Trifluoro-1,3,5-tridehydrobenzene

Article abstract of DOI:10.1002/anie.200700536

(Chemical Equation Presented) Three stay, three go: Flash vacuum pyrolysis (FVP) of 1 and subsequent trapping of the products in solid argon at 3 K produces trifluoro-1,3,5-tridehydrobenzene (2). IR spectroscopy, the photochemical behavior, and quantum chemical calculations confirm that for the first time a derivative of the elusive 1,3,5-tridehydrobenzene could be isolated and spectroscopically characterized.

Full text of DOI:10.1002/anie.200700536

Communications
DOI: 10.1002/anie.200700536
Dehydroarenes
Trifluoro-1,3,5-tridehydrobenzene**
Sugumar Venkataramani, Michael Winkler, and Wolfram Sander*
Dedicated to Professor Rolf Gleiter on the occasion of his 70th birthday
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Chemie
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Triradicals[1] combine high reactivity with complex electronic
structures, and therefore their investigation poses special
challenges to experiment and theory. Tridehydrobenzenes 1–
3, formally derived from benzene by removal of three
hydrogen atoms, are prototypes of this class of reactive
intermediates and have been subject to recent experimental
and computational studies.[2–4]
Scheme 1. Formation and subsequent reactions of 1,2,3-tridehydroben-
zene (1).
According to calculations by Krylov and co-workers, 2
and 3 are less stable than 1 by 3 and 17 kcalmolÀ1,
respectively.[2] All three systems are predicted to exhibit
doublet ground states, indicative of bonding interactions
between the formally unpaired electrons. The adiabatic
doublet-quartet splittings have been calculated to be 49, 41,
and 28 kcalmolÀ1 for 1, 2, and 3, respec-
tively. These values show that coupling of
the unpaired spins is most effective in 1, in
which all three radical centers are adjacent,
and weakest in 3.[2]
bicyclic geometry with a weakly bonding distance of 170 pm
between the formal radical centers at C1 and C3.[3]
The existence of two low-lying doublet states is readily
understood by considering the frontier molecular orbitals
shown schematically in Figure 1a. At small C1–C3 distances,
the singly occupied molecular orbital (SOMO) is of a1
Recently, we described the isolation and
IR spectroscopic characterization of 1,2,3-
tridehydrobenzene (1) in a cryogenic neon
matrix at 3 K.[3] Consecutive photolysis of 3-
iodophthalic anhydride with 308 and
248 nm laser light led to the formation of
1, which could clearly be identified by
subsequent trapping with iodine atoms
upon annealing the matrix at 8 K
(Scheme 1). The most significant finding
Figure 1. Frontier molecular orbitals in the two lowest electronic states of a) 1 and b) 3.
was the identification of a bicyclic 2A1
ground state for 1, while previous compu-
tations assumed a monocyclic 2B2 ground state for that
molecule. According to high-level computations, the former is
adiabatically only 1–2kcalmol À1 lower in energy. On the basis
of a comparison of calculated and measured IR data, the 2B2
state could undoubtedly be discarded in favor of a more
symmetry, whereas at larger distances the antibonding b2
orbital drops below in energy. For 1,3,5-tridehydrobenzene
(3), a similar situation can be envisaged (Figure 1b). In D3h
symmetry, the two doublet states arising from occupation of
either the a1 or b2 orbital are degenerate (symmetry labels in
C2v). The degeneracy is lifted by a Jahn–Teller distortion to
either the 2A1 or 2B2 state. Similar to 1, both doublet states are
close in energy, the latter being 3–4 kcalmolÀ1 less stable.[2] In
contrast to 1, however, the 2B2 isomer of 3 does not
correspond to a minimum on the C6H3 potential energy
surface (PES), but is instead predicted to be a transition state
[*] S. Venkataramani, Prof. Dr. W. Sander
Lehrstuhl für Organische Chemie II
Ruhr-Universität Bochum
Universitätsstrasse 150, 44780 Bochum (Germany)
Fax: (+49)234-321-4353
E-mail: wolfram.sander@rub.de
2
for interconversion of equivalent A1 minima.[2a]
Dr. M. Winkler
Institut für Organische Chemie
Universität Würzburg
Thermochemical data for 3 were determined in the gas
phase by Wenthold and co-workers.[4] According to these
measurements, the heat of formation of the triradical is (179 Æ
4.6) kcalmolÀ1, which leads to the bond dissociation energies
(BDEs) given in Scheme 2for successive hydrogen removal
from benzene.
