Dinitreno pentaradicals: organic sextet molecules

Article abstract of DOI:10.1002/poc.3621

A new sextet ground state molecule, the 2,4,6-trichloro-1,3-dinitrenophenoxyl radical, was obtained upon UV photolysis of the corresponding diazido precursor in argon at 5 K. The sextet molecule is formed stepwise with the triplet nitrene, the quartet nitreno radical, and the quintet dinitrene as intermediates that were detected by EPR spectroscopy. IR and UV–Vis measurements only allowed us to observe the main product, the quintet dinitrene. The coupling between the two nitrene centers and the oxygen centered radical in the sextet state results in zero-field splitting (zfs) parameters of |D/hc| = 0.088 cm^?1 and |E/hc| = 0.009 cm^?1, considerably larger than in previously reported organic sextet molecules. An analogous sextet dinitrene formed by replacing the oxygen centered radical by a carbon centered radical was also studied by EPR spectroscopy, and in this case the zfs parameters |D/hc| = 0.125 cm^?1 and |E/hc| = 0.023 cm^?1 indicate even larger spin localization. Copyright

Full text of DOI:10.1002/poc.3621

Special issue article
Received: 10 June 2016,
Accepted: 26 June 2016,
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/poc.3621
Dinitreno pentaradicals: organic sextet
molecules
Joel Mieres-Péreza, Stefan Henkela, Enrique Mendez-Vegaa, Tim Schleifa,
Thomas Lohmillerb,a, Anton Savitskyb and Wolfram Sandera*
A new sextet ground state molecule, the 2,4,6-trichloro-1,3-dinitrenophenoxyl radical, was obtained upon UV photolysis of
the corresponding diazido precursor in argon at 5 K. The sextet molecule is formed stepwise with the triplet nitrene, the quar-
tet nitreno radical, and the quintet dinitrene as intermediates that were detected by EPR spectroscopy. IR and UV–Vis mea-
surements only allowed us to observe the main product, the quintet dinitrene. The coupling between the two nitrene
centers and the oxygen centered radical in the sextet state results in zero-field splitting (zfs) parameters of |D/hc|
= 0.088 cmÀ1 and |E/hc| = 0.009 cmÀ1, considerably larger than in previously reported organic sextet molecules. An analogous
sextet dinitrene formed by replacing the oxygen centered radical by a carbon centered radical was also studied by EPR spec-
troscopy, and in this case the zfs parameters |D/hc| = 0.125 cmÀ1 and |E/hc| = 0.023 cmÀ1 indicate even larger spin localization.
Copyright © 2016 John Wiley & Sons, Ltd.
Keywords: EPR; matrix isolation; nitrenes; organic radicals; sextet
substituents in 1 are expected to efficiently suppress rearrange-
ments of the phenylnitrene units to seven-membered ring
INTRODUCTION
The magnetic properties of organic polyradicals have attracted
the attention of the scientific community for many years, as
these species are discussed as building blocks for organic mag-
netic materials.[1–7] Among the organic high-spin molecules re-
ported so far, only very few exhibit a sextet (S = 5/2) spin
state.[8,9] Rajca et al.[8] synthesized pentaradical P1 with benzyl
radical units as spin bearing centers (Chart 1). Using SQUID
magnetometry they found S ≥ 2.3. In the EPR spectra, two sets
of signals corresponding to S = 5/2 and zero field splitting
(zfs) parameters of |D/hc| = 0.0027 cmÀ1, |E/hc| = 0.0009 cmÀ1
systems according to Scheme 1.
EXPERIMENTAL SECTION
Matrix isolation spectroscopy
Matrix isolation experiments were performed by standard tech-
niques as reported previously.[12,13] The matrices were generated
by co-deposition of the precursors and a large excess of argon
(Messer Griesheim, 99.99%).
and |D/hc| = 0.0027 cmÀ1
, , respectively,
|E/hc| = 0.0000 cmÀ1
were assigned to two conformers of the pentaradical P1. Two
years later, Fujita et al.[9] reported on the stable pentaradical
P2, based on nitroxyl radicals. P2 shows a sextet ground state
with zfs parameters of |D/hc| = 0.0039 cmÀ1 and |E/hc|
= 0.0013 cmÀ1. The structure of P2 was determined by XÀray
structure analysis.
