Matrix isolation and spectroscopic characterization of 2,5,6- trifluoropyridylnitren-3-yl

Article abstract of DOI:10.1002/ejoc.201101866

The photochemistry of 3-iodo-2,5,6-trifluoropyridyl azide (6c), matrix-isolated in argon, was investigated by IR and EPR spectroscopy. The primary photoproduct is 3-iodo-2,5,6-trifluoropyridylnitrene (7c) in its triplet ground state. Further irradiation produced very low yields of nitrene radical 3c, which shows a characteristic EPR spectrum. The yield of 3c, however, was too low for IR detection. Instead, azirinyl radical 11, formed from 3c by ring closure, could clearly be identified in the IR spectra. Other products observed were azirine 8c' and the ketenimine 9c', which are formed in a photostationary equilibrium together with nitrine 7c. Copyright

Full text of DOI:10.1002/ejoc.201101866

Job/Unit: B11866
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FULL PAPER
DOI: 10.1002/ejoc.201101866
Matrix Isolation and Spectroscopic Characterization of
2,5,6-Trifluoropyridylnitren-3-yl
Dirk Grote,[a] Christopher Finke,[a] Patrik Neuhaus,[a] and Wolfram Sander*[a]
Keywords: Radicals / EPR spectroscopy / IR spectroscopy / Matrix isolation / Photochemistry
The photochemistry of 3-iodo-2,5,6-trifluoropyridyl azide
(6c), matrix-isolated in argon, was investigated by IR and
EPR spectroscopy. The primary photoproduct is 3-iodo-2,5,6-
trifluoropyridylnitrene (7c) in its triplet ground state. Further
irradiation produced very low yields of nitrene radical 3c,
which shows a characteristic EPR spectrum. The yield of 3c,
however, was too low for IR detection. Instead, azirinyl radi-
cal 11, formed from 3c by ring closure, could clearly be iden-
tified in the IR spectra. Other products observed were azirine
8cЈ and the ketenimine 9cЈ, which are formed in a photo-
stationary equilibrium together with nitrine 7c.
usually large zero field splitting (zfs) parameters E in these
molecules, in accordance with quantum-chemical calcula-
tions.[1a,7c]
Introduction
High-spin molecules are the basis for the development of
organic magnets and are intensively studied in many
laboratories throughout the world.[1] A common motif used
to stabilize high-spin states are π radical centers coupled by
a meta-phenylene unit such as in m-xylylene 1 with a triplet
ground state[2] or trimethylenebenzene 2 with a quartet
ground state.[3] The meta-phenylene unit has proven to be
a robust coupling unit to achieve ferromagnetic coupling
between π radical centers.[4]
Precursors of nitrene radicals 3–5 are iodophenyl azides,
which, on photolysis, split off N2 to give phenyl nitrenes as
primary photoproducts. In a second step, requiring pro-
longed photolysis, the C–I bond is cleaved to produce the
corresponding nitrene radicals. Unwanted side reactions re-
ducing the yields of phenyl nitrenes are the photochemical
rearrangements to azirines 8 and ketenimines 9. These re-
arrangements have been extensively studied over the last
decades.[9] During the photolyis of phenyl azides, phenyl
nitrenes 7, azirines 8, and ketenimines 9 are formed in
photostationary equilibria with yields that depend on the
irradiation conditions (Scheme 1).[9a]
Fluorine substituents in the ortho position of the aro-
matic ring raise the barriers for these rearrangements,[10]
and thus fluorinated phenylnitrenes are formed in higher
yields. Because the efficiency of cleavage of the C–I bonds
in an inert matrix is low due to in-cage recombinations, the
only nitrene radicals that could be isolated in a matrix so
far bear at least one fluorine substituent in the ortho posi-
tion.[1a,7]
A different approach to molecules with high-spin ground
states is the combination of π and σ radical centers.[5] Ab-
initio and DFT calculations suggest that ortho- and para-
nitrene radicals 3 and 5 have a high-spin quartet ground
state, whereas meta-nitrene radical 4 is a doublet ground
state molecule.[6] This has been confirmed by matrix isola-
tion studies of several derivatives of 3 and 5, which all show
quartet ground states.[1a,7] Attempts to synthesize deriva-
tives of 4 failed, and only ring-opened products were ob-
served.[8] The electronic structure of 3 and 5 can be de-
scribed as σ,σ,π triradicals resembling both nitrenes and
carbenes, as shown in the resonance structures below. The
carbene character of nitrene radicals 3 and 5 results in un-
2,3,5,6-Tetrafluorophenylnitren-4-yl (5b) was the first ni-
trene radical that could be matrix-isolated and charac-
terized as a quartet ground state molecule by IR and EPR
spectroscopy.[1a,7a] In the following years, several phen-
ylnitren-4-yl compounds with a large variety of fluorine
substitution were synthesized and characterized by IR and
EPR spectroscopy.[7b]
[a] Lehrstuhl für Organische Chemie II, Ruhr-Universität
Bochum,
44780 Bochum, Germany
Fax: +49-234-321-4353
E-mail: wolfram.sander@rub.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101866.
