CH)
νas(CH)
νas(CH)
3153 (2)
3126 (<1)
3051 (11)
3010 (9)
1539 (36)
1494 (104)
1404 (20)
1343 (71)
1294 (4)
1283 (89)
1204 (6)
1122 (2)
1041 (4)
987 (9)
971 (22)
913 (11)
854 (15)
837 (39)
793 (2)
726 (<1)
641 (10)
614 (38)
525 (20)
3107 (4)
3107 (<1)
3061 (2)
3059 (23)
1515 (3)
1453 (22)
1419 (1)
1384 (10)
1268 (<1)
1259 (26)
1180 (10)
1169 (1)
1065 (<1)
1027 (6)
974 (0)
952 (<1)
944 (3)
825 (10)
820 (0)
794 (48)
701 (<1)
644 (1)
3053.3 vw
1489.8 vw
1459.2 m
ν(C=C)
ν(C-C)
δ(CH)
δ(CH)
δ(CH)
ν(C-Nnirene
ν(C=N)
δ(CH)
δ(CH)
δ(CH)
γ(CH)
γ(CH)
δ(C=N)+δ(C=C)
Ring breathing
γ(CH)
γ(CH)
γ(CH)
1398.9 m
1271.1 m
1169.2 m
)
1035.9 vw
956.7 vw
838.3 w
799.5 s
γ(ring)
δ(C=C)
503 (1)
aCalculated harmonic IR frequencies (cm–1, scaled by a factor of 0.9679)54) and intensities (km mol–1) in
parentheses at the B3LYP/6-311++G(3df,3pd) level. bObserved band positions for the most intense matrix sites in
N2-matrix at 15 K. The relative band intensities: s = strong, m = medium strong, w = weak, vw = very weak.
cTentative assignment based on the calculated vibrational displacement vectors for the triplet state.
Quantum chemical calculations
To understand the mechanism for the decomposition of the two azides 1 and 5, the ground-state potential energy
surfaces were computationally explored with the CBS-QB3 method (Figure 8). For both azides 1 and 5, the
activation barriers (TS1) for their concerted Curtius-rearrangement (1: 28.6 kcal mol–1; 5: 28.5 kcal mol–1) to 3
and 7 are lower than those (TS3) for the stepwise pathway via the intermediacy of the nitrene intermediates 2
(34.4 kcal mol–1) and 6 (34.3 kcal mol–1), respectively. Therefore, the former pathway should play a major role in
the thermal decomposition of the two azides. Unlike the ground-state thermal decomposition, previous
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time-resolved spectroscopy studies17,19,28,29,34 on the photolytic decomposition of carbonyl azides in solutions have
revealed that carbonyl nitrenes in the closed-shell singlet state can be generated photochemically through
decomposition of the azides in the excited states. The thermal stability of the initially generated singlet nitrenes 2
and 6 can be estimated by calculating the barriers (TS4) 20.2 and 21.8 kcal mol–1 for the highly exothermic
rearrangement to the corresponding isocyanates 3 and 7, respectively. The moderate barriers reasonably account
for their isolation and IR spectroscopic detection in cryogenic matrices. In contrast, given such low barriers, these
two nitrenes can hardly survive under conditions of pyrolysis of the azide precursors (450 °C). However, when the
ground state is a triplet, the nitrene can sometimes be isolated from a pyrolysis reaction. For example, the
thermally persistent carbonyl nitrene FC(O)N in the triplet ground state has been generated in the gas phase by
pyrolysis of the azide precursor, here the barrier for the rearrangement of the initially generated singlet nitrene is
17.0 kcal mol–1 at the CBS-QB3 level.24
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Figure 8. Calculated relative Gibbs free energy profile (kcal mol–1, including zero-point vibrational energy
corrections) for the decomposition of nicotinoyl azide (1) and isonicotinoyl azide (5) (in parentheses) at CBS-QB3
level. For clarity, only the molecular structures for the species involved in the decomposition of 1 are depicted.
