Halogen derivatives of m-phenylene(carbeno)nitrene: A switch in ground-state multiplicity

Article abstract of DOI:10.1021/jo025877q

m-Phenylene-coupled carbenonitrenes [(3-nitrenophenyl)methylene (2-H), (3-nitrenophenyl)fluoromethylene (2-F), (3-nitrenophenyl)chloromethylene (2-Cl), (3-nitrenophenyl)bromomethylene (2-Br)] have been investigated computationally (with B3LYP, MCSCF, CASPT2, ROMP2, and QCISD(T) methods) and experimentally (with IR, U]V, and ESR spectroscopy). For each species, five electronic states were considered. At the highest level of theory explored, the parent compound (2-H) has a quintet ground state, but its halogen derivatives (2-X, X = F, Cl, and Br) have triplet ground states. A linear relationship between the Q-T energy gap of 2-X and the T-S gap of the corresponding phenylcarbenes 8-X is found, which can be helpful in rationalizing and predicting ground-state multiplicities in m-phenylene-linked carbenonitrenes and similar species. Precursors for the photochemical generation of 2-X (X = H, F, Cl, and Br) were synthesized and photolyzed in matrixes (Ar, triacetin) at low temperatures. IR (Ar, 13 K) and ESR (triacetin, 77 K) data are compatible with the generation of triplet halocarbenonitrenes 2-X, (X = F, Cl, and Br). All four compounds upon further irradiation undergo isomerization to substituted cyclopropenes 5-X (X = H, F, Cl, and Br), as suggested by their IR spectra.

Full text of DOI:10.1021/jo025877q

Ha logen Der iva tives of m -P h en ylen e(ca r ben o)n itr en e: A Sw itch in
Gr ou n d -Sta te Mu ltip licity
Tomonori Enyo,† Athanassios Nicolaides,*,‡ and Hideo Tomioka*,†
Department of Chemistry, University of Cyprus, Nicosia 1678, Cyprus, and Chemistry Department for
Materials, Faculty of Engineering, Mie University, Tsu, Mie 514-8507, J apan
athan@ucy.ac.cy
Received April 30, 2002
m-Phenylene-coupled carbenonitrenes [(3-nitrenophenyl)methylene (2-H), (3-nitrenophenyl)fluoro-
methylene (2-F ), (3-nitrenophenyl)chloromethylene (2-Cl), (3-nitrenophenyl)bromomethylene (2-
Br )] have been investigated computationally (with B3LYP, MCSCF, CASPT2, ROMP2, and
QCISD(T) methods) and experimentally (with IR, UV, and ESR spectroscopy). For each species,
five electronic states were considered. At the highest level of theory explored, the parent compound
(2-H) has a quintet ground state, but its halogen derivatives (2-X, X ) F, Cl, and Br) have triplet
ground states. A linear relationship between the Q-T energy gap of 2-X and the T-S gap of the
corresponding phenylcarbenes 8-X is found, which can be helpful in rationalizing and predicting
ground-state multiplicities in m-phenylene-linked carbenonitrenes and similar species. Precursors
for the photochemical generation of 2-X (X ) H, F, Cl, and Br) were synthesized and photolyzed in
matrixes (Ar, triacetin) at low temperatures. IR (Ar, 13 K) and ESR (triacetin, 77 K) data are
compatible with the generation of triplet halocarbenonitrenes 2-X, (X ) F, Cl, and Br). All four
compounds upon further irradiation undergo isomerization to substituted cyclopropenes 5-X (X )
H, F, Cl, and Br), as suggested by their IR spectra.
In the search for high-spin organic molecules, carbenes
and/or nitrenes linked via the m-phenylene linker are
popular building blocks.1 This is because m-phenylene
usually acts as a ferromagnetic coupler between the
reactive centers. For example, m-phenylene biscarbene
(1-CH),2a m-phenylene bisnitrene (1-N),2b and the corre-
sponding “mixed” carbenonitrene 2-H2c are all known to
have quintet ground states. From the study of such
systems, the idea has evolved that the meta topology
generally leads to high-spin ground states. Such empiri-
cal concepts are useful in the design of new compounds,
and therefore it is important to gain a deeper insight and
understanding of their origins. This is usually achieved
by examining and testing model compounds at or close
to the limits of the various empirical predictions. Thus,
it has been reported that, when the reactive centers are
radicals (for example, as with 1-CH2), the m-phenylene
specific type of geometrical distortion is not applicable
when the reactive centers are carbenes and/or nitrenes.
In this case, it has been argued that the coupling
orientation of this linker may be controlled by chemical
substitution.1a The essence of this idea is that, if the
carbene is designed to have a strong intrinsic preference
for a singlet ground state, then the tendency of the linker
to promote a high-spin ground state may be overridden.
In this context, Sheridan reported the formation of what
is believed to be m-phenylenebischloromethylene (1-CCl).
On the basis of its reactivity and IR and UV/vis spectra,
linker can be transformed to an antiferromagnetic cou-
pler if the radicals are twisted out of conjugation.3 This
shows the importance of geometrical factors and has been
used to explain the low-spin ground states of related
m-phenylenebis(tert-butyl nitroxides).4 However, this
† Mie University.
‡ University of Cyprus.
(1) (a) Zuev, P. S.; Sheridan, R. S. Tetrahedron, 1995, 51, 11337.
(b) Iwamura, H.; Koga, N. Acc. Chem. Res. 1993, 26, 346. (c) Dougherty,
D. Acc. Chem. Res. 1991, 24, 88.
(2) (a) Wasserman, E.; Murray, R. W.; Yager, W. A.; Trozzolo, A.
M.; Smolinsky, G. J . Am. Chem. Soc. 1967, 89, 5076. (b) Itoh, K. Chem.
Phys. Lett. 1967, 1, 235. (c) Tukada, H.; Mutai, K.; Iwamura, H. J .
Chem. Soc., Chem. Commun. 1987, 1159.
(4) (a) Dvolaitzky, M.; Chiarelli, R.; Rassat, A. Angew. Chem., Int.
Ed. Engl. 1992, 31, 180. (b) Kanno, F.; Koga, N.; Iwamura, H. J . Am.
Chem. Soc. 1993, 115, 847.
(3) Fang, S.; Lee, M. S.; Hrovat, D. A.; Borden, W. T. J . Am. Chem.
Soc. 1995, 117, 6727.
10.1021/jo025877q CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/13/2002
5578
J . Org. Chem. 2002, 67, 5578-5587