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D. Grote, C. Finke, P. Neuhaus, W. Sander
FULL PAPER
low temperature matrices. For this reason, it was concluded
that the ring closure of 3b is a photochemical rather than
thermal reaction.[7c] Other products formed during the pho-
tolysis of 6b were the azirines 8b and 8bЈ and the keten-
imines 9b and 9bЈ (Scheme 2).
Substitution of a CF group in 3b by a nitrogen atom
results in a heterocyclic nitrene radical. These unique trirad-
icals would allow us to study the influence of a nitrogen
atom on the spin distribution in nitrene radicals and, in
particular, determine whether these systems still maintain a
high-spin ground state. Here, we report on the matrix isola-
tion and spectroscopic characterization of the first pyr-
idylnitrene radical.
Scheme 1. Photochemistry of phenyl azide 6.
Results
UV Irradiation (λ = 254 nm) of an argon matrix (16 K)
doped with small amounts of 3-iodo-4,5,6-trifluoropyridyl
azide (6c) results in a rapid decrease of the IR bands of 6c
and a rapid increase of newly formed strong absorptions
at 1572, 1487, and 1395 cm–1. By comparison with the IR
spectrum calculated at the (U)B3LYP/6-311G(d,p) level of
theory, these bands could be assigned to triplet 3-iodo-
2,5,6-trifluoropyridylnitrene T-7c (Table 1, Figure 1). The
calculated IR spectrum is in reasonable agreement with the
Recently, we reported on the matrix isolation and EPR experimental spectrum. At short irradiation times, 7c is the
spectroscopic characterization of the first ortho nitrene radi- only detectable photoproduct.
cal 3,4,5,6-tetrafluorophenylnitren-2-yl (3b).[7c] The EPR
Upon further irradiation with UV light of 254 nm, a new
spectrum of 3b clearly corroborates a quartet ground state set of IR absorptions at 1586, 1559, and 1276 cm–1 is
for this nitrene radical. By the less selective and less sensi- formed, while the bands assigned to triplet nitrene 7c con-
tive IR spectroscopic analysis, however, 3b could not be de- currently decreased. The absorption at 1831 cm–1 is charac-
tected. Instead, the thermodynamically more stable re- teristic of the cumulene C=C=N st. vibration of keten-
arranged azirinyl radical 10 was found (Scheme 2). Because imines.[7b,7c,9b,11] Detailed investigation of the IR spectrum
10 is a radical with a doublet ground state, whereas 3b has revealed that only one of the two possible ketenimines 9cЈ
a quartet ground state, the ring closure of 3b requires a spin is formed. No trace of the isomeric ketenimine 9c was ob-
inversion. This should result in an activation barrier of at served in the matrix (Figure 2, Table 2).
least the magnitude of the doublet–quartet gap of 3b, calcu-
The IR spectrum revealed additional bands that formed
lated to be 7.0 kcal/mol, and thus not occur thermally in simultaneously with the absorptions of the ketenimine. A
Scheme 2. Photochemistry of aryl azide 6b.
