Direct detection of a triplet vinylnitrene, 1,4-naphthoquinone-2-ylnitrene, in solution and cryogenic matrices

Article abstract of DOI:10.1021/jacs.5b00998

The photolysis of 2-azido-1,4-naphthoquinone (1) in argon matrices at 8 K results in the corresponding triplet vinylnitrene ³2, which was detected directly by IR spectroscopy. Vinylnitrene ³2 is stable in argon matrices but forms 2-cyanoindane-1,3-dione (3) upon further irradiation. Similarly, the irradiation of azide 1 in 2-methyltetrahydrofuran (MTHF) matrices at 5 K resulted in the ESR spectrum of vinylnitrene ³2, which is stable up to at least 100 K. The zero-field splitting parameters for nitrene ³2, D/hc = 0.7292 cm^-1 and E/hc = 0.0048 cm^-1, verify that it has significant 1,3-biradical character. Vinylnitrene ³2 (λ_max ～ 460 nm, τ = 22 μs) is also observed directly in solution at ambient temperature with laser flash photolysis of 1. Density functional theory (DFT) calculations support the characterization of vinylnitrene ³2 and the proposed mechanism for its formation. Because vinylnitrene ³2 is relatively stable, it has potential use as a building-block for high-spin assemblies.

Full text of DOI:10.1021/jacs.5b00998

Article
pubs.acs.org/JACS
Direct Detection of a Triplet Vinylnitrene, 1,4-Naphthoquinone-2-
ylnitrene, in Solution and Cryogenic Matrices
Sujan K. Sarkar,† Asako Sawai,‡ Kousei Kanahara,‡ Curt Wentrup,§ Manabu Abe,‡
and Anna D. Gudmundsdottir*,†
†Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172, United States
‡Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima
739-8526, Japan
§School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland 4072, Australia
S
* Supporting Information
ABSTRACT: The photolysis of 2-azido-1,4-naphthoquinone
(1) in argon matrices at 8 K results in the corresponding triplet
3
vinylnitrene 2, which was detected directly by IR spectroscopy.
Vinylnitrene 32 is stable in argon matrices but forms 2-
cyanoindane-1,3-dione (3) upon further irradiation. Similarly,
the irradiation of azide 1 in 2-methyltetrahydrofuran (MTHF)
3
matrices at 5 K resulted in the ESR spectrum of vinylnitrene 2,
which is stable up to at least 100 K. The zero-field splitting
3
parameters for nitrene 2, D/hc = 0.7292 cm−1 and E/hc =
0.0048 cm−1, verify that it has significant 1,3-biradical character.
3
Vinylnitrene 2 (λmax ∼ 460 nm, τ = 22 μs) is also observed
directly in solution at ambient temperature with laser flash photolysis of 1. Density functional theory (DFT) calculations support
the characterization of vinylnitrene 32 and the proposed mechanism for its formation. Because vinylnitrene 32 is relatively stable,
it has potential use as a building-block for high-spin assemblies.
cryogenic matrices yielded ketenimine derivatives (Scheme 2),
not vinylnitrenes, although the ketenimines are likely to be
INTRODUCTION
■
Nitrenes are electron-deficient reactive intermediates that have
two unpaired electrons and can thus have triplet or singlet
configurations.1,2 Singlet nitrenes are mostly short-lived and
decay by intramolecular reactivity. However, some singlet
nitrenes are sufficiently long-lived to undergo bimolecular
reactions and have therefore been used in various applications,
such as the cross-linking of polymers,3 surface alteration,4 and
photoaffinity labeling.5 In comparison, the high-spin properties
of triplet nitrenes make them potential building units for high-
spin assemblies.6 Because of the various applications of
nitrenes, there is a general interest in them.