Among organic polyradicals, nitrenes exhibit the largest D
values and the strongest ferromagnetic exchange interactions
between the unpaired electrons. In addition, nitrenes have large
energy gaps (5–10 kcal/mol) between the high-spin ground
state and low-spin excited states.[10] Therefore, they are well
suited as molecular models for organic molecular magnets.[11]
The design of a pentaradical based on nitrenes, rather than on
carbon-centered radicals, is of high interest since such
pentaradicals are expected to show considerably larger D values
than the carbon-centered pentaradicals.
Pulse EPR spectroscopy in frozen solution
Sample preparation
Compound 3 was dissolved in methylcyclohexane, and oxygen
was removed by purging the solution with argon. 3 μl of the
solution were transferred into a 0.4 mm inner diameter quartz
tube for W-band measurements, and 30 μl were transferred into
a Q-band quartz capillary EPR tube with an inner diameter of
1.6 mm. For UV irradiation a high pressure mercury lamp with
quartz optics (L.O.T. Oriel) was used.
* Correspondence to: Wolfram Sander, Lehrstuhl für Organische Chemie II,
Ruhr-Universität Bochum, D-44780 Bochum, Germany.
E-mail: wolfram.sander@rub.de
In order to synthesize a sextet pentaradical bearing two
nitrene units and a single radical center, a 1,3-dinitrene phenyl
motive with ferromagnetic coupling between the nitrene units
was chosen as basis. As additional radical center, a phenoxyl
radical as in 2,4,6-trichloro-1,3-dinitreno-phenoxyl radical 1 or a
benzyl radical as in 2 were selected (Chart 2). The chlorine
a J. Mieres-Pérez, S. Henkel, E. Mendez-Vega, T. Schleif, T. Lohmiller, W. Sander
Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, D-44780,
Bochum, Germany
b T. Lohmiller, A. Savitsky
Max-Planck-Institut für Chemische Energiekonversion, D-45470, Mülheim an
der Ruhr, Germany
J. Phys. Org. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

J. MIERES-PÉREZ ET AL.
[M – 2 N2 – Cl]+, 165 (30), 163 (50), 154 (18) [M – 2 N2 – 2Cl]+,
128 (36), 47 (32), 31 (27). HRMS (EI-TOF) calculated for C6Cl3FN6
279.93492; Found: 279.92341. IR (Nujol): eν = 2220 m cmÀ1
,
2168 s, 2121vs (N3), 1572 m, 1555 m, 1537 m (C = C), 1415vs (C–F),
1256 m, 1237 m, 1207 m, 958w, 808 m, 789 m, 525w.
1-(Allyloxy)-3,5-diazido-2,4,6-trichlorobenzene (3)
0.163 g (1.45 mmol) of KOtBu were suspended in 2 ml of dry di-
oxane. 0.13 ml (0.111 g, 1.90 mmol) of 2-propenol were added
and the suspension was stirred for 15 min at room temperature
upon which the mixture turned intense yellow. 0.140 g
(0.50 mmol) of 1,3-diazido-2,4,6-trichloro-5-fluorobenzene were
added and the reaction mixture was stirred for 3.5 h at room
temperature. 5 ml of destilled water were added to the orange
solution and it was stirred for 10 min at room temperature before
extraction with MTBE. Evaporation of the solvent gave 0.135 g of
orange-brown oil as crude product, which was purified by HPLC
with hexane as eluent to give 0.045 g (0.14 mmol, 28% yield) of 3
as colorless solid.