2
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Isolation and Characterization of 2,5,6-Trifluoropyridylnitren-3-yl
Table 1. IR spectroscopic data of 3-iodo-2,5,6-trifluoropyridyl-
nitrene (7c).
[a,b]
[b,c]
Mode Symm.
ν
[cm–1 [a]
]
Irel., exp.
ν
[cm–1 [c]
]
Irel., calc
˜
˜
exp.
calcd.
14
16
17
18
19
20
21
22
23
24
AЈ
AЈ
AЈЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
614.2
686.3
723.0
830.8
0.03
0.04
0.03
0.23
0.11
0.23
612.8
694.2
724.2
831.7
1057.9
1113.8
1223.5
1260.2
1282.3
1403.5
0.01
0.01
0.02
0.22
0.13
0.14
0.05
0.48
0.24
1.00
1048.4
1098.3
1220.5[d]
1266.8[d]
1275.7[d]
1381.3
1385.0
1395.3
1487.0
1571.5
[d]
–
[d]
–
[d]
–
0.08
0.14
1.00
0.76
0.95
25
26
27
AЈ
AЈ
AЈ
1415.5
1528.0
1591.6
0.53
0.46
0.54
Figure 2. IR spectra showing the photochemistry of nitrene T-7c
after 5 min irradiation at 254 nm (argon, 10 K). (a) Calculated
spectrum of nitrene T-7c (vibrational modes 17 to 27, see Table 1).
(b) Difference IR spectrum: Bands of T-7c pointing downwards
disappear and bands of ketenimine 9cЈ pointing upwards appear
during irradiation. (c) Calculated IR spectrum of 9cЈ. (d) Calcu-
lated spectrum of 9c (vibrational modes 16 to 27, see Table 2). All
calculations at the (U)B3LYP/6-311G(d,p) level of theory.
[a] Ar, 3 K. [b] Band intensity relative to the strongest absorption.
[c] UB3LYP/6-311G(d,p), unscaled. [d] Assignment tentative due to
some deviations in intensity and band positions between experi-
mental and calculated spectrum probably caused by Fermi reso-
nance.
Table 2. IR spectroscopic data of ketenimine 9cЈ.
[a,b]
[b,c]
Mode
ν
[cm–1][a]
Irel., exp.
ν
[cm–1][c] Irel., calc
˜
˜
exp.
calcd.
16
17
18
19
20
21
22
23
24
25
26
27
681.0
689.3
808.4
964.2
1125.9
1198.7
1264.5
1275.9
1320.4
1558.7
1586.3
1830.6
0.13
0.09
0.46
0.36
0.66
0.63
0.21
1.00
0.25
0.71
0.70
0.04
700.3
708.9
819.6
970.2
1143.5
1232.6
1274.9
1291.9
1342.9
1566.9
1624.9
1893.3
0.04
0.04
0.33
0.18
0.35
0.39
0.46
1.00
0.27
0.97
0.53
0.35
Figure 1. IR spectra showing the 254 nm photochemistry of azide
6c in argon at 10 K after 1 min irradiation. (a) Difference IR spec-
trum: Bands of 6c pointing downwards disappear and bands of
triplet pyridylnitrene T-7c pointing upwards appear during irradia-
tion. (b) IR spectrum of T-7c calculated at the UB3LYP/6-
311G(d,p) level of theory (vibrational modes 14 to 27, see Table 1).
[a] Ar matrix, 3 K. [b] Relative Intensity based on the strongest ab-
sorption. [c] B3LYP/6-311G(d,p), unscaled.
strong band appeared at 1450 cm–1, and some weaker bands
at 1366, 1102, 953, and 818 cm–1. These bands increased
upon subsequent UV irradiation, whereas bands assigned
to pyridyl nitrene decreased. After 3.5 h irradiation at
254 nm, the newly formed species was the final photoprod-
uct found in the matrix (Figure 3). By comparison with
UB3LYP/6-311G(d,p) calculations (Table 3) the newly
formed compound was assigned to azirinyl radical 11. Ac-
cording to DFT calculations, radical 11 is 15.5 kcal/mol
more stable than quartet nitrene radical Q-3c.