The molecular structures of nicotinoyl nitrene (2) and isonicotinoyl nitrene (6) in the closed-shell singlet (CSS),
open-shell
singlet
(OSS),
and
triplet
(T)
states,
calculated
with
the
CASPT2(12,11)/cc-pVTZ//CASSCF(12,11)/cc-pVTZ method, are shown in Figure 9. The singlet–triplet energy
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gaps (ΔEST) for both nitrenes calculated at B3LYP, CCSD(T), CASPT2, and CBS-QB3 methodologies are
collected in Table 3.
7
8
9
All the B3LYP, CCSD(T), and CASPT2 methods predict that the triplet state is lower in energy than the
closed-shell singlet state for nitrenes 2 and 6, whereby the calculated energy gaps are close to zero in the case of
the CCSD(T) calculations. In contrast, the most accurate CBS-QB3 method, which relies on a B3LYP optimized
geometry and a CCSD(T) calculated energy as an estimate for electron correlation with an extrapolation to the
infinite basis set limit, correctly predicts the preference of a closed-shell singlet state as the ground state (ΔEST < 0
kcal mol–1, Table 3). The ΔEST values for 2 and 6 are –3.9 and –2.4 kcal mol–1, respectively. They are larger than
those for the two magnetically bistable furoylnitrenes (2-furoylnitrene: –1.2 kcal mol–1; 3-furoylnitrene: –1.1 kcal
mol–1)31 but close to that for the intensively studied singlet benzoylnitrene (–4.7 kcal mol–1).31 Nevertheless, such
narrow energy gaps between the singlet and triplet states suggest that the spin multiplicity might be switchable by
heat, light, or weak intermolecular interactions, as has been recently observed for arylcarbenes61-63 and
pentafluorophenylnitrene.64 Similarly, attempts were made to generate the two nitrenes 2 and 6 in the triplet state
by irradiating with different light sources (365 and 830 nm), but no distinguishable IR bands for triplet 2 and 6
could be identified by IR spectroscopy.
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Table 3. Calculated singlet–triplet energy gaps ΔEST (kcal mol–1) for nicotinoyl nitrene (2) and isonicotinoyl
nitrene (6).
Species B3LYPa
CCSD(T)b
0.1
CASPT2c
1.7
CBS-QB3
–3.9
2d
6d
5.0
5.0
0.1
1.5
–2.4
b
c
aAt the 6-311++G(3df,3pd) basis set. At the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) level. At the
d
CASPT2(12,11)/cc-pVTZ//CASSCF(12,11)/cc-pVTZ level. For the more stable conformer of the two states.
Positive values refer to the triplet being more stable.
Structurally, the OCN moieties in the two nitrenes are quite similar but change strongly with the electronic states.
For instance, the O–C–N angles in CSS nitrenes 2 (anti: 92.7°; syn: 92.7°) and 6 (93.0°) are significantly smaller
than those in the OSS (anti-2: 117.1°; syn-2: 117.2°) and 6 (117.4°) and in the triplet states (anti-2: 117.5°; syn-2:
117.5°) and 6 (117.7°). The small O–C–N angles in closed-shell singlet state of carbonyl nitrenes can be
understood in terms of a contribution by the oxazirine structure arising from an intramolecular interaction between
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the electron deficient nitrene center (Nnitrene) and the neighboring carbonyl oxygen atom (C=O).27 In line with this
structural motif, the two nitrenes display characteristic C=N stretching vibrations at around 1760 cm–1
15
(N2-matrices) with significant N-isotopic shifts of 15 cm–1 (B3LYP/6-311++G(3df,3pd)). In sharp contrast, the
calculated C=O stretching vibrations for triplet 2 and 6 are predicted at ca. 1500 cm–1 with a negligible
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15N-isotopic shifts of 1 cm–1 (B3LYP/6-311++G(3df,3pd)).