We have reported the first detection of triplet vinylnitrenes in
solution by performing laser flash photolysis of azirine and
isoxazole derivatives (Scheme 1).7−9 The vinylnitrenes were
formed by using built-in triplet sensitizers, which made it
possible to form the triplet vinylnitrenes without having to
depend on intersystem crossing from singlet to triplet
vinylnitrenes. However, the photolysis of these azirines in
Scheme 2. Formation of Ketenimines in Argon Matrices
formed from vinylnitrene intermediates. Similarly, Inui et al.
have shown that the irradiation of naphthyl azirines in
cryogenic matrices yields ketenimine products. Nunes et al.
reported that the photolysis of isoxazole derivatives in
cryogenic matrices can be used to convert them to the
corresponding azirines, presumably through triplet vinyl-
nitrenes.10−13 However, no reported studies have verified that
these conversions take place through triplet vinylnitrene
intermediates, as vinylnitrenes have not been directly detected
in cryogenic matrices. Thus, the question arises why simple
triplet vinylnitrenes are not stable in cryogenic matrices, unlike
other triplet nitrenes, such as alkyl- and phenylnitrenes. Triplet
alkyl- and arylnitrenes are generally long-lived intermediates
Scheme 1. Photochemistry of Azirine and Isoxazole
Derivatives
Received: January 29, 2015
© XXXX American Chemical Society
A
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that decay by dimerization rather than intersystem crossing or
reacting with the solvent and have been characterized directly
in both solution and cryogenic matrices.14,15 Intriguingly,
Wentrup and Kvaskoff reported that highly conjugated triplet
dienylnitrenes are stable in cryogenic matrixes.16
Recently, we established that the triplet sensitization of a
vinyl azide can be used to form a vinylnitrene intermediate.17
As vinyl azides are more versatile precursors than azirine or
isoxazole derivatives, they enable the formation of a wide
variety of vinylnitrenes with different and complex structures.
In this Article, we studied the vinylnitrene formed from the
bicyclic 2-azido-1,4-naphthoquinone 1 (Scheme 3). By
Scheme 3. Proposed Mechanism for Forming Triplet
3
Vinylnitrene 2 and Nitrile 3 from 1
Figure 2. (A) IR spectra obtained before (red) and after (blue to
black) irradiating 1 in argon matrix at 8 K. Calculated IR spectrum at
irradiating the rigid vinyl azide 1 with a built-in triplet
sensitizer, it becomes possible to form triplet vinylnitrene 2,
3
3
the B3LYP/6-31+G(d) level for (B) vinylnitrene 2 and (C) nitrile 3
after scaling by 0.9613.18 The difference IR spectra after 60 and 1230 s
which can be detected directly in cryogenic matrices using IR
and ESR (electron spin resonance) spectroscopy, as it is stable
up to at least 100 K. Under the irradiation conditions, the
are displayed in Supporting Information Figure S1.
3
triplet nitrene 2 was found to rearrange to a nitrile 3.
comparisons with the calculated spectrum (Figure 2B). The
most intensive band at 1705 cm−1 is assigned to the CO
stretch of the carbonyl group next to the nitrene center, which
is calculated to be located at 1761 cm−1, and scaling by 0.961318
places it at 1693 cm−1. We assign the band at 1345 cm−1 to the
RESULTS
■
Matrix IR Spectroscopy. Vinyl azide 1 was deposited into
an argon matrix at 8 K by heating it to 50 °C while using argon
to entrain the vapor. The matrix was irradiated with a xenon
lamp through a 320−360 nm filter for 20.5 min. The
characteristic IR bands of the azide chromophore at 2129,
2118, and 2107 cm−1 were almost fully depleted after 2 min of
irradiation (Figure 1), along with bands at 1683, 1664, 1661,
3
C−N stretch in vinylnitrene 2, which is calculated to be
located at 1375 cm−1, and scaling places it at 1322 cm−1. The
band at 1325 cm−1 is assigned to the coupled C−C stretches
calculated at 1321 cm−1, which scaling places at 1270 cm−1. A
set of bands formed at 1774, 1743, 1288, 1233, 1135, 938, 930,
3
825, and 682 cm−1 as the bands due to vinylnitrene 2 were
depleted, and these bands are assigned to nitrile 3 on the basis
of comparison with its calculated spectrum (Figure 2C). The
bands at 1774 and 1743 cm−1 are assigned to the symmetrical
and antisymmetrical stretches of CO bands, which are
calculated at 1831 and 1800 cm−1, respectively, and 1760 and
1730 cm−1, respectively, with scaling. Furthermore, we assign
the bands at 1288 and 1233 cm−1 to C−C stretching, calculated
to be at 1298 and 1255 cm−1, and scaled to be at 1248 and
1206 cm−1, respectively. The CN stretching in nitrile 3 is
located at 2271 cm−1 as a very weak band, which matches the
calculated and scaled band at 2268 cm−1.