Irradiation of the matrix containing ketenimine 9cЈ with
visible light (λ = 450 nm) yields new IR signals at 1509,
1359, 1304, and 1201 cm–1, while the bands assigned to 9cЈ
decrease at the same time (Figure 4, Table 4). By compari-
son with absorptions calculated at the B3LYP/6-311G(d,p)
level of theory, the new bands are assigned to azirine 8cЈ.
The isomeric azirine 8c was not found in the matrix. The
band of 8cЈ calculated at 1620 cm–1 was not observed in
Figure 3. IR spectra showing the photochemistry of azide 6c (3.5 h
irradiation at 254 nm, argon, 3 K). (a) Difference IR spectrum:
Bands pointing upwards assigned to 11 appear, and bands pointing
downwards disappear during irradiation. (b) IR spectrum of 11
calculated at the UB3LYP/6-311G(d,p) level of theory (vibrational
modes 15 to 23, see Table 3).
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D. Grote, C. Finke, P. Neuhaus, W. Sander
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Table 3. IR spectroscopic data of azirinyl radical 11.
Table 4. IR spectroscopic data of azirine 8cЈ.
[a,b]
[b,c]
[a,b]
[b,c]
Mode Symm. ν
[cm–1][a] Irel., exp.
ν
[cm–1][c] Irel., calc
Mode
ν
[cm–1][a]
Irel., exp.
ν
[cm–1][c]
Irel., calc
˜
˜
˜
˜
exp.
calcd.
exp.
calcd.
13
14
15
16
17
18
19
20
21
22
23
24
AЈЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
–
–
–
–
715.4
724.5
763.9
945.2
1075.4
1191.4
1353.8
1390.7
1444.9
1465.0
1506.4
1784.8
0.01
0.01
0.04
0.04
0.08
0.12
0.03
0.08
0.08
1.00
0.36
0.02
15
16
17
18
19
20
21
22
23
24
25
26
27
683.0
693.5
753.3
881.3
1018.2
–
0.08
0.07
0.37
0.24
0.48
–
701.7
715.1
764.1
891.5
1029.3
1081.2
1121.4
1200.7
1333.0
1368.3
1525.1
1620.4
1774.4
0.03
0.03
0.08
0.09
0.09
0.10
0.11
0.24
0.36
1.00
0.38
0.44
0.08
644.8
817.5
952.8
1101.9
–
1365.9
–
1450.4
–
0.05
0.12
0.13
0.35
–
0.33
–
1.00
–
–
–
1200.5
1303.8
1359.0
1508.9
–
0.41
0.54
1.00
0.44
–
–
–
–
–
[a] Argon matrix, 3 K. [b] Relative intensity based on the strongest
absorption. [c] UB3LYP/6-311G(d,p), unscaled.
[a] Argon matrix, 3 K. [b] Relative intensities based on the strong-
est absorption. [c] B3LYP/6-311G(d,p), unscaled.
the difference spectrum because it overlaps with a broad
absorption of ketenimine 9cЈ at 1586 cm–1. UV irradiation
of 8cЈ yields ketenimine 9cЈ and the azirinyl radical 11.
Figure 5. EPR spectra showing the photochemistry of pyridyl
nitrene T-7c. (a) EPR spectrum after UV irradiation (argon, 5 K)
of azide 6c. The signals x2 and y2 are assigned to nitrene T-7c with
a D value of |D/hc| = 1.110 cm–1 (g = 2.003, ν = 9.60883 GHz). A
very strong signal at g = 2 indicates the formation of radical species.
(b) Simulated spectrum of quartet nitrene radical Q-3c with ZFS
parameters |D/hc| = 0.384 cm–1 and |E/hc| = 0.0088 cm–1 (g = 2.003,
ν = 9.60883 GHz).