Chart 1. Previously reported organic sextet ground state molecules
1H-NMR (200 MHz, CDCl3): δ = 6.12 ppm (ddt, J = 16.3, 10.3,
6.0 Hz, 1H, CH = CH2), 5.43 (dd, J = 17.1, 1.4 Hz, 1H, CH = CHtrans),
5.32 (dd, J = 10.3, 1.2 Hz, 1H, CH = CHcis), 4.54 (dt, J = 6.0, 1.2 Hz,
2H, OCH2). 13C-NMR (50 MHz, CDCl3): δ = 151.5 ppm (C1), 134.5
(C3/C5), 132.3 (CH = CH2), 122.3 (C2/C6), 120.0 (C4), 119.8
(CH = CH2), 74.9 (OCH2). MS (EI, 70 eV): m/z (rel. int.) = 322
(1), 320 (3), 318 (3) [M]+, 294 (1), 292 (3), 290 (3) [M – N2]+,
225 (2), 223 (5), 221 (4) [M – 2 N2 – C3H5]+, 197 (2), 195 (4),
193 (4) [M – 2 N2 – C3H5 – CO]+, 134 (6), 132 (8), 120 (7), 85
(5), 73 (6), 41 (100) [C3H5]+, 39 (25) [C3H3]+, 28 (7) [N2/CO]+.
HRMS (EI-TOF) calculated for C9H5Cl3N6O 317.91274; Found:
317.95904.
Chart 2. Pentaradicals studied in this work
Scheme 1. Ring expansion process in nitrenes
3,5-Diaminobenzyl alcohol
3,5-Diaminobenzyl alcohol was prepared according to a modi-
fied literature procedure[17] (see SI).
Q-band and W-band pulse EPR spectroscopy
1H-NMR (200 MHz, DMSO-d6): δ = 5.77 ppm (d, J = 1.7 Hz, 2H,
H2/H6), 5.68 (t, J = 1.8 Hz, 1H, H4), 4.79 (t, J = 5.8 Hz, 1H, OH),
4.61 (s, 4H, NH2), 4.19 (d, J = 5.7 Hz, 2H, CH2OH).
Q-band (≈34 GHz) pulse EPR experiments were performed at
5 K using a Bruker ELEXSYS E580 spectrometer equipped with
a homebuilt TE011 microwave cavity[14] and a cryogen-free
closed-cycle cryostat (Cryogenic Limited) with a Cryomech
liquid He compressor. W-band (≈94 GHz) pulse EPR
measurements were carried out at 5 K using a Bruker ELEXSYS
E680 spectrometer employing a homebuilt microwave cavity,
which comprised a solenoid of Teflon-coated silver wire inte-
3,5-Diazidobenzyl alcohol
3,5-Diazidobenzyl alcohol was prepared by a modified literature
procedure[17] (see SI).
1H-NMR (250 MHz, CDCl3): δ = 6.83 ppm (d, J = 1.9 Hz, 2H,
H2/H6), 6.57 (t, J = 1.9 Hz, 1H, H4), 4.69 (d, J = 3.5 Hz, 2H, CH2OH).
13C-NMR (62.5 MHz, CDCl3): δ = 144.7 ppm (C3/C5), 142.2
(C1), 113.6 (C2/C6), 109.1 (C4), 64.5 (CH2OH). MS (EI, 70 eV):
m/z (rel. int.) = 190 (30)[M]+, 162 (24) [M – N2]+, 155 (20), 133
(6) [M – 2 N2 – H]+, 121 (24), 107 (35) [M – 2 N3 + H]+, 105 (42)
[M – 2 N3 – H]+, 79 (48), 64 (38), 52 (100) [C4H4]+, 39 (55)
[C3H3]+, 31 (70) [CH2OH]+. HRMS (EI-TOF) calculated for
C7H6N6O 190.01668; Found: 190.06031.
grated into
a commercial W-band probe head (Bruker).
ESE-detected field-swept EPR spectra were recorded by use
of the pulse sequence π/2-τ-π-τ-echo[15] with microwave pulse
lengths π = 40 ns and inter-pulse distances τ = 300 ns (Q-band)
and 200 ns (W-band).
