Is Magnetic Bistability of Carbenes a General Phenomenon? Isolation of Simple Aryl(trifluoromethyl)carbenes in Both Their Singlet and Triplet States

Article abstract of DOI:10.1021/jacs.7b06868

p-Tolyl(trifluoromethyl)carbene and the related fluorenyl(trifluoromethyl)carbene were synthesized in solid argon and characterized by IR, UV-vis, and electron paramagnetic resonance spectroscopy as well as by quantum mechanical calculations. The carbenes

Full text of DOI:10.1021/jacs.7b06868

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Article
Is Magnetic Bistability of Carbenes a General Phenomenon? Isolation of
Simple Aryl(trifluoromethyl)carbenes in Both their Singlet and Triplet States
Yetsedaw A. Tsegaw, Pritam Eknath Kadam, Niklas Tötsch, Elsa Sanchez-Garcia, and Wolfram Sander
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06868 • Publication Date (Web): 08 Aug 2017
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Journal of the American Chemical Society
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Is Magnetic Bistability of Carbenes a General Phenomenon? Isolation
of Simple Aryl(trifluoromethyl)carbenes in Both their Singlet and
Triplet States
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†
†
§‡
§‡*
Yetsedaw A. Tsegaw , Pritam E. Kadam , Niklas Tötsch , Elsa Sanchez-Garcia , and Wolfram
Sander
†
*
†
Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, 44780 Bochum, Germany
§Max-Planck-Institut für Kohlenforschung, 45470 Mülheim an der Ruhr, Germany
Universität Duisburg-Essen, Universitätsstr. 2, 45141 Essen, Germany
‡
KEYWORDS carbene; matrix isolation; EPR; IR; spin state
ABSTRACT: p-Tolyl(trifluoromethyl) carbene and the related fluorenyl(trifluoromethyl)carbene were synthesized in solid
argon and characterized by IR, UV-vis, and EPR spectroscopy as well as by quantum mechanical calculations. The car-
benes can be generated in both their triplet and singlet states, and both states coexist under the conditions of matrix
isolation. According to our calculations, the singlet and triplet states of these carbenes are energetically nearly degenerate
in the gas phase. Warming of matrices containing pure triplet p-tolyl(trifluoromethyl) carbene from 3 to 25 K leads to an
interconversion of up to 20% of the triplet into the singlet state. This interconversion is thermally irreversible, and cool-
ing back to 3 K does not change the ratio of singlet to triplet. Irradiation at 365 nm results in a complete interconversion
of singlet to triplet, whereas 450 nm irradiation produces again up to 20% of the singlet state. An alternative way to gen-
erate the singlet carbene is the reaction of the triplet with water molecules by annealing water-doped matrices at 25 K.
This results in the irreversible formation of a hydrogen-bonded complex between the singlet carbene and water. For fluo-
renyl(trifluoromethyl)carbene very similar results were obtained, only the yield of the singlet state is even higher. Mag-
netic bistability of carbenes seems to be a general phenomenon that only depends on the singlet-triplet gap rather than
on the nature of the carbene.
perpendicular to the naphthyl group) and T-1 (both
groups in one plane).
INTRODUCTION
A paradigm of carbene chemistry is that carbene reac-
tions are spin-specific. Since the intersystem crossing
Toscano et al. investigated carbene 1 by time-resolved
1
IR spectroscopy and detected both T-1 and S-1 as transi-
(
ISC) rates between the singlet and triplet states of aryl-
4
ent species in Freon-113. From these experiments, ꢀG
=
2
ST
carbenes are generally faster than diffusion processes in
solution, the reaction pattern of these carbenes mainly
depends on the population of the equilibrated spin states,
and thus on the free energy singlet-triplet gap ꢀGST and
the temperature. At cryogenic temperatures, excited spin
states are not populated, and thus carbenes are isolated in
low-temperature matrices in their ground spin states,
either singlet or triplet, depending on the nature of the
carbene. In contradiction to this general belief, McMahon
0.2 ± 0.1 kcal/mol was estimated, which is much smaller
than the singlet-triplet splitting ꢀEST = 4.52 kcal/mol
calculated by DFT. This discrepancy reflects both limita-
tions of the DFT calculations as well as solvent effects
preferentially stabilizing S-1. In the more polar acetoni-
5
trile as solvent, S-1 is slightly preferred. Later, Platz et al.