Figure 4. IR spectra showing the 450 nm photochemistry of keteni-
mine 9cЈ in argon at 3 K after 10 seconds irradiation with visible
light (λ = 450 nm). (a) Calculated spectrum of ketenimine 9cЈ (vi-
brational modes 13 to 27, see Table 2). (b) Difference IR spectrum:
Bands of 9cЈ pointing downwards disappear and bands of azirine
8cЈ pointing upwards appear during irradiation. No traces of 8c
were found in the matrix. (c) Calculated IR spectrum of 8c (vi-
brational modes 13 to 27, see Table 4); (d) Calculated IR spectrum
of 8cЈ (vibrational modes 11 to 27). All calculations at the B3LYP/
6-311G(d,p) level of theory.
of the spin Hamiltonian, and are typical of EPR spectra of
high-spin systems with S Ͼ 3/2. These transitions are as-
signed to orientations of about 25° in the zx plane and 30°
in the zy plane (Figure 7). Transitions parallel to z are weak
and were not observable in the experimental spectrum un-
To detect the paramagnetic species formed during the der these conditions. Figure 7 also shows the slightly off-
photolysis of 6c, EPR spectra of the photoproducts were axis behavior of transition z4,[1a] which becomes allowed if
recorded in argon at 5 K. A set of very weak signals charac- the z axis is rotated slightly away from the direction of the
teristic of a quartet species with ZFS parameters |D/hc| = magnetic field.[12]
0.384 cm–1 and |E/hc| = 0.0088 cm–1 (g = 2.003, ν =
The EPR signals of 3c disappear upon annealing of the
9.60883 GHz) suggests the formation of quartet nitrene matrix at 30 K, which is typical of matrix-isolated radical
radical 3c. By solving the spin Hamiltonian for the orienta- pairs that recombine at temperatures at which the matrix
tion of the quartet spin system parallel to the axes x and y, becomes soft, and trapped species are able to diffuse
the EPR signals were assigned to the transitions y1, x1, x3, rapidly. If the matrix is cooled back to 5 K and irradiated
and y3 (see Figures 5, 6).
with UV light, the signals of 3c appear again.
Two EPR transitions at 6240 and 6500 G cannot be as-
In addition to the signals of 3c, the EPR spectrum shows
signed to the canonical orientations. These “off-axis” tran- very weak x2 and y2 transitions of a triplet system, assigned
sitions (labeled “A” in Figure 6) are higher order solutions to nitrene T-7c. By simulating a triplet species with a D
4
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Isolation and Characterization of 2,5,6-Trifluoropyridylnitren-3-yl
Figure 7. Angular dependence of the EPR signals of Q-3c when
varying the field orientation from parallel to x to parallel to z and
parallel to y. The signals A are assigned to off-axis transitions. The
plot also shows the slightly off-axis behavior of transition z4.
Figure 6. The four sublevels of quartet Q-3c with the external field
H parallel to the z, x, and y-axis, respectively, calculated using the
ZFS parameters |D/hc| = 0.384 cm–1, |E/hc| = 0.0088 cm–1 (g =
2.003).
photostationary equilibrium with 7c and 9cЈ. The isomeric
azirine 8c and ketenimine 9c are not observed under these
conditions, which indicates that the rearrangement of 7c is
highly selective (Scheme 3). The same type of selectivity, al-
though less pronounced, was previously found for the re-
arrangement of 7b, for which 8b and 9b were the minor
products (Scheme 2).[7c]
The photochemical rearrangement of triplet 7c to the
singlet products 8cЈ and 9cЈ involves an intersystem crossing
step from the triplet to the singlet surface. Although the
rearrangement of 7c is induced by light, we cannot discrimi-
nate true photochemical reactions from “hot” ground state
reactions. Presumably, the irradiation of T-7c results in the
formation of vibrationally excited S-7c, which subsequently
rearranges to 8cЈ and 9cЈ.
value of |D/hc| = 1.110 cm–1 (g = 2.003, ν = 9.60883 GHz),
the spectrum of T-7c is reproduced. Due to the low signal
intensity and overlap with the quartet signals, the simula-
tion is only tentative and no reliable E value could be de-
rived. A very strong doublet signal appears at g = 2, indicat-
ing that radicals are formed as the final products of the UV
irradiation. Due to the lack of resolved hyperfine splitting,
the radicals could not be assigned.