Figure 1. Depletion of the azide bands of azide 1 upon irradiation in
argon matrices at 8 K.
Therefore, the matrix isolation experiments demonstrate that
3
the photolysis of azide 1 yields triplet vinylnitrene 2, which is
stable in the argon matrix, as it does not undergo intersystem
crossing to form singlet products. However, vinylnitrene 2 is
photolabile, as it forms nitrile 3 upon prolonged irradiation.
3
1605, 1600, 1578, 1372, 1324, 1273, 944, 885, 741, 716, 644,
and 594 cm−1 (Figure 2A). Concurrently with the depletion of
these bands, two sets of new bands were formed. The
intensities of the bands that formed at 1705, 1345, 1325,
1270, 962, and 701 cm−1 increased with irradiation time for the
first 120 s and then decreased with further irradiation until they
were almost fully depleted after 20.5 min of irradiation. These
Because the photolysis of azide 1 produces triplet vinyl-
3
nitrene 2, which, upon further irradiation, yields nitrile 3, we
propose that the mechanism for forming vinylnitrene 32 on the
triplet surface of 1 is as shown in Scheme 3. Upon excitation,
azide 1 forms its first singlet excited state (S1), which
intersystem crosses to its first triplet excited state (T1). The
3
bands are assigned to the triplet vinylnitrene 2 on the basis of
B
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3
T1 of 1 extrudes a N2 molecule to yield triplet vinylnitrene 2,
which is stable at 8 K (Scheme 3).
ESR Spectroscopy. A solution of azide 1 in MTHF was
degassed by three freeze and thaw cycles at 2.5 × 10−2 Pa. The
resulting solution was cooled to 5 K and irradiated for 10 s, and
an ESR spectrum was recorded between 0 and 10 000 G. The
spectrum showed X2 and Y2 lines typical of a nitrene at 6008
and 6224 G (Figure 3a). In addition, a broad signal at ∼4500 G
Figure 4. Transient UV−vis spectrum obtained by the laser flash
photolysis of 1 in argon-saturated acetonitrile (top). Calculated
electronic transitions (TD-DFT/B3LYP/6-31+G(d)) of triplet vinyl-
nitrene 32 (middle). Calculated electronic transitions (TD-DFT/
B3LYP/6-31+G(d)) for T1 of 1 (bottom).
Figure 3. ESR spectra obtained by irradiating 1 in MTHF at (a) 5 K.
(b) ESR signals as a function of temperature. Microwave frequency:
9.398 896 GHz; H0 = 3353.5 G. X2 = 6008 G; Y2 = 6224 G; Z1 ∼ 4500
G; Δms = 2−1685 G. D/hc = 0.7292 cm−1. E/hc = 0.0048 cm−1.
the basis of the similarity between the observed transient
spectrum and the time-dependent density functional theory
(TD-DFT) calculated absorption spectrum of 32. The TD-DFT
3
is found in the position expected for the Z1 transition. The
broadness of the signals is undoubtedly due to the use of an
MTHF matrix. This makes the exact position of the Z1 line
uncertain, and it was not used in the calculation of the D and E
values. A weak signal at ∼1685 G is also observed in Figure 3a,
and this is most likely due to Δms = 2 (expected at 1676 cm−1).