studied carbene 1 using ultrafast UV-vis and IR spectros-
6
copy. They found that in CHCl , CH CN, and Freon-113
3
3
only S-1 is present, whereas in the unpolar cyclohexane S-
1 decays with a half-lifetime of 3 ns to T-1. Thus, in both
cyclohexane at room temperature and in solid argon at 10
K, T-1 is the ground state, but the lifetime of S-1 increases
dramatically by more than 12 orders of magnitude.
et
al.
reported
in
1998
that
2-
naphthyl(carbomethoxy)carbene 1 can be matrix-isolated
and spectroscopically characterized in both its singlet (S-
3
1
) and triplet states (T-1). The singlet state S-1 was popu-
lated by visible light irradiation (λ > 515 nm) of T-1 and
has a lifetime of several hours under these conditions
Recently, we reported on bis(para-methoxyphenyl) car-
bene 2, a second carbene that could be isolated in rare gas
(
Scheme 1). The energy barrier preventing the expected
7
matrices both in its S and T state (Scheme 1). At tempera-
rapid ISC back to T-1 was attributed to pronounced con-
formational differences between S-1 (carbomethoxy group
tures below 10 K, both spin states coexist indefinitely, and
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irradiation at 450 nm results in an increase of S-2 and
Page 2 of 9
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irradiation under the conditions of matrix isolation.
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decrease of T-2, whereas 365 nm irradiation shifts the
photostationary equilibrium towards T-2. Annealing at
higher temperatures (> 20 K) leads to an increase of S-2,
which is not reversible by cooling back to very low tem-
peratures (3 K). This indicates a thermal activation barrier
for the S – T interconversion. This magnetic bistability is
astonishing, since the geometries of S-2 and T-2 are very
similar, differing mainly in the bond angle at the carbene
center (estimated to 119° for S-2 and 141° for T-2), and
therefore the spin interconversion doesn’t require a large
geometrical change as in 1.
Carbene 4a rapidly reacts with molecular oxygen, as ex-
pected for a triplet ground state carbene.
ground state was further confirmed by EPR spectroscopy.
According to calculations by Song and Sheridan, the tri-
fluoromethyl group stabilizes the singlet state by a small
1
0-11
The triplet
9
1
2
amount (ꢀEST 3: 5.4 kcal/mol, 4a: 4.0 kcal/mol). Ar-
yl(trifluoromethyl)carbenes 4 are of special interest since
they found wide applications in photoaffinity labelling.
1
3
The photochemistry and the excited state dynamics of
these carbenes was extensively studied by time-resolved
0
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4-19
spectroscopy.
The S – T gap in 4a is still too large to expect magnetic
bistability, but it can be further lowered by electron-
donating substituents in the para-position of the phenyl
ring. Here, p-tolyl(trifluoromethyl)carbene 4b was select-
ed as a feasible candidate for a low-gap carbene. Platz et
al. reported the synthesis of carbene 4b by photolysis of
Scheme 1. Magnetically bistable carbenes 1 and 2.
diazirine 8 in a perfluorated organic glass at 77 K (Scheme
14
2
). Under these conditions, a typical triplet carbene EPR
spectrum with zero field splitting (zfs) parameters D =
-1
0
.5215 and E = 0.0249 cm was observed, which suggests
that 4b either has a triplet ground state or such a small S -
T gap that the excited triplet state is thermally populated
even at 77 K. From laser flash photolysis studies of 8 in
the presence of various quenchers it was concluded that
at room temperature the singlet state S-4b and triplet
state T-4b are in a rapid thermal equilibrium, and thus 4b
can be trapped with both typical singlet and triplet
quenchers. From these experiments, the S – T gap of 4b
An open question is if the magnetic bistability is a gen-
eral phenomenon that occurs in all carbenes with small S
–
T gaps at cryogenic temperatures, or if it requires specif-
ic substituents and specific electronic structures to slow
down the ISC. This question is of fundamental im-
portance for the understanding of carbene reactions, but
also for the development of applications such as optically
switchable organic magnetic materials or tailored pho-
toaffinity labeling.