Figure 9. Calculated molecular structures and relative energies (kcal mol–1) of nicotinoyl nitrene (2) and
isonicotinoyl nitrene (6) in the closed-shell singlet (CSS), open-shell singlet (OSS), and triplet (T) states at the
CASPT2(12,11)/cc-pVTZ//CASSCF(12,11)/cc-pVTZ level. The calculated bond lengths (Å) and angles (deg) of
the O–C–N moieties are shown.
Conclusions
The stepwise photodecomposition reactions of both nicotinoyl azide (1) and isonicotinoyl azide (5) in solid
N2-matrices at 15 K have been studied by combining matrix isolation IR spectroscopy and quantum chemical
calculations. In line with the very recent time-resolved IR spectroscopy study,34 nicotinoyl nitrene (2) in the
closed-shell singlet has been generated in the 266 nm laser photolysis of 1, and its IR spectrum in the range of
4000–500 cm–1 has been obtained. Subsequent irradiation of 2 with purple light (400 ± 20 nm) results in
rearrangement to 3-pyridyl isocyanate (3). Similarly, isonicotinoyl nitrene (6), the key intermediate in the
decomposition of 5, has also been identified by matrix-isolation IR spectroscopy in the closed-shell singlet ground
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state and it undergoes rearrangement to 4-pyridyl isocyanates (7) upon green light irradiation (532 nm). Upon 193
nm laser irradiation, both isocyanates 3 and 7 eliminate CO and furnish ring-opening products ketenimines 12 and
13 via the intermediacy of triplet pyridyl nitrenes 4 and 8, respectively. In particular, the 4-pyridyl nitrene (8) in
the triplet ground state, and its reversible photoisomerization with ring-expansion product diazacycloheptatetraene
(9) have been directly observed by IR spectroscopy. Computationally, the potential energy surfaces (PES) for the
decomposition of azides reveals a preference for a concerted Curtius-rearrangement to isocyanates over the
stepwise pathway of forming singlet acylnitrenes in the singlet ground state. However, nitrenes 2 and 6 in the
singlet state can be generated through the photolytic decomposition of the azides in the excited state, and the
moderate barriers (ca. 20 kcal mol–1) associated with their further rearrangements to the corresponding
isocyanates 3 and 7 permit their isolation in cryogenic matrices for spectroscopic detection.
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ASSOCIATED CONTENT
Author Information
Corresponding Author
*X. Q. Zeng: e-mail, xqzeng@suda.edu.cn
*C. Wentrup: e-mail, wentrup@uq.edu.au
Notes
The authors declare no competing financial interest.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21673147, 21422304) and the
project of scientific and technologic infrastructure of Suzhou (SZS201708).
Supporting Information
Experimental and calculated IR spectra, calculated vertical transitions, energies and atomic coordinates for all
species discussed in the paper. This material is available free of charge via the internet at http://pubs.acs.org.
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References
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(1) Falvey, D. E.; Gudmundsdottir, A. D. Nitrenes and Nitrenium Ions (Wiley Series of Reactive Intermediates in
Chemistry and Biology); Wiley: New York, 2013; Vol. 6.
9
(2) Gritsan, N. P.; Platz, M. S.; Borden, W. T. The Study of Nitrenes by Theoretical Methods in Theoretical
Methods in Photochemistry; Kutateladze, A., Ed.; Taylor and Francis: Boca Raton, 2005.
(3) Moss, R. A.; Platz, M. S.; Jones, M., Jr. Reactive Intermediate Chemistry; Wiley-Interscience: New York,
2004.
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N2-matrix IR spectra at 15 K. (a) IR spectra showing the photolytic behavior of nicotinoyl azide (1) after
irradiating 266 nm and subsequently 400 ± 20 nm. (b) IR spectra showing the photolytic behavior of 3-
pyridyl isocyanate (3) after irradiating 266 nm and subsequently 400 ± 20 nm. (c) IR spectra showing the
photolytic behavior of nicotinoyl nitrene (2) after irradiating 266 nm and subsequently 400 ± 20 nm.
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