Am Hubland, 97074 Würzburg (Germany)
[**] This work was financially supported by the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie. M.W.
thanks the Fonds der Chemischen Industrie for a Liebig fellowship.
The similarity of the first and third BDEs indicates that 3
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
is most appropriately described as a meta-benzyne[5,6] moiety
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3 K to 7.5 K results in a recombination of the radicals and
iodine atoms to yield back the precursor 5. Unfortunately,
derivative 7 turned out to be photostable, and even upon
prolonged irradiation no evidence for the formation of 4
could be obtained in these experiments.[8]
Alternatively, pyrolysis of 5 in the gas phase with
subsequent trapping of the products in a cryogenic matrix
could also lead to the elusive triradical 4. In a similar way, a
number of didehydroarenes were synthesized by FVP of aryl
diiodocompounds.[6f,g] A drawback of this method is the
fragmentation of fluorinated aromatic compounds that con-
tain radical centers. In these experiments, complex mixtures
of small fluorinated molecules (CF2, CF3, CF4, C2F3, etc.) are
observed.[9] The mechanism of this fragmentation is not yet
clear and currently under investigation in our laboratory.
The FVP of 5 was investigated at temperatures between
500 and 7508C (Figure 2and Figure S1 in the Supporting
Scheme 2. Successive hydrogen removal from benzene to 1,3,5-tridehy-
drobenzene (3) and corresponding bond dissociation energies (BDEs).
that interacts only weakly with the third radical center on the
opposite side of the ring.[4] This finding is in full agreement
with the calculated 2A1 ground state of 3. However, given the
similar energy of the two lowest doublet states of 3, and taking
into account that several computational methods predict the
2B2 state to be a second minimum instead of a transition state
(see below), more direct information about the ground-state
properties of 3 and its derivatives is desirable.
Herein, we describe the matrix isolation and IR spectro-
scopic characterization of trifluoro-1,3,5-tridehydrobenzene
(4), a simple derivative of triradical 3. Fluorine substitution
often leads to kinetic stabilization of reactive species that are
otherwise too reactive to be isolated even under low-temper-
ature conditions.[7] A suitable precursor of 4 is 1,3,5-triiodo-
À
2,4,6-trifluorobenzene (5), which has three labile C I bonds
that on UV photolysis or flash vacuum pyrolysis (FVP) should
be cleaved to produce the corresponding mono-, di-, and
triradicals 6, 7, and 4, respectively (Scheme 3).
Figure 2. FVP of trifluorotriiodobenzene 5. a) IR spectrum of a matrix
(Ar, 3 K) containing the FVP products (6758C) of 5. b) IR spectrum of
a matrix containing the FVP products (7258C) of 5. Absorptions of
meta-benzyne 7 are labeled by dots. c) Difference spectrum: Bands
pointing downwards decrease, and bands pointing upwards increase
in intensity upon irradiation witha low-pressure mercury lamp of a
matrix containing the FVP products (7258C) of 5. Absorptions of
monoradical 6 are labeled withcrosses; the strongest absorptions of
5
are labeled withcircles. d) Calculated vibrational spectrum (UBLYP/cc-
pVTZ) of triradical 4 in its 2A1 ground state. Note that most of the
unassigned peaks belong to low-molecular-weight fragmentation prod-
ucts suchas SiF , CF4, CF3, CF2 etc. Assignments of these by-products
4
are provided in the Supporting Information.
Scheme 3. Formation of triradical 4 from 1,3,5-triiodo-2,4,6-trifluoro-
benzene (5).
Information). The thermal decomposition of 5 begins at
temperatures above 6008C. However, even above 7008C the
product mixture still contains undecomposed precursor 5.