The zero-field splitting parameters D/hc = 0.7292 cm−1 and E/
hc = 0.0048 cm−1 were calculated on the basis of the X2 and Y2
lines using Wasserman’s equations.19 Warming of the matrix
between 5 and 100 K caused the signal intensities to decrease,
but the intensities were recovered upon recooling (Figure 3b).
Thus, it can be concluded that vinylnitrene 32 is stable up to at
least 100 K.
The D and E values derived from Figure 3 are indicative of a
delocalized nitrene. A linear correlation of D with the natural
spin density on nitrogen ρ calculated at the UB3LYP/EPRIII
level for over 100 nitrenes has been reported,20 and the data for
32 fit the graph precisely (see Supporting Information).
However, the D and ρ values of naphthoquinon-2-ylnitrene
32 (0.729 and 1.409) are significantly lower than those for 2-
naphthylnitrene (0.925 and 1.521) but just slightly below those
for 1-naphthylnitrene (0.793 and 1.449), which means that 32 is
more delocalized, i.e., more biradical-like, but it is definitely a
nitrene. Other conjugated vinylnitrenes 4-cyanobutadienylni-
trenes and 4-cyano-4-azabutadienylnitrenes have significantly
lower D and ρ values 0.40−0.55 and 1.25−1.27, respectively.16
Transient Absorption Spectroscopy. The laser flash
photolysis (excimer laser, 308 nm, 17 ns)21 of 1 in argon-
saturated acetonitrile produced a broad transient spectrum with
λmax ∼460 nm and a smaller shoulder at ∼350 nm (Figure 4).
We assign this transient absorption to triplet vinylnitrene 32 on
calculation places the major electronic transitions for 2 at 311
nm (f = 0.0522), 364 nm (f = 0.0126), and 484 nm (f =
0.0122) (Figure 4), which fit nicely with the observed
spectrum. In comparison, TD-DFT calculations predicted the
most intense electronic transitions for the precursor T1 of 1
(Figure 4) as being at 309 nm (f = 0.0352), 376 nm (f =
0.0605), 385 nm (f = 0.0611), 393 nm (f = 0.1872), and 521
nm (f = 0.0324), which does not fit the observed spectrum.
Kinetic analysis further supports the assignment of the
3
transient absorption to triplet vinylnitrene 2, as the kinetic
traces obtained at 350 and 460 nm in argon-saturated
acetonitrile on shorter time scales can be fitted as
monoexponential growth to yield a rate constant of 1.76 ×
107 s−1 (τ ∼57 ns) for the formation of vinylnitrene 32 from T1
of 1. At longer time scales, the decay of vinylnitrene 32 yields a
rate constant of 4.46 × 104 s−1 (τ ∼22 μs, Figure 5). A residual
absorption that does not decay significantly on a millisecond
time scale is ascribed to product formation.
In oxygen-saturated acetonitrile, the formation of vinyl-
3
nitrene 2 is faster than the time resolution of the laser flash
apparatus (17 ns, Figure 5). Therefore, oxygen must reduce the
lifetime of T1 of 1, leading to a much reduced yield of triplet
3
vinylnitrene 2. No new absorption was detected that can be
attributed to the addition of oxygen to vinylnitrene 32 to form a
peroxy radical, and it is concluded that the quenching of T1 of 1
by oxygen is more efficient than the trapping of triplet
3
vinylnitrene 2.
Low-Temperature UV−Vis Absorption Spectroscopy.
Azide 1 was dissolved in MTHF with N2 bubbled through the
solution at room temperature for 10 min. The resulting
solution was placed in liquid N2, and its absorption spectrum
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the first and second triplet excited states (T1 and T2) of 1 at 49
and 54 kcal/mol above the S0 of 1, respectively.