14
was estimated to ꢀGST = 0.5 – 1.5 kcal/mol. The related
fluorenyl(trifluoromethyl)carbene 4c was studied by Platz
et al. using ultrafast time resolved absorption spectrosco-
16
py. In acetonitrile the singlet carbene S-4c was charac-
terized as a transient species with an initial absorption
maximum at 427 nm that shifts to 432 nm after vibration-
al cooling.
Carbenes 4b and 4c are simple arylcarbenes that should
allow us to answer the question whether or not magnetic
bistability is a general phenomenon in carbenes with
small S – T gaps. We therefore studied these carbenes
under the conditions of matrix isolation. The principal
results are identical for both carbenes, and therefore we
here discuss only 4b in detail, for 4c we refer to the Sup-
porting Information.
A disadvantage of carbenes 1 and 2 for studying mag-
netic bistability is that both carbenes are rather flexible
and form several energetically close lying conformers. In
addition, 1 shows secondary photochemistry, which re-
sults in complex product mixtures that complicate the
interpretation of the spectra. We therefore were searching
for conformationally simple, photostable arylcarbenes
with nearly degenerate S and T states that would allow us
to matrix-isolate both spin states. Phenylcarbene 3 is
highly photolabile towards rearrangement to 1,2,4,6-
8
cycloheptatetraene, but phenyl (trifluoromethyl)carbene
4a is photochemically rather stable towards visible light
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back to T-4b. It is tempting to assign this EPR-silent
Scheme 2. Synthesis and chemistry of carbene 4b.
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product to the singlet state S-4b.
N
2
CF
3
H
3
C
CF
3
435 nm
OPO
10
4
50 nm
H
3
C
254 nm
9
- N2
ISC
CF
3
CF
3
3
65 nm
3
65 nm
H
3
C
450 nm
3
H C
N
N
S-4b
T-4b
-
N
2
CF
3
+
2
H O
3
65 nm
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H
3
C
OH
H
8
H
OH
CF
4
50 nm
CF
3
3
H
3
C
3
H C
S-4b...H-OH
11
Figure 1. X-band CW EPR spectra of an argon matrix show-
ing the thermal behavior of T-4b after warming from 4 to 25
K and cooling back to 4 K. (a) Argon matrix at 4 K showing
the spectrum of T-4b (read line; zfs parameters |D/hc| =
RESULTS AND DISCUSSION
-
1
-1
0.5219 cm and |E/hc| = 0.0304 cm ). (b) After subsequent
annealing at 25 K for 10 min and cooling back to 4 K, 36% of
the signal intensity is lost (blue line). Inset shows the loss of
Carbene 4b was generated by either UV (365 nm) pho-
tolysis of p-tolyl(trifluoromethyl)diazirine 8 or by visible
light
(450
nm)
photolysis
of
p-
the T-4b signal at the z canonical position after cooling back
1
tolyl(trifluoromethyl)diazomethane 9 in neon, argon,
xenon or nitrogen matrices at 3 - 4 K. The carbene 4b was
characterized by IR, UV-vis, and EPR spectroscopy. Pho-
tolysis of 8 produced small amounts of 9, otherwise, the
same products were obtained from both precursors
from 8, 12, 16, 20 and 25 K to 4 K. The bands marked * are
assigned to products of carbene-carbene rearrangements.