Apart from impurities (CO, CO2, H2O) common in pyrolysis
experiments, significant amounts of fragments (especially
CF2, CF3, and CF4) are observed under these conditions
(Figure S1 in the Supporting Information). At higher temper-
atures, the concentration of the fragments increases and
several additional compounds are found. Two of those are
readily identified as phenyl radical 6, produced in very low
yields, and didehydrobenzene 7. This observation is in line
The photochemistry of trifluorotriiodobenzene 5 in neon
at 3 K has been described previously.[8] Irradiation at 248 nm
(KrF excimer laser) or 254 nm (low-pressure mercury arc
lamp) produced the expected phenyl radical 6 and meta-
benzyne 7 that were identified by comparison of their matrix
IR spectra with density functional theory (DFT) calculations.
As the radicals 6 and 7 are formed in proximity to iodine
atoms in the same matrix cage, warming of the matrix from
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with previous FVP studies of aromatic diiodo compounds,[6f,g]
where the concentrations of the monoradicals are generally
low compared to that of the diradicals. This indicates a higher
À
thermal lability of the C I bond in monoradicals compared to
the diiodo compounds.[10] A third species with characteristic
absorptions at 831, 875, 1422, and 2324 cmÀ1 is identified as
difluorohexatriyne 8.[11] The formation of 8 clearly indicates
À
that the cleavage of C F bonds occurs at least to some degree
in the FVP experiments. These three molecules were identi-
fied by comparison of the matrix IR spectra with data from
the literature and DFT calculations.
produced monoradical 6, diradical 7, and triradical 4, which
confirms the assignment of 4.
To investigate the effect of fluorine substitution on the
molecular and electronic structure of tridehydrobenzene 3,
extensive quantum chemical computations were carried out
employing density functional theory (DFT) as well as ab initio
methods.[13] Structures of 3 and 4 obtained at the UB3LYP,
UBLYP,[14] and RO-CCSD(T)[15] level, and PES scans along
the C1–C3 distance coordinate[16] are shown in Figure 3. For
both systems, the three methods give rather similar results
regarding the structure and energetic separation of the two
lowest doublet states. In agreement with previous calcula-
A number of additional IR signals at 954, 1030, 1266, 1560,
and 1738 cmÀ1 were assigned to a new species A. These signals
appear only at high temperatures of the FVP (above 7008C)
and show always the same relative intensity, as expected for a
single compound. Species A proved to be photolabile, and on
UV irradiation (254 nm) all IR absorptions assigned to A
decrease in intensity. Simultaneously, all IR bands of 5
(remaining precursor after FVP) decrease in intensity as
previously reported.[8] The products formed during photolysis
are phenyl radical 6 and hexatriyne 8. As photolysis of 5
results in the formation of 6 but not in 8, the hexatriyne must
be a product of the photolysis of A.
2
tions,[2] 3 is found to have a A1 ground state with a C1–C3
separation almost identical to meta-benzyne,[5,6] whereas the
2B2 excited state is 2–3 kcalmolÀ1 higher in energy. Interest-
ingly, the B3LYP method (in contrast to BLYP) predicts the
latter to be a minimum rather than a transition state that is
By comparison of these five IR absorptions with DFT
calculations (BLYP and B3LYP), compound A is identified as
trifluoro-1,3,5-tridehydrobenzene 4 in its A1 state (Table 1).
2
2
1.4 kcalmolÀ1 less stable than the A1 ground state. Despite
A high-spin quartet state can clearly be excluded based on the
poor agreement with the computed vibrational spectrum.[12]
To gain additional support for this assignment, we also
investigated the photo- and thermochemistry of the isomeric
triiodobenzenes 9 and 10 under identical FVP conditions.
Whereas several common products could be identified after
FVP of 9, 10, and 5 (fragments such as CF2 and CF3), only 5
several attempts, no additional stationary point could be
localized in this region of the B3LYP PES. In particular, no
transition state for the interconversion of both isomers could
be found.