In comparison, the optimization of T1 of 1 places it 46 kcal/
mol above the S0 of 1. As expected, the calculated vertical
energy of T1 of 1 is only a few kcal/mol higher in energy than
its relaxed structure. The optimized structure of T1 of 1 has a
(π,π*) configuration, as the CαCβ bond has single bond
character and the carbonyl groups have similar bond lengths as
in the S0 of 1 (Figure 7). Spin density calculations further
Figure 5. Kinetic traces obtained at 460 nm from the laser flash
photolysis of 1 in argon-saturated (red) and oxygen-saturated (black)
acetonitrile.
measured at 84 K showed a broad absorption between 320 and
410 nm with maximum absorption at approximately 350 and
390 nm due to the absorption of ground state of azide 1, which
is similar to the absorption spectrum of 1 at ambient
temperature (Figure 6).
Figure 7. Optimized structures of 1, T1 of 1, triplet vinylnitrene 2, and
1
singlet vinylnitrene 2. The calculated vinyl CC and CN bond
lengths and spin densities (in parentheses) are listed.
support this assignment, as the unpaired spins are located on
the α-C and β-C atoms and both the carbonyl oxygen atoms
and the terminal N atom of the azido group, which are in
conjugation with the α-C and β-C atoms.
Figure 6. Absorption spectra obtained by irradiating 1 in (A) glassy
MTHF matrix at 84 K and (B) MTHF solution at ambient
temperature.
3
The calculated spin density of vinylnitrene 2 demonstrates
that the unpaired spin is located mainly on the N atom (1.42)
and the β-carbon atom (0.63). In addition, there is a small spin
density on the carbonyl O atom (0.21), which is conjugated
with the β-carbon atom (Figure 7). From these calculations, it
can be concluded that vinylnitrene 32 has significant 1,3-
biradical character.
Irradiation of the glassy matrix with 340 nm light resulted in
a broad new absorption band with λmax at ∼450 nm with small
shoulders at approximately 370 and 520 nm (Figure 6). Further
irradiation of the glassy MTHF matrix increased the intensity of
these bands with simultaneous depletion of the absorptions at
350 and 390 nm ascribed to azide 1. Because the absorption
band at ∼450 nm matches the transient absorption obtained by
the laser flash photolysis of 1, the absorption band at ∼450 nm
The calculated transition state for the formation of
3
vinylnitrene 2 from the T1 of 1 was located only 2 kcal/mol
above the triplet precursor and is therefore easily accessible.
The stationary points on the triplet surface of 1 are plotted in
Figure 8, which show that, upon the population of S1 of 1, it
can undergo intersystem crossing to yield T2 of 1, which has an
(n,π*) configuration, and then internal conversion to form T1
of 1 with a (π,π*) configuration.
3
is assigned to triplet vinylnitrene 2. Thus, both the photolysis
of azide 1 at ambient temperature and that in glassy matrices at
3
84 K resulted in the formation of triplet vinylnitrene 2.
Quantum Chemical Calculations. To better understand
the reactivity of 1, density functional theory (DFT) calculations
were performed at the B3LYP/6-31+G(d) level of theory using
Gaussian09.22
Time-dependent density functional theory (TD-DFT)
calculations on the optimized structure of 1 places the first
excited singlet state (S1) of 1 at 63 kcal/mol above its S0 and
The open shell configuration of vinylnitrene 12 was
optimized using the broken-symmetry method with guess=mix
as a keyword as featured in Gaussian09, resulting in a singlet
nitrene with S2 value of 1.01. Optimization of the singlet
1
vinylnitrene 2 places it 16 kcal/mol higher in energy than
triplet vinylnitrene 32. Therefore, the singlet−triplet energy gap
D
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9A). The calculated transition state for the α-cleavage of
3
vinylnitrene 2 to form this ketenimine radical is 23 kcal/mol
and is therefore easily accessible photochemically. The
transition state for the rearrangement of ketenimine radical 6
to form T1 of 3 is only 8 kcal/mol above 6 and is thus also very
feasible. Because the formation of nitrile 3 requires intersystem
crossing to the singlet surface, it is reasonable that ketenimine
radical 6 intersystem crosses to form the S0 of nitrile 3 rather
than T1 of 3. Thus, the calculations support the observation
that vinylnitrene 2 forms nitrile 3 photochemically, and the
formation of this nitrile in a stepwise reaction seems more
favorable.