IR Experiments. This assumption was confirmed by
following the thermal and photochemical S-T intercon-
version of 4b by IR spectroscopy. Visible light irradiation
(450 nm) of matrix-isolated 9 rapidly results in the com-
plete photolysis of the diazo precursor and formation of a
major product assigned to T-4b and a minor product
assigned to S-4b with characteristic absorptions at 1578.8
(
Scheme 2).
EPR Experiments. Photolysis of 9 in argon at 4 K re-
sults in the formation of T-4b with an EPR spectrum typi-
cal of a triplet carbene (Figure 1). The zfs parameters
D/hc| = 0.5219 and |E/hc| = 0.0304 cm are similar to
-
1
|
1
4
that reported for T-4b in an organic glass at 77 K. The D
values are almost identical, whereas the E value in argon
is slightly larger than that recorded in the organic glass,
which might reflect small differences in the conformation
of T-4b generated in different matrices. In an attempt to
measure the Curie-Weiss behavior and thus to determine
the ground state of 4b, the matrix was stepwise annealed
for 10 min at temperatures of 8, 12, 16, 20, and 25 K. An
EPR spectrum was recorded at each of these tempera-
tures, and before increasing the temperature to the next
higher level, the matrix was cooled back to 4 K to record a
second EPR spectrum (Figure 1, inset). These experiments
clearly demonstrate an irreversible loss of T-4b at each
temperature step, summing up to more than 30% loss
after warming to 25 K. While keeping the matrix at 4 K for
several hours did not result in any increase of the signals
of T-4b, several minutes UV irradiation (365 nm) resulted
in a partial signal recovery. In these experiments, we took
care that 9 was completely photolyzed, and therefore
remaining 9 was not the precursor of T-4b under these
conditions. Subsequent annealing again irreversibly re-
duced the amount of T-4b, and this behavior was repro-
ducible in repeated annealing/irradiation cycles. These
experiments indicate that T-4b thermally rearranges to an
EPR-silent product, that upon UV irradiation rearranges
-1
and 1599.5 cm , respectively (Figure 2). Annealing at 25 K
or prolonged 450 nm irradiation leads to an increase of
the bands of the minor component S-4b. UV irradiation
(365 nm) results in the almost complete bleaching of all
IR bands of S-4b and concurrently in an increase of the
absorptions of the major product T-4b. This assignment
was confirmed by comparing the experimental IR spectra
to calculations at the B2PLYP-GD3BJ/aug-cc-pVTZ (Fig-
ure 2 and S2, Tables S1 and S2) and B3LYP-D3/Def2-TZVP
levels of theory (Figure S6) which nicely reproduce the
experimental data.
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chromatic light of an OPO laser system in the range be-
tween 435 and 444 nm (Scheme 2). Under these condi-
tions, both the singlet and triplet states of carbene 4b
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9
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decrease
and
1-trifluoromethyl-5-methyl-1,2,4,6-
cycloheptatetraene 10 is formed in high yields (Figure S7).
Short-wavelength UV irradiation (254 nm) leads 10 to
partially rearrange back to 4b. Interestingly, even pro-
longed irradiation of 4b with the low-intensity LED light
source with a maximum emission at 450 nm does not
result in the rearrangement to 10.
0
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0
UV-vis Experiments. The singlet and triplet state of 4b
are easily distinguished in their UV-vis spectra. Visible
light photolysis (450 nm) of 9 produces the UV-vis spec-
trum shown in Figure 3 with a series of absorptions
stretching from 250 to 450 nm. A broad band with λmax
=
3
26 nm completely disappears after 365 nm irradiation
and re-appears after 450 nm irradiation, and thus is as-
signed to S-4b. The triplet carbene T-4b shows an absorp-
tion with a pronounced progression with λmax = 436 nm,
and irradiation into this band results in the rearrange-
ment of T- to S-4b. To confirm the assignment of the UV-
vis absorptions of the two states of 4b, we recorded UV-
vis and IR spectra from the same matrix (Figures 3 and
S4). These experiments allow us to correlate changes in
band intensities in the UV-vis and the IR region and con-
firm the band assignments.