Discussion
Photochemsitry of 7c
To gain insight into the mechanisms of these rearrange-
The photochemistry of 7c is quite similar to that of 2- ments, we compared the calculated energies and activation
iodo-3,4,5,6-tetrafluorophenyl azide (7b) described pre- barriers of 7c and its photoproducts (Figure 8) and found
viously.[7c] Upon irradiation with UV light, triplet nitrene that 8cЈ is 6.6 kcal/mol more stable than 8c. The barriers
T-7c is the first photoproduct formed. Subsequent UV irra- for the formation of 8c and 8cЈ are 27.6 and 24.9 kcal/mol,
diation results in the formation of ketenimine 9cЈ, which is respectively, and thus the thermodynamically preferred azir-
formed through ring opening of azirine 8cЈ. When the ma- ine 8cЈ is also kinetically preferred by 2.7 kcal/mol. Similar
trix is irradiated with visible light (λ = 420 nm), azirine 8cЈ correlations were observed for a large number of substi-
is produced in yields high enough for IR detection in a tuted phenylnitrenes.[7b,7c] In all cases the preferred pho-
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Scheme 3. Photochemistry of 3-iodo-2,5,6-trifluoropyridyl azide (6c) in argon or neon matrix.
toproduct depends on the relative stabilities of the formed value could not be determined. Due to the heavy atom ef-
azirines and ketenimines, as well as on the activation barri- fect of iodine, the ZFS parameters of 7b and 7c are signifi-
ers for the formation of azirines.
cantly influenced by spin-orbit coupling (SOC).
Nitrene radical 3c is formed from nitrene 7c upon subse-
quent irradiation with UV light in very low yield. The D
value obtained from the EPR spectrum of 3c by simulation
(|D/hc| = 0.384 cm–1) is larger than that found for nitrene
radical 3b (|D/hc| = 0.357 cm–1), while the E value of 3c
(|E/hc| = 0.0088 cm–1) is smaller than that of 3b (|E/hc| =
0.0136 cm–1).
DFT calculations indicate that nitrene radicals such as
3b and 3c simultaneously exhibit a nitrene and a carbene
character that both contribute to the overall quartet D ten-
sor. The magnetic z axis of the nitrene subunit is oriented
parallel to the C–N bond, whereas the easy axis of the carb-
ene subunit lies parallel to a hypothetical 180° bond angle
at the spin center. The nitrene center, with larger dipolar
interactions, dominates the overall spin system, but the di-
polar field of the carbene moiety contributes to the y direc-
tion perpendicular to the z axis of the nitrene. Thus, the
easy axis of the D tensor of the quartet spin system of 3b
is rotated about 1° in the molecular plane away from the
C–N bond. The carbene contribution in the y direction
leads to magnetically nonequivalent x and y axes, which is
reflected in the E value of the quartet system. This qualita-
tive description of the magnetic properties can also be ap-
Figure 8. Energies of 7c and its isomers calculated at the
(U)B3LYP/6-311G(d,p) level of theory.
Nitrene Radical
As expected, the primary photoproduct of the photolysis plied to 3c. The difference between 3b and 3c is the replace-
of azide 6c is pyridyl nitrene 7c. However, 7c is formed in ment of a CH group in the aromatic ring by a nitrogen
lower yields than phenyl nitrene 7b under comparable con- atom in 3c, which changes the spin distribution in the ring.