MATERIALS
1,3-Diazido-2,4,6-trichloro-5-fluorobenzene
3,5-Diazidobenzyl mesylate
1,3-Diazido-2,4,6-trichloro-5-fluorobenzene was prepared by a
modified literature procedure[16] (see SI).
3,5-Diazidobenzyl mesylate was prepared following a literature
procedure[18] (see SI).
13C-NMR
(50 MHz,
CDCl3):
δ = 153.8 ppm
(C5,
d,
J = 250.7 Hz), 134.7 (C1/C3, d, J = 1.9 Hz), 119.6 (C2, d,
J = 4.2 Hz), 114.9 (C4/C6, d, J = 21.1 Hz). MS (EI, 70 eV): m/z
(rel. int.) = 284 (8), 282 (25) [M]+, 280 (27), 256 (9), 254
(41) [M – N2]+, 252 (42), 224 (3) [M – 2 N2]+, 191 (65), 189 (100)
1H-NMR (400 MHz, CDCl3): δ = 6.84 ppm (d, J = 1.8 Hz, 2H,
H2/H6), 6.67 (t, J = 1.8 Hz, 1H, H4), 5.17 (s, 2H, CH2O), 3.02 (s, 3H,
CH3). 13C-NMR (100 MHz, CDCl3): δ = 142.6 ppm (C3/C5), 137.2
(C1), 115.3 (C2/C6), 110.4 (C4), 69.7 (CH2O), 38.4 (CH3). MS (EI,
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J. Phys. Org. Chem. (2016)

DINITRENO PENTARADICALS
70 eV): m/z (rel. int.) = 268 (10) [M]+, 240 (6) [M – N2]+, 185 (9), 173
(30) [M – OMs]+, 133 (19), 116 (30), 105 (35), 79 (100) [Ms]+, 63
(35), 52 (36) [C4H4]+, 39 (30) [C3H3]+. HRMS calculated for
C8H8N6O3S 267.96057; Found: 268.03786.
3,5-Diazidobenzyl iodide (8)
3,5-Diazidobenzyl iodide (8) was prepared according to a modi-
fied literature procedure[18] (see SI).
1H-NMR (200 MHz, CDCl3): δ = 6.80 ppm (d, J = 2.0 Hz, 2H,
H2/H6), 6.53 (t, J = 2.0 Hz, 1H, H4), 4.36 (s, 2H, CH2I). 13C-NMR
(50 MHz, CDCl3): δ = 142.9 ppm (C3/C5), 142.2 (C1), 115.7 (C2/C6),
109.4 (C4), 3.0 (CH2). MS (EI, 70 eV): m/z (rel. int.) = 300 (60)
[M]+, 272 (12) [M – N2]+, 173 (37) [M – I]+, 127 (42) [I]+, 117
(87) [M – 2 N2 – I]+, 90 (92) [C7H6]+, 63 (86), 52 (53), 39 (45).
HRMS calculated for C7H5N6I: 299.91390; Found: 299.96204.
Computational details
Figure 1. IR spectra showing the photochemistry of 3 in argon at 8 K. a)
IR spectrum of 3. b) IR spectrum obtained after 12 h irradiation of 3 with
λ = 365 nm. c) Calculated IR spectrum of 4 (B3LYP/def2-TZVP). Three con-
formers of 4 with different orientations of the allyloxy group are consid-
ered (see SI for details)
Geometry optimizations, vibrations analysis, and excited state
calculations were carried out using the B3LYP and M06 func-
tionals and the def2-TZVP basis set as implemented in Gaussian
09.[19] A spin-unrestricted formalism was used for the open-shell
species. Tight convergence criteria for gradients and an integra-
tion grid having 99 radial shells per atom and 590 angular points
per shell were used. Experimental EPR spectra were analyzed
using the Easyspin program package.[20]
Calculations of the magnetic properties were conducted using
the ORCA program package.[21] See SI for more detailed
information.