Figure
2. IR spectra showing the interconversion of S-4b
and T-4b in argon matrices. (a) IR spectrum after several
hours of 450 nm irradiation of 9 at 3 K (dark green). (b) Dif-
ference IR spectrum at 3 K showing changes after 60 min of
365 nm irradiation of the same matrix. Bands pointing
downward, assigned to S-4b, are disappearing, and bands
pointing upward, assigned to T-4b are appearing. (c) Differ-
ence IR spectrum of the same matrix after 10 min annealing
to 30 K and cooling back to 3 K. Bands pointing downward,
assigned to T-4b, are disappearing, and bands pointing up-
ward, assigned to S-4b, are appearing. (d) Difference IR spec-
trum at 3 K showing changes after 60 min of 365 nm irradia-
tion of the same matrix. Bands pointing downward, assigned
to S-4b, are disappearing, and bands pointing upward, as-
signed to T-4b are appearing. (e) Difference IR spectrum at 3
K showing changes after 60 min of 450 nm irradiation of the
same matrix. Bands pointing downward, assigned to T-4b,
are disappearing, and bands pointing upward, assigned to S-
4b, are appearing. (f) Superimposed IR spectrum showing
the interconversion of T-4b (blue) to S-4b (red) calculated at
the B2PLYP-GD3BJ/aug-cc-pVTZ level of theory. Equal con-
tributions of S-4b and T-4b were used to simulate the IR
spectra.
The highest yield of S-4b of approximately 20% was ob-
tained in a nitrogen matrix, whereas in solid argon only
1
3% was obtained after 450 nm irradiation and annealing
at 25 K. Further 450 nm irradiation or annealing did not
increase yields of S-4b. The interconversion between T-
and S-4b was reproducible in several 450 nm/365 nm
irradiation cycles. At 3 K and in the absence of irradiation,
T-4b and S-4b coexist without noticeable changes in their
spectra even after several days.
Figure 3. UV−vis (top) and IR (bottom) spectra recorded
from the same nitrogen matrix showing the S - T intercon-
version of 4b. (a) T-4b and S-4b obtained after several hours
of 450 nm irradiation of 9 at 3 K (blue line). (b) Spectra ob-
tained after 60 min irradiation with λ = 365 nm at 3 K (red
A different photochemistry is observed if the S/T mix-
ture of 4b is irradiated with the intense, highly mono-
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line). (c) Spectra obtained after subsequent annealing for 30
changes of an argon matrix (without water) containing T-4b
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min at 20 K and cooling back to 3 K (gray line). The weak
band of T-4b in the visible region of the UV−vis spectrum is
shown in the inset.
and S-4b at 3 K after annealing for 10 min at 25 K (black line).
Bands pointing downward, assigned to T-4b, are disappear-
ing, and bands pointing upward, assigned to S-4b, are ap-
pearing. Intensity divided by 5. (b) Difference IR spectrum of
a similar experiment in argon doped with 1% of water (blue
line). Bands pointing downward are disappearing and as-
signed to T-4b, bands pointing upward are appearing and
assigned to S-4b···H-OH. (c) Difference IR spectrum of the
same matrix showing changes after 3 h irradiation (λ = 450)
(blue line). Bands pointing downward are assigned to S-
Reaction of Carbene 4b with Water. Previously, we
demonstrated that the spin state of triplet ground state
2
0-21
carbenes such as diphenylcarbene 5
6
or fluorenylidene
22
is switched from triplet to singlet upon formation of
hydrogen bonds with methanol or water under the condi-
tions of matrix isolation. In a similar experiment, 9 was
photolyzed (450 nm, 3 K) in argon matrices doped with
0
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0
4b···HOH, and bands pointing upward are assigned to p-
tolyl(trifluoromethyl)ethanol 11. (d) Reference spectrum of
11, matrix-isolated in argon at 3 K.