ditions from phenyl azide 6b, indicating the sensitivity of 7c The spin population at the nitrene center of 3c is calculated
towards rearrangements to ketenimines and azirines. The to be 1.64, which is slightly higher than in 3b (1.60),
EPR spectrum of 7c exhibits the characteristic pattern and whereas the spin population at the carbene center is lower
ZFS parameters of a triplet phenylnitrene, similar to that in 3c (1.24) than in 3b (1.26). This finding is in accordance
of iodinated phenyl azides investigated previously.[1a,7,8] The with the EPR results. A comparison of the EPR spectra
xy2 transition is quite broad, indicating a nonzero E value reveals that the D value of 3c is larger than that of 3b,
that is smaller than that of 7b (|E/hc| = 0.0071 cm–1). How- whereas the E value is smaller. The D value is strongly influ-
ever, because no splitting of the signal was observed, and enced by the delocalization of the π electron into the aro-
because the signal overlapped with quartet signals, the E matic ring. Because this delocalization is smaller in 3c, due
6
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Isolation and Characterization of 2,5,6-Trifluoropyridylnitren-3-yl
to the nitrogen atom in the ring, the π electron is more a larger D value of the quartet system, which can be de-
localized at the nitrene center, which results in the larger D tected by EPR spectroscopy. The thermodynamic sink for
value of 3c. The E value, in turn, is strongly influenced by ortho nitrene radicals such as 3c is the formation of a bond
the carbene contribution to the overall spin system. This between the nitrogen atom and the ortho carbon atom to
influence is smaller in 3c because of the lower spin popula- give azirinyl radicals. With radical 11 the second example
tion at the carbene center. The larger spin population at the of these elusive radicals has now be spectroscopically char-
nitrene center adds to the nitrene contribution exceeding acterized.
the carbene contribution, resulting in the smaller E value in
3c compared to 3b.
Experimental Section
Azirinyl Radical
3-Iodo-2,5,6-trifluoropyridyl Azide (6c): Synthesized analogously to
2-iodo-3,4,5,6-tetrafluorophenyl azide[7c] (6b). Yield 55%. IR (Ar,
Azirinyl radical 11 is most likely formed from nitrene
radical 3c in an excited doublet state by ring closure be-
tween the nitrogen atom and the adjacent radical center in
the ortho position.
At the UB3LYP/6-311G(d,p) level of theory, 11 was cal-
culated to be 14.5 kcal/mol more stable than the nitrene
radical 3c. A similar azirinyl radical 10 was described pre-
viously as the final photoproduct of the UV photolysis of
azide 6b.[7c] According to DFT calculations, both azirinyl
radicals are planar. With a spin population of 0.62, the azir-
ine nitrogen atom bears the major part of the spin density
in the radical. The rest of the spin density is delocalized
into the aryl rings of 10 and 11, respectively (Figure 9).
4 K): ν (%) = 2120.8 (85), 1506.6 (100), 1481.1 (45), 1405.1 (7),
˜
1283.9 (6), 1252.1 (10), 1119.3 (23), 1065.7 (24), 934.5 (23), 818.8
(22) cm–1. 19F NMR (400 MHz, CDCl3): δ = –56.1 (s), –88.8 (s),
–155.9 (s) ppm. 13C NMR (100 MHz, CDCl3): δ = 64.8 (d, J =
50.1 Hz), 136.5 (d, J = 258.1 Hz), 144.6 (s), 149.7 (d, J = 229.4 Hz),
155.3 (d, J = 237.4 Hz) ppm. MS: m/z (%) = 300 (100) [M+], 272
(7) [M+ – N2], 145 (35) [M+ – I – N2], 127 (20) [M+ – N2 – I – F],
76 (10) [M+ – I – N3 – 3F]. C5F3IN4 (302.00): calcd. C 19.8, N
18.5; found C 19.99, N 18.35.
Matrix Isolation: Matrix isolation experiments were performed by
using standard techniques[13] with closed-cycle helium cryostats al-
lowing cooling of a CsI spectroscopic window to 4 K. FTIR spectra
were recorded with a standard resolution of 0.5 cm–1, using a N2(l)-
cooled MCT detector in the range 400–4000 cm–1. X-band EPR
spectra were recorded with a Bruker Elexsys E500 EPR spectrome-
ter with an ER077R magnet (75 mm pole cap distance), an ER047
XG-T microwave bridge, and an oxygen-free high-conductivity cop-
per rod (75 mm length, 3 mm diameter) cooled by a closed-cycle
cryostat.