RESULTS AND DISCUSSION
IR spectroscopy
1-(Allyloxy)-3,5-diazido-2,4,6-trichlorobenzene (3) was isolated in
argon at 8 K, and its photochemistry was investigated by IR
spectroscopy. The IR spectrum of matrix-isolated 3 shows the
characteristic azide stretching vibration at 2124 cmÀ1, which
upon UV photolysis (λ = 365 nm) decreases in intensity.
Simultaneously, new bands with the strongest absorptions at
1361, 1288, 940, and 786 cmÀ1 are formed. After prolonged
photolysis (12 h) all IR absorptions of 3 disappear completely
(Fig. 1). By comparison with DFT calculations, the new
bands are assigned to the quintet dinitrene 2,4,6-trichloro-3,5-
dinitreno-(allyloxy)-benzene 4. The formation of larger amounts
of rearranged products such as benzazirines or azepines can be
excluded because the characteristic absorptions of these species
in the region between 1650 and 1900 cmÀ1 are missing in the IR
spectra. The characteristic azide stretching vibrations are
bleached upon photolysis, which excludes the presence of triplet
mononitrene 5. Additional UV photolysis (λ = 365 nm) does not
result in further changes in the spectrum, indicating that 4 is
stable towards 365 nm irradiation. In particular, the allyl radical
(Scheme 2) with its characteristic IR absorption at
801 cmÀ1[22,23] is not formed under these conditions.
Scheme 2. Photochemistry of 3
UV–Vis spectroscopy
Matrix-isolated 3 exhibits a broad band in the UV–Vis spectrum
centered at 229 nm, which is decreasing in intensity on
λ = 365 nm irradiation. Simultaneously, new bands with maxima
at 481, 450, 327, 312, 285, 235, and 212 nm are formed. In order
to analyze the UV–Vis data, TD-DFT calculations were carried out
using a variety of functionals and basis sets. The best match with
the experimental results was found for the quintet state of 4 cal-
culated at the M06-2X/def2-TZVP level of theory. The calculation
predicts the major electronic transitions of 4 at 404, 267, 226,
and 207 nm, in reasonable agreement with the most intense
observed bands (Fig. 2). However, the presence of other
photoproducts as minor constituents of the matrix cannot be
ruled out.
Further irradiation of the matrix with 266 nm light leads to a
slight decrease of all remaining IR bands of 4 and appearance
of a very low intensity band at 801 cmÀ1. However, due to the
low yield of O-allyl cleavage, other photoproducts, in particular
1, could not be identified by IR spectroscopy.
EPR spectroscopy
UV irradiation (λ = 365 nm) of 3 in argon at 5 K produces an EPR
spectrum typical of quintet dinitrenes, which therefore is
assigned to 4 (Fig. 3a). The signals at 66, 142, 308, 374, 460,
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J. MIERES-PÉREZ ET AL.
IR experiments, λ = 365 nm irradiation does not lead to products
expected from the O-allyl cleavage. In particular, the absence of
a strong signal around 342 mT indicates that the allyl radical is
not formed.
Further photolysis of the matrix with λ = 254 nm leads to a
decrease in intensity of all signals assigned to triplet 5 and
quintet 4 and formation of signals at 22, 225, 255, and 290
mT. In addition, a strong radical signal centered at 342 mT
and the characteristic transitions of hydrogen atoms at 315
and 365 mT are found (Fig. 3c). The new signals are assigned
to sextet dinitreno phenoxyl radical 1 with the zfs parameters
|D/hc| = 0.088 cmÀ1 and |E/hc| = 0.009 cmÀ1 obtained from a
simulation (Fig. 3d). Because of overlap with the signals of
quintet 4, identification of further bands of 1 was not
possible.