Figure 8. Calculated stationary points on the triplet surface of 1. The
energies of the S1 (63), T1 (49), and T2 (54) of 1 were obtained from
the TD-DFT calculations, whereas the energies of S0 (0), T1 (46), TS
(48), and triplet vinyl nitrene 32 were obtained by optimization
calculations. Energies are in kcal/mol.
3
Product Analysis. Naruta et al. have shown previously that
the photolysis of azide 1 in the presence of 2,4-hexadiene yields
cycloaddition products 7 and 8 (Scheme 5).23 We repeated this
for 2 is similar to the values calculated for several other
vinylnitrene derivatives.7−9,17
Although triplet vinylnitrene 2 did not produce azirine 4 or
3
3
Scheme 5. Trapping of Triplet Vinylnitrene 2 with 2,4-
ketenimine 5 in cryogenic matrices, we optimized the structures
of these compounds to determine whether their formation is
energetically feasible (Scheme 4). Ketenimine 5 is 16 kcal/mol
Hexadiene
Scheme 4. Potential Product of the Intersystem Crossing of
3
Triplet Vinylnitrene 2
experiment and obtained the same results. Adducts 7 and 8
3
must be formed by trapping triplet vinylnitrene 2 with the
diene. In comparison, the photolysis of azide 1 in chloroform,
methanol, and acetonitrile at room temperature results in a
polymeric tar. The product studies indicate that vinylnitrene 32
is sufficiently long-lived to be intercepted in bimolecular
reactions, whereas, in the absence of a trapping agent, it forms
products that are not stable enough to be isolated.
more stable than vinylnitrene 32, whereas azirine 4 is
isoenergetic with vinylnitrene 2. Thus, azirine 4 must possess
3
considerable ring strain and is expected to be highly reactive.
We calculated the stationary points on the triplet surface of
3
DISCUSSION
vinylnitrene 2 for the formation of nitrile 3 (Figure 9). The
■
Intramolecular sensitization makes it possible to form triplet
3
vinylnitrene 2 efficiently from azide 1. The T1 of 1 has a
(π,π*) configuration, which resembles a triplet 1,2-biradical
that is stabilized by the adjacent carbonyl and azido groups. T1
3
of 1 is an excellent precursor to triplet vinylnitrene 2, as the
biradical centers are lined up perfectly to reform the vinylic
CC bond upon extrusion of the N2 molecule (Scheme 6). In
Scheme 6. Formation of Triplet Vinylnitrenes from the
(π,π*) Triplet Excited State of 1 and 1,2-Biradical Forms of
Triplet Excited Vinyl Azides
Figure 9. Calculated energy surfaces for triplet vinylnitrene 32 forming
nitrile 3 on the triplet surface: (A) stepwise and (B) concerted.
Energies are in kcal/mol.
3
transition state for vinylnitrene 2 forming the T1 of nitrile 3
comparison, the formation of vinylnitrenes by the intra-
molecular sensitization of simple azidoalkenes yield orthogonal
1,2-biradicals to minimize the repulsion between the two radical
centers; therefore, these two radical centers have to rotate to
reform the vinylic CC bond and extrude the N2 molecule.17
Because triplet vinylnitrene 32 is stable at cryogenic
temperatures, it was observed directly by IR and ESR
3
was located 44 kcal/mol above vinylnitrene 2 (Figure 9B).
Thus, the formation of nitrile 3 is accessible upon the excitation
3
of vinylnitrene 2.