Fluorination has only a small effect on the equilibrium
geometry and relative stability of both states. This finding is in
contrast to meta-benzyne, where fluorination leads to for-
Table 1: IR spectroscopic data of matrix-isolated trifluoro-1,3,5-tridehydrobenzene 4 (2A1). Calculated data for the low-spin (2A1, 2B2) states and high-spin quartet
state[12] are given for comparison.
Ar, 3 K
Mode n˜exp [cmÀ1 [a]
4 (2A1)
Sym. n˜calcd [cmÀ1 [a] Icalcd [kmmolÀ1 [b]
4 (2B2)[c]
Sym. n˜calcd [cmÀ1 [a] Icalcd [kmmolÀ1 [b]
4 (4A’’)
Sym. n˜calcd [cmÀ1 [a]
]
Iexp,rel
]
]
]
]
]
Icalcd [kmmolÀ1 [b]
]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
b1
b1
b2
a1
a2
a1
b2
b1
a1
a2
b2
b1
a1
b2
a1
b2
a1
b2
a1
b2
a1
163.0
250.5
271.3
286.6
323.1
379.9
435.0
451.7
546.4
556.0
569.3
578.7
767.0
921.7
997.6
1221.8
1310.6
1361.3
1378.0
1530.0
1726.6
0 (0)
1 (0)
1 (0)
0 (0)
0 (0)
1 (0)
29 (9)
8 (2)
0 (0)
0 (0)
b2
b1
a2
b1
a1
b2
a1
b1
a2
a1
b1
b2
a1
b2
a1
a1
b2
b2
a1
a1
b2
310.2i
125.2
247.9
270.8
279.2
309.7
484.3
484.4
502.5
568.0
573.1
573.6
808.6
976.2
1016.0
1291.3
1325.4
1413.1
1427.7
1572.6
1690.6
36 (12)
0 (0)
0 (0)
1 (0)
0 (0)
2 (0)
9 (2)
14 (4)
0 (0)
0 (0)
a’’
a’
a’
a’’
a’
a’’
a’
a’’
a’
a’’
a’
a’
a’
122.2
122.4
184.9
297.3
307.5
330.3
471.0
549.2
555.2
598.9
612.5
651.7
872.8
976.3
985.8
1021.9
1171.7
1275.7
1299.1
1426.6
1525.2
31 (6)
0 (0)
2 (0)
22 (5)
0 (0)
4 (1)
1 (0)
2 (0)
0 (0)
0 (0)
3 (0)
7 (2)
7 (2)
0 (0)
7 (2)
2 (0)
13 (3)
0 (0)
17 (5)
193 (57)
183 (54)
117 (35)
54 (16)
17 (5)
3 (0)
954
1030
1266
60
30
40
173 (57)
166 (55)
99 (33)
90 (30)
1 (0)
0 (0)
297 (100)
262 (88)
a’
60 (13)
18 (4)
477 (100)
332 (70)
124 (26)
32 (7)
211 (44)
186 (39)
a’’
a’’
a’
a’’
a’
1560
1738
70
100
246 (73)
336 (100)
a’’
a’
[a] UBLYP/cc-pVTZ. [b] Relative intensities are given in parenthesis. [c] Contains imaginary frequency.
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À
Figure 3. Energy as a function of the distance of radical centers C1 and C3 in a) 3 and b) 4. Selected C C bond lengths of stationary points are
given in pm (normal print: UB3LYP/cc-pVTZ; bold: UBLYP/cc-pVTZ; italics: RO-CCSD(T)/cc-pVTZ). Note that the PES scans yield two-
dimensional cuts through a Jahn–Teller triple-cone potential, where the three equivalent 2B2 states are transition states for the interconversion of
2
three equivalent A1 minima. This interconversion corresponds to a pseudorotation around the cone apex.