An additional signal at 177 mT indicates the formation of a
quartet species. The quartet is assigned to 6 (Scheme 3), proba-
bly formed from precursor 3 by cleavage of only one azide group
in addition to the O-allyl bond. The large concentration of quin-
tet 4 in the matrix indicates that the major pathway is the cleav-
age of both azide groups. However, we cannot exclude that the
quartet is a secondary photoproduct of sextet 1. Again, the over-
Figure 2. UV–Vis spectra showing the photochemistry of 3 in argon at
8 K. Dashed blue line: UV–Vis spectrum of 3. Red solid line: UV–Vis spec-
trum obtained after 12 h of irradiation of 3 with λ = 365 nm. Black line:
UV–Vis spectrum of 4, calculated at the M06-2X/def2-TZVP level of theory
544, 613, 658, 848, and 946 mT are nicely reproduced by a simu-
lation of a quintet species with zfs parameters of |D/hc|
= 0.190 cmÀ1 and |E/hc| = 0.043 cmÀ1 (Fig. 3b). An additional line
at 690 mT indicates the presence of triplet mononitrene 5 with |
D/hc| = 1.00 cmÀ1 and |E/hc| = 0.002 cmÀ1. In agreement with the
Scheme 3. Triplet, quartet, quintet, and sextet species formed by pho-
tolysis of 3
Figure 3. X-band CW EPR spectra of the photoproducts of matrix-iso-
lated 3. a) Spectrum obtained upon λ = 365 nm photolysis (50 min) of 3
in argon at 5 K. Lines assigned to the quintet are labeled as ‘q’, the triplet
signal is labeled as ‘t’. b) Simulated spectrum of a quintet species with |D/
hc| = 0.190 cmÀ1 and |E/hc| = 0.043 cmÀ1 (ν = 9.575 GHz). A signal cen-
tered at 690 mT indicates the presence of a triplet nitrene with |D/hc|
= 1.00 cmÀ1 and |E/hc| = 0.002 cmÀ1 (simulation shown as dashed line).
c) Spectrum obtained after additional photolysis of the matrix with
λ = 254 nm (1.5 h). Signals assigned to sextet species are labeled as ‘s’.
The asterisk denotes an additional band that is attributed to a quartet
species (see text for details). d) Simulated spectrum of a sextet species
with |D/hc| = 0.088 cmÀ1 and |E/hc| = 0.009 cmÀ1 (ν = 9.678 GHz)
Scheme 4. Photochemistry of 8
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J. Phys. Org. Chem. (2016)

DINITRENO PENTARADICALS
lap with the intense quintet signals precludes assignment of
other signals of the quartet compound. The position of the tran-
sition assigned to the quartet species is not very sensitive to the
D values, and therefore the zfs values cannot be determined
reliably.
The photochemistry of precursor 3 in methylcyclohexane at
5 K was studied by W-band pulse EPR (Scheme 4). After UV pho-
tolysis, intense signals assigned to triplet mononitrene 5 were
observed together with additional bands assigned to quintet 4.
The signals of 4 increased during overnight irradiation, while
that of 5 decreased, but no signals could be assigned to 1 (Figure
S1, SI). Simulations confirmed the zfs parameters from the X-
band experiments, and especially the W-band simulations identi-
fied the sign of the D value of quintet 4 to be positive,
D/hc = 0.190 cmÀ1 (Figure S2, SI).
Figure 5. Spin density distribution of sextet pentaradicals 1 and 2. Blue
color indicates an excess of α-spin density at the atom (positive spin den-
sity), and green color shows an excess of β-spin density at the atom (neg-
ative spin density)
In addition to 1, the pentaradical 2, in which the oxygen-
centered radical is replaced by a carbon-centered radical,
was investigated by X-band CW EPR spectroscopy. Photolysis
of matrix-isolated 8 with λ = 308 nm leads to formation of a
quintet species with transitions at 86, 146, 279, 309, 394,
623, and 860 mT (Fig. 4a). The spectrum could be simulated
using the zfs parameters |D/hc| = 0.205 cmÀ1 and |E/hc|
centered at 340 mT indicates the formation of radical species
by cleavage of the C-I bond of 8. An intense line at 701 mT
was assigned to triplet mononitrene 10 with a D value of ap-
proximately 1.00 cmÀ1. Additional bands at 79, 178, 251, 268,
and 568 mT are assigned to species containing a nitrene unit
as well as a radical center as in 11 or 12 (Scheme 4). How-
ever, short-wavelength UV photolysis (λ = 254) nm leads to
lines at 5, 202, 229 238, and 430 mT that are in agreement
= 0.041 cmÀ1 (Fig. 4b) and was assigned to quintet
9
(Scheme 4). In addition to the quintet signals, a strong line
with
a sextet species with zfs parameters of |D/hc|
= 0.125 cmÀ1 and |E/hc| = 0.023 cmÀ1 (Fig. 4c,4d). Therefore,
these signals are tentatively assigned to 2.