0
.5 – 1% of water. Similar to the photolysis in argon in the
absence of water, mixtures T-4b as major and S-4b as
minor constituent were formed. Annealing at 20 - 25 K for
several minutes allowed for the diffusion of water mole-
Computational Studies on Carbene 4b. There are
20
cules which can be easily followed in the IR spectra by
observing the formation of water dimers and oligomers
from the water monomer. Simultaneously, all IR bands
assigned to T-4b decrease in intensity, and singlet car-
bene S-4b is formed. However, in comparison to S-4b
obtained in the absence of water, all IR bands of the sin-
only few accurate thermochemical gas-phase data availa-
23-28
ble for carbenes.
CCSD(T) calculations with large basis
sets are often used as “gold standard” for calculating ꢀE
ST
29
of carbenes and for calibrating less expensive DFT re-
sults. Here, our calculations of the singlet-triplet splitting
of 4b at the CCSD(T)/cc-pV(D/T)Z//B97D-GD3/def2-
TZVP level of theory result in ꢀEST = 0.0 kcal/mol (exclud-
ing ZPVE), thus predicting degeneracy of the spin states
in the gas phase. We tested several density functionals
(see SI for details) and found that B2PLYP-GD3BJ predicts
-
1
glet carbene are blue-shifted by 3 – 9 cm . This is a clear
indication that the water complex S-4b…H-OH and not
the free singlet carbene is the preferred product under
these conditions. A further proof for the water complex S-
4b…H-OH is its 450-nm photochemistry, which rapidly
ꢀEST = -0.4 kcal/mol (ZPVE correction is 0.03 kcal/mol at
leads to the insertion product, p-
this level of theory) in excellent agreement with the
CCSD(T) result. Thus, unless otherwise specified, we are
using this functional for discussing the structures, ener-
gies, and spectra of 4b.
tolyl(trifluoromethyl)ethanol 11 (Figure 4, Table 1). In
contrast, as described above, 450 nm irradiation of mix-
tures of T- and S-4b leads to an increase of S-4b and not
to the destruction of the singlet carbene. At temperatures
below 20 K the water complex S-4b…H-OH is thermally
stable and does not show OH insertion via quantum
chemical tunneling as in the water complexes of diphe-
While the adiabatic singlet – triplet gap is very small,
the vertical gaps of S-4b and T-4b are -8.7 and 8.5
kcal/mol at their respective minimum geometries. The
relaxed one-dimensional scan of S-4b and T-4b reveals an
intersection at a carbene C-C-C bond angle of 127°, the
triplet minimum at 135° and the singlet minimum at 115°
(Figure 5). Apart from the carbene C-C-C angle, the ge-
ometries of S-4b and T-4b differ in the rotation of the
methyl group (Table S7). The other degrees of freedom
are very similar in both states. Unlike S-4b and T-4b, the
complexes S-4b…HOH and T-4b…HOH differ geometri-
cally because the carbene-water interaction is dependent
on the electronic state. As expected, interaction with a
water molecule results in a large stabilization of the sin-
glet state (10.1 kcal/mol without and 8.2 kcal/mol includ-
ing ZPVE correction), whereas the triplet state is only
weakly stabilized (-2.9 kcal/mol without and 2.0 kcal/mol
including ZPVE correction).
20
22
nylcarbene 5 and fluorenylidene 6. This is presumably
due to the higher barrier for the OH insertion of the wa-
ter complex of 4b compared to that of 5 and 6 (see be-
low).
Figure 4. IR spectra of the water complex S-4b…H-OH ma-
trix-isolated in argon. (a) Difference IR spectrum showing
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calculations, the major difference between the singlet and
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triplet states of carbene 4b is the C-C-C bond angle at the
carbene center which increases from 115° in S-4b to 135° in
T-4b. The rotation of the methyl group differs for S-4b
and T-4b (Table S7), otherwise, the structures are very
similar. Despite this geometric similarity, both states
indefinitely coexist in cryogenic matrices and can be in-
terconverted by irradiation into the absorption maxima of
S-4b in the UV or T-4b in the visible range, respectively.