The formation of nitrile 3 can also take place by the
photoinduced α-cleavage of vinylnitrene 32 to form a
ketenimine radical 6, which rearranges into nitrile 3 (Figure
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spectroscopy, although further irradiation causes 32 to
rearrange to nitrile 3. Wenthold and Hossain have demon-
strated that radical stabilization by electron delocalization
effectively reduces the triplet−singlet energy gap of vinyl-
nitrenes.24 However, the calculated energy gap between the
open shell singlet and triplet configuration of vinylnitrene 2 of
16 kcal/mol is similar to the calculated energy gap for
vinylnitrenes 9, 10, and 11, which are not stable in cryogenic
argon matrices (Scheme 7), as they form ketenimines.7−9
Therefore, the stability of vinylnitrene 32 cannot be credited to
an unusually large energy gap between its singlet and triplet
configurations.
crossing to form products more efficiently than vinylnitrene 32.
Conversely, triplet phenylnitrene derivatives do not decay by
intersystem crossing, as the phenyl ring also prohibits rotation
around the aromatic bonds. The calculated spin density of
3
vinylnitrene 2 explains nicely why it reacts efficiently with
alkenes to form adducts 7 and 8 (Scheme 5). Similarly, the spin
density of phenylnitrene also reflects that it undergoes
dimerization to form azobenzene (Scheme 8).
Scheme 8. Comparison of the Calculated Spin Densities for
3
Triplet Vinylnitrene 2 and Phenylnitrene
Scheme 7. Calculated Rotation Barrier around the CC
CN Bond in Triplet Vinylnitrenes 9, 10, and 11 and Spin
Densities on N and C Atoms
CONCLUSION
■
We have shown that triplet vinylnitrene 32 is formed efficiently
through intramolecular triplet sensitization upon irradiation of
vinyl azide 1. Triplet vinylnitrene 2 was thermally stable in
3
cryogenic matrices up to at least ∼100 K and was characterized
directly by ESR and IR spectroscopy. Under the irradiation
conditions in the Ar-matrix at 8 K, a novel photoinduced
3
rearrangement reaction of vinylnitrene 2 was found to give
3
The properties of triplet vinylnitrene 2 are reminiscent of
nitrile 3. The ESR spectroscopy demonstrates that triplet
vinylnitrene 32 has a significant 1,3-biradical character, as
previously perceived from calculations. Because triplet vinyl-
triplet phenyl- and alkylnitrenes, which are also stable in
cryogenic matrices and have been identified directly using ESR,
UV−vis, and IR spectroscopy.14,25−28 The significant difference
between triplet vinylnitrenes and alkyl- and phenylnitrenes is
that the latter ones do not react via intersystem crossing to the
singlet states but rather by dimerization. Because the singlet−
triplet energy gap for alkylnitrenes is large (31.2 kcal/mol for
methylnitrene),29 they are not expected to undergo intersystem
cross efficiently, as intersystem crossing is enhanced by small
energy gaps and spin−orbit coupling. In comparison, the
singlet−triplet energy gap for phenylnitrene has been measured
as 15 kcal/mol,30 which is similar to the calculated energy gap
for vinylnitrenes 2, 9, 10, and 11, which raises the question of
why these nitrenes react so differently. A comparison of the
calculated spin density for triplet phenylnitrene and vinyl-
3
nitrene 2 is part of a bicyclic ring system, it is rigid, which
restricts the rotation around the vinylic CC bond and
renders it more stable than other vinylnitrenes due to less facile
intersystem crossing. Thus, we have demonstrated that the
unique reactivity of triplet vinylnitrenes is due to their
flexibility, which is a reflection of their significant 1,3-biradical
character. Triplet vinylnitrenes can be stabilized by limiting the
flexibility of the vinyl CC bond; thus, it may be possible to
design various triplet vinylnitrenes as potential building blocks
for high-spin assemblies.