The calculated zfs parameters (see SI table TI) for sextets 1
(|D/hc| = 0.093) and 2 (|D/hc| = 0.096) follow the same trend as
the ones obtained experimentally, that is, higher D values in
the case of sextet 2 than for sextet 1, which indicates a
stronger localization of the electron density at the radical
centers in the case of 2. This is in agreement with the higher
values of the calculated spin densities for 2 (Fig. 5).
CONCLUSION
Pentaradicals with sextet electronic ground states are interesting
high spin species. Nitrenoradical 1 could be produced by photol-
ysis of precursor 3 and characterized by EPR spectroscopy.
Because the cleavage of the O-allyl group is very inefficient, only
low yields of 1 were obtained, and therefore it was not possible
to identify this pentaradical by the less sensitive UV–Vis or IR
spectroscopy. Dinitrene 4, on the other hand, was formed in very
high yield and could therefore be observed by EPR, UV–Vis, and
IR spectroscopy. The chlorine substituents inhibit the
phototchemical rearrangement of the phenylnitrene derivatives
1, 4, 5, or 6 to the corresponding keteneimines, and therefore
high yields of nitrenes are achieved.
In contrast, nitrenes such as 9 or 10 can easily undergo photo-
chemical rearrangements to keteneimines, and therefore the
yields are low. Pentaradical 2 was therefore only investigated
by EPR spectroscopy.
These sextet radicals 1 and 2 exhibit D values more than 10
times larger than previously reported organic sextets, indicating
a large degree of spin-localization at the nitrene units. These
sextet species could be used as new building blocks for the
synthesis of organic magnetic materials.
Figure 4. EPR spectra of the photoproducts of matrix-isolated 8. a)
Spectrum obtained upon λ = 308 nm photolysis (17 min) of 8 in argon
at 5 K. Lines assigned to the quintet are labeled as ‘q’, triplet lines are la-
beled as ‘t’. b) Simulated spectrum of a quintet species with |D/hc|
= 0.205 cmÀ1 and |E/hc| = 0.041 cmÀ1 (ν = 9.566 GHz). A strong line cen-
tered at 701 mT indicates the presence of a triplet nitrene with |D/hc|
= 1.04 cmÀ1 (simulation shown as dashed line). Asterisks denote addi-
tional bands that are attributed to quartet species (see text for details).
c) Spectrum obtained after additional photolysis of the matrix with
λ = 254 nm (1 h). Signals assigned to the sextet species are labeled as
‘s’. d) Simulated spectrum of
a
sextet species with |D/hc|
= 0.125 cmÀ1 and |E/hc| = 0.023 cmÀ1 (ν = 9.564 GHz)
J. Phys. Org. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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J. MIERES-PÉREZ ET AL.
[17] N. Umezawa, N. Matsumoto, S. Iwama, N. Kato, T. Higuchi, Bioorg.
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Acknowledgements
[18] T. Hosoya, A. Inoue, T. Hiramatsu, H. Aoyama, T. Ikemoto, M. Suzuki,
Bioorg. Med. Chem. 2009, 17, 2490.
This work was supported by the Cluster of Excellence RESOLV
(EXC 1069) funded by the Deutsche Forschungsgemeinschaft
(DFG).
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