Under all experimental conditions, T-4b is the major
constituent. S-4b is never obtained in more than 20%
yield (thermally or upon 450 nm irradiation), whereas 365
nm irradiation transforms S-4b quantitatively to T-4b.
Annealing of matrices containing T-4b results in the
thermal formation of S-4b, however the reversed thermal
interconversion of S-4b into T-4b is not observed, similar
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to S-2 that thermally does not rearrange to T-2. In con-
trast, S-1 is only metastable and thermally completely
3
rearranges to T-1.
Since the triplet state is threefold degenerate, we expect
a population of 75% triplet and 25% singlet if the singlet –
triplet gap ꢀEST = 0. From the observed maximum yield of
20% S-4b we estimate ꢀEST = 0.014 kcal/mol under the
assumption of a Boltzmann distribution between the spin
states. The SI contains more details on how we calculated
ꢀEST from the populations. In contrast, much higher
yields of S-4c are obtained on annealing of matrices con-
taining S/T mixtures of 4c (see Figures S8 – S10). This
hints to a singlet ground state of 4c and a kinetically sta-
bilized excited triplet state.
Figure 5. Relaxed one dimensional scan of the C-C-C car-
bene angles of 4b and the water complexes 4b...H-OH calcu-
lated at the B3LYP-GD3BJ/aug-cc-pVTZ level of theory. Adia-
batic energy differences are empirically corrected using the
B2PLYP-GD3BJ results (see SI for details). Solid lines repre-
sent 4b, dotted lines 4b…HOH. The irregularity at 132° in the
scan of S-4b…HOH is caused by the rotation of the CF3
group. Please note that vertical excitations cannot be extrap-
olated from this figure.
The singlet – triplet splitting of 4b estimated from our
experiments is in good agreement with the computational
results. However, the prediction of such small energy
differences is clearly beyond the accuracy of the calcula-
tions. Moreover, rare gas matrices will interact with the
trapped carbenes, and the highly polar singlet states pre-
sumably will be more stabilized than the less polar triplet
states. Since the stabilization of the carbenes depends on
the matrix environment (different matrix sites), we expect
a distribution of singlet – triplet splitting values in the
order of 1 – 2 kcal/mol rather than a well-defined value as
in the gas phase as reported in our previous studies of
The calculations also allow us to rationalize the higher
thermal stability of the water complex of the singlet state
of 4b compared to that of diphenylcarbene 5 and fluo-
renylidene 6. While the barriers for the OH insertion of S-
5…HOH and S-6…HOH are reported as 6.6 and 5.1
kcal/mol, respectively, at the B3LYP-D3/6-311++G(d,p)
3
0
diphenylcarbene 5 in argon matrices. Thus, matrix site
effects might exceed the intrinsic energy differences be-
tween the singlet and triplet states of the trapped car-
benes, resulting in the coexistence of carbenes in both
spin states.
22, 30
level of theory,
that of S-4b…HOH is predicted to 9.2
kcal/mol at the same level and to 10.0 kcal/mol at the
CCSD(T)/cc-pV(D/T)Z//B97D-GD3/def2-TZVP level of
theory (Figure S13, SI), and thus considerably higher. We
assume that the higher activation barriers result in tun-
neling rates that are too slow to be observable in our
experiments.
Matrix effects not only change the thermodynamic sta-
bility of trapped species, but also that of transition states
and therefore strongly influence the kinetics. This results
in a distribution of rate constants and in the dispersive
kinetics observed in our experiments (see Table S4). In
addition, at the very low temperatures used in our exper-
iments (3 K), rare gas matrices are very rigid, and confor-
mational changes as required for the singlet – triplet in-
terconversion of carbenes are kinetically hindered. To be
efficient, spin interconversion must be coupled with
changes of the C-C-C bond angle, and this process is hin-
dered in very rigid matrices at low temperatures. Again,
CONCLUSION
Carbene 4b is the structurally simplest carbene that has
been demonstrated to show magnetic bistability under
the conditions of matrix isolation. According to the DFT
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transition state stabilization and steric hindrance depends
on the local matrix environment resulting in a distribu-
tion of rates for the ISC as observed in our experiments.