EXPERIMENTAL SECTION
■
Matrix IR Spectroscopy. First, argon was deposited at 20 K for 5
min on a CsI target inside the cryostat cold head. The sample was then
sublimed at 50 °C and deposited with argon at 20 K for 10 min in the
vacuum of a diffusion pump. The cryostat cold head was cooled to 8 K
for sample irradiation. The argon matrix containing the deposited
molecules of 1 was irradiated through quartz windows using a 320−
360 nm filter, and IR spectra were collected after various irradiation
times.
ESR Spectroscopy. A 0.15 mM solution of 1 in anhydrous MTHF
was degassed by three freeze/thaw cycles using diffusion and rotary
pumps. The tube containing the solution was sealed after degassing
and used for ESR spectroscopy measurements. ESR spectra were
recorded on a Bruker instrument.
Quantum Chemical Calculations. The ground states, triplet
excited states, vinyl nitrene (2), and product (3) were optimized by
performing density functional theory (DFT) calculations using
Gaussian09 at the B3LYP level of theory with the 6-31+G(d) basis
set.22,32,33 The natural spin densities were calculated at the UB3LYP/
EPRIII level as implemented in Gaussian03.34 To obtain the calculated
UV−vis spectra for the excited states and intermediates, time-
dependent DFT (TD-DFT) calculations were conducted at the
B3LYP level of theory with the 6-31+G(d) basis set. The same type of
calculation was used to locate the excited singlet (S1) and triplet states
(T1, T2) of the azide 1. The transition states were confirmed to
contain one imaginary vibrational frequency by the analytical
3
nitrene 2 demonstrates that the spin density in vinylnitrene 2
is localized to a greater extent on the β-C atom than in the case
for phenylnitrene. We have previously reported the calculated
spin densities of vinylnitrenes 9, 10, and 11, which are similar
to those of vinylnitrene 32. Thus, the most significant difference
between triplet vinylnitrenes 2, 9, 10, and 11 on one hand and
triplet phenylnitrene on the other is that the vinylnitrenes have
more 1,3-biradical character, which makes rotation around the
vinylic CC bond feasible for vinylnitrenes 9, 10, and 11. The
3
ring structure of vinylnitrene 2, however, severely limits the
rotation around its vinylic bond. The calculated rotational
barriers around the vinylic CC bond for triplet vinylnitrenes
9, 10, and 11 are only between 7.5 and 11 kcal/mol due to their
significant 1,3-biradical characters (Scheme 7). Thus, we
propose that a major reason for the stability of vinylnitrene
32 in matrices is due to its rigid cyclic structure, which limits
rotation around the vinylic CC bond. For triplet biradicals to
intersystem cross and form ground-state products, they need to
acquire the conformation best suited for intersystem crossing,
concurrently with new bond formation.31 Because the triplet
vinylnitrenes 9, 10, and 11 can rotate around their vinylic C
C bond, they are flexible and should undergo intersystem
F
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determination of the second derivative of the energy with respect to
the internal coordinates. Intrinsic reaction coordinate (IRC)
calculations were used to verify that the transition state is correlated
with the vinyl nitrene (2) and the triplet excited state of starting
material 1.35,36
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR and IR spectra of 1; graph showing the correlation of
D with the natural spin densities ρ for several nitrenes;
Cartesian coordinates and energies of 1−5; difference IR
spectra in Ar matrices. This material is available free of charge
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AUTHOR INFORMATION
■
Corresponding Author
anna.gudmundsdottir@uc.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation and the Ohio
Supercomputer Center for their generous support of this work.
A.D.G. is grateful to the Fulbright Foundation for giving her the
opportunity to be a Fulbright Scholar in Professor Abe’s
laboratory at Hiroshima University. This work was supported
by a Grant-in-Aid for Science Research on Innovative Areas
“Stimuli-responsive Chemical Species” (No. 24109008) from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan. ESR and MS measurements were
performed at N-BARD, Hiroshima University.
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DOI: 10.1021/jacs.5b00998
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