mation of a bicyclic structure and, accordingly, to a significant
increase in the singlet–triplet splitting.[8] This effect has been
rationalized in terms of eclipsing strain of the three adjacent
fluorine atoms in tetrafluoro-m-benzyne, rather than by
electronic effects of the substituents.[8c] In line with this
interpretation, a similar effect is not found for fluorination of
3, because no steric interaction exists between the fluorine
atoms in 4. However, a thorough investigation of substituent
effects in aromatic bi- and triradicals seems to be necessary to
fully understand the interplay of electronic and steric factors
influencing the ground- and excited-state properties of these
intermediates.[17]
In summary, trifluoro-1,3,5-tridehydrobenzene (4) was
generated by FVP of trifluorotriiodobenzene (5) at temper-
atures above 7008C. Despite the formation of several side
products and low-molecular-weight fragments, the IR absorp-
tions of triradical 4 can readily be assigned on the basis of
their photolability upon irradiation at 254 nm. The measured
IR spectrum is in good agreement with the vibrational
2
spectrum calculated at the BLYP level for the A1 ground
state of 4. Fluorination of 3 has only a marginal effect on the
structures and relative stabilities of the two lowest doublet
2
states. According to high-level calculations, the B2 states of
both 3 and 4 are transition states for the interconversion of
2
2
2
Analogous to 3, the B3LYP functional predicts the B2
degenerate A1 minima lying 2–3 kcalmolÀ1 above the A1
ground states. A detailed investigation aimed at a deeper
understanding of substituent effects in aromatic bi- and
triradicals is currently in progress in our laboratories.
state to be a minimum on the C6F3 PES. The vibrational
spectra calculated at the B3LYP level for both states of 4 are
similar. To allow for a more quantitative comparison with the
experimental IR spectrum, anharmonic corrections to har-
monic vibrational frequencies for both isomers were calcu-
lated perturbatively (Table S1 in the Supporting Informa-
tion). Surprisingly, a low-frequency vibrational mode of b2
symmetry becomes imaginary in the fundamental IR spec-
trum of the 2B2 state of 4, corresponding to a change of
380 cmÀ1 (from 100 cmÀ1 in the harmonic approximation to
283i cmÀ1 with inclusion of anharmonicity effects). Thus, in
agreement with the BLYP calculations, only the three
equivalent 2A1 states of 4 correspond to minimum-energy
structures, whereas the three equivalent 2B2 states are
transition states for their mutual interconversion.
Received: February 6, 2007
Keywords: dehydroarenes · IR spectroscopy · matrix isolation ·
.
radicals · triradicals
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4892 www.angewandte.org
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level, the 4A’’ state of 4 is adiabatically 24.9 kcalmolÀ1 higher in
energy than the 2A1 ground state.
[5] For recent reviews, see: a) W. Sander, Acc. Chem. Res. 1999, 32,
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[7] Generally, fluorine atoms tend to avoid acetylenic positions and
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rearranges to acetylene: a) J. Breidung, H. Bürger, C. Kötting,
R. Kopitzky, W. Sander, M. Senzlober, W. Thiel, H. Willner,
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[16] Distances of two of the radical centers have been frozen and all
other coordinates optimized at the DFT level within the
constraint to C2v symmetry. Subsequently, CCSD(T) calculations
have been carried out on the geometries obtained from these
calculations.
[10] The higher reactivity of phenyl radicals compared to benzynes is
well established. See, for example: a) M. J. Schottelius, P. Chen,
J. Am. Chem. Soc. 1996, 118, 4896; b) C. F. Logan, P. Chen, J.
Am. Chem. Soc. 1996, 118, 2113; c) P. Chen, Angew. Chem. 1996,
108, 1584; Angew. Chem. Int. Ed. Engl. 1996, 35, 1478; d) J. H.
Hoffner, M. J. Schottelius, D. Feichtinger, P. Chen, J. Am. Chem.
Soc. 1998, 120, 376.
[17] For 1, which already has a rather bicyclic structure,[3] fluorination
leads to a stabilization of the 2B2 state which becomes the ground
state of trifluoro-1,2,3-tridehydrobenzene, without changing the
structure of either state significantly. On the basis of steric
effects alone, the opposite tendency would be expected. Thus,
electronic effects are likely to play an important role here as
well.
Angew. Chem. Int. Ed. 2007, 46, 4888 –4893ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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