ASSOCIATED CONTENT
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Supporting Information. Experimental and computational
details, supplementary figures and tables and Cartesian co-
ordinates are available free of charge via the Internet at
http://pubs.acs.org.
From our experiments, we conclude that carbenes with
small S – T gaps can coexist in both their singlet and tri-
plet states under the conditions of matrix isolation. The
matrix plays an important role, and magnetic bistability
results both from thermodynamic and kinetic effects of
the matrix on the trapped carbene. In addition, small
intrinsic barriers that influence the ISC rates must be
considered. Annealing at higher temperatures results in
soft matrices which enable conformational relaxation of
the carbenes and allow to overcome small intrinsic barri-
ers. We assume that this results in equilibrium popula-
tions of the singlet and triplet states. Irradiation at low
temperature (3 K), on the other hand, results in non-
equilibrium populations of the spin states. Thus, the
combination of thermo- and photochemistry allows to
shift the spin states of the carbenes in a controlled way.
We are currently using this phenomenon to study the
spin-specific intermolecular reactivity of carbenes.
AUTHOR INFORMATION
Corresponding Author
wolfram.sander@rub.de
elsa.sanchez-garcia@uni-due.de
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Author Contributions
The manuscript was written through contributions of all
authors.
Funding Sources
This work was supported by the Cluster of Excellence
RESOLV (EXC 1069) funded by the Deutsche Forschungsge-
meinschaft (DFG).
ACKNOWLEDGMENT
In summary, magnetic bistability of carbenes is a gen-
eral phenomenon that only depends on the S – T gap of
the carbene. This opens the way for the development of
new optically switchable magnetic materials.
This work was supported by the Cluster of Excellence
RESOLV (EXC 1069) funded by the Deutsche For-
schungsgemeinschaft (DFG). E. S-G. also acknowledges a
Plus3 grant by the Boehringer-Ingelheim Foundation.
EXPERIMENTAL SECTION
General procedures. All chemicals required to pre-
pare the precursors were commercially available. For
materials and synthesis details of the precursors see SI.
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Table 1. Experimental and Calculated Vibrational Frequencies and Shifts of the S-4b···H O Complex.
2
a
Calculated
S-4b
Experimental
e
Assignment
c
d
Argon
Ar/H O
2
S-4b…HOH
ν/cm (Iabs)
b
-1
-1
b
-
1
-1
Shift
ν/cm (I )
ν/cm (I )
Shift
+8.6
ν/cm (Iabs)
rel
rel
1
1
071.6 (278.7)
137.4 (139.3)
1070.8 (278.6)
1131.2 (191.3)
-0.8 1086.7(32)
-6.2 1139.2 (39)
+1.3 1171.7(100)
+5.2 1206.4(44)
+6.8 1599.5 (43)
1095.3(40)
1147.5 (50)
1178.6(100)
1211.1(86)
C-CF bend (out of plane)
3
+8.3
+6.9
+4.7
+2.7
C-CF bend (in plane)
3
1191.5 (645.0)
1192.8 (656.8)
1230.5 (49.4)
1648.9 (290.3)
C-C-C str. (carbene center)
C-H def. (in plane)
C=C str. Ring
1
1
225.3 (115.5)
642.1 (247.8)
1602.2 (75)
a
b
Calculated at the B2PLYP-GD3BJ (S-4b) or B2PLYP-GD3BJ//B3LYP/aug-cc-pVTZ (S-4b…HOH) level of theory. Fre-
-
1
c
d
e
quency shift relative to the monomers (cm ). In argon matrix at 3 K. In argon matrix doped with 1% H O at 3 K. Ten-
2
tative assignment.
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SYNOPSIS TOC
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p-Tolyl(trifluoromethyl) carbene can be isolated in low-temperature matrices in both its singlet and triplet states, and in
the absence of irradiation at 3 K, both states coexist without noticeable changes in their spectra even after several days.
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