Broadband irradiation was carried out with mercury high-pressure
arc lamps in housings equipped with quartz optics and dichroic
mirrors in combination with cutoff filters (50% transmission at the
wavelength specified). IR irradiation from the lamps was absorbed
by a 10 cm path of water. A low pressure mercury arc lamp was
used for 254 nm irradiation.
Figure 9. Spin-density distribution of azirinyl radical 11 [UB3LYP/
6-311G(d,p)], which shows Cs symmetry; the spin density is delo-
calized into the aromatic π system.
Computational Methods: Optimized geometries and vibrational fre-
quencies of all species were calculated at the B3LYP level[14] em-
ploying the 6-311G(d,p) polarized valence-triple-ξ-basis set.[15]
A
spin-unrestricted formalism was used for all high-spin systems and
for singlet biradicals, whenever an external instability[16] was ob-
served. All DFT calculations were carried out with the Gaussian
03 suite of programs.[17] Computer simulation of the EPR spectra
were performed by using the XSophe computer simulation software
suite (version 1.0.4),[18] developed by the Centre for Magnetic Reso-
nance and Department of Mathematics, University of Queensland,
Brisbane (Australia) and Bruker Analytik GmbH, Rheinstetten
(Germany).
Conclusions
The chemistry of triplet nitrene 7c and quartet nitrene
radical 3c nicely fits into the general scheme that has been
developed for these reactive intermediates. The yields of
photochemically generated nitrene radicals such as 7c is di-
minished by rearrangements to the corresponding azirines
and ketenimines.[7b,7c,8,9] By carefully selecting the irradia-
tion conditions (wavelength and monochromatic vs. broad-
band source), the yield of the nitrenes can be optimized, but
generally remains quite poor. Nitrene radicals with radical
centers in the ortho or para positions with respect to the
nitrene unit show quartet ground states, and nitrene radical
3c is no exception to this rule. The presence of the ring
nitrogen atom results in a shift of charge density towards
this nitrogen atom, and an increased spin density at the
nitrene center, compared to nitrene radical 3b. The in-
Supporting Information (see footnote on the first page of this arti-
cle): Additional figures and tables, and synthetic, spectroscopic and
analytical data. Geometries, total energies and IR spectroscopic
data of the photoproducts. Relative energies of all rearrangement
1
products, and H and 13C NMR spectra.
Acknowledgments
This work was financially supported by the Deutsche Forschungs-
creased spin density at the nitrene center, in turn, results in gemeinschaft (DFG) and the Fonds der Chemischen Industrie.
Eur. J. Org. Chem. 0000, 0–0
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Job/Unit: B11866
/KAP1
Date: 26-04-12 15:02:35
Pages: 9
D. Grote, C. Finke, P. Neuhaus, W. Sander
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Received: December 30, 2011
Published Online: ᭿
8
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Job/Unit: B11866
/KAP1
Date: 26-04-12 15:02:35
Pages: 9
Isolation and Characterization of 2,5,6-Trifluoropyridylnitren-3-yl
High-Spin Molecules
D. Grote, C. Finke, P. Neuhaus,
W. Sander* ....................................... 1–9
Matrix Isolation and Spectroscopic
Characterization of 2,5,6-Trifluoropyridyl-
nitren-3-yl
Keywords: Radicals / EPR spectroscopy /
IR spectroscopy / Matrix isolation / Photo-
chemistry
The combination of an aryl radical with an
arylnitrene results in new motives for the
construction of organic high-spin mol-
ecules. Photolysis of 3-iodo-2,5,6-trifluoro-
pyridyl azide under the conditions of ma-
trix isolation results in the formation of a
nitrene radical with a quartet ground state.
The electronic structure of this nitrene rad-
ical is described best as a σ,σ,π-triradical.
The nitrene radical is stable at cryogenic
temperatures but, upon irradiation, re-
arranges to a thermodynamically more
stable azirinyl radical.
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