Halogen Derivatives of m-Phenylene(carbeno)nitrene
TABLE 1. Rela tive En er gies (k ca l/m ol) of Va r iou s
Sta tes of 2-X (X ) H, F , Cl, a n d Br ) Rela tive to th e 5A′
Sta te (Ha r tr ee)
m -P h en ylen e(ca r ben o)n itr en e (2-H). Selected com-
puted bond lengths and bond angles for 2-H (anti isomer)
are shown in Figure 1.7 The two computational methods
utilized for geometry optimizations (B3LYP and MCSCF)
d
species
B3LYPa
MCSCFb
CASPT2c
EQCI
5
are in reasonable agreement as far as the A′ and 3A′′
2-H (1A′′)
2-H (1A′)
2-H (3A′′)
2-H (3A′)
2-H (5A′)
2-F (1A′′)
2-F (1A′)
2-F (3A′′)
2-F (3A′)
2-F (5A′)
2-Cl (1A′′)
2-Cl (1A′)
2-Cl (3A′′)
2-Cl (3A′)
2-Cl (5A′)
2-Br (1A′′)
2-Br (1A′)
2-Br (3A′′)
2-Br (3A′)
33.4
10.1
15.0
5.8
38.0
10.3
18.0
5.3
states of 2-H are concerned. However, they differ con-
12.3
11.9
siderably (Figure S2) when it comes to the other states
1
(3A′, A′, and 1A′′). The difference is most likely due to
-324.20098 -322.37862 -323.30523 -323.58826
the inability of the B3LYP method to properly account
for the multiconfigurational character of these states.8,9
Inspection of the geometries (Figure 1) of the A′ states
shows that, independent of the overall multiplicity, the
angle at the divalent carbon center is approximately 130°,
close to that calculated for triplet phenylcarbene (Figure
S3). On the other hand, the A′′ geometries are character-
ized by a narrower bond angle at the carbene center and
close to that of singlet phenylcarbene. The C-C bond
lengths between the carbene center and the benzene ring
follow a similar pattern, i.e., those of the A′′ states
resemble singlet phenylcarbene and those of the A′ states
triplet phenylcarbene (although this is not so clear for
10.2
10.0
-7.9
5.9
14.3
9.8
-5.3
5.2
-8.5
-11.8
-423.43976 -421.22559 -422.32602 -422.72349
21.9
9.6
3.9
25.3
9.5
5.8
0.4
-2.9
5.7
5.1
-783.81262 -781.28135 -782.35371 -782.72518
22.3
9.7
4.2
25.5
9.5
5.9
0.4
-1.5
5.7
5.0
2-Br (5A′) -2895.32284 -2891.68888 -2892.76209 -2895.56012
1
a
the A′ geometry). The C-N bond length of the A′ and
Includes zero-point energy corrections using B3LYP/6-31G(d)
3A′′ states is close to that of triplet phenylnitrene (3A2-9,
b
frequencies scaled by 0.981. Active space includes 10 (11) MOs
1
and 10 (12) electrons for 2-H (2-X, X ) F, Cl, and Br). The 6-31G(d)
Figure S3), whereas the bond length of A′′ is closer to
basis set was used. c Using the MCSCF wave function described
that of open-shell singlet phenylnitrene (1A2-9). These
geometrical similarities between subparts of 2-X on one
hand and the corresponding phenylcarbene (8-X) and
phenylnitrene on the other are in line with analogies
drawn on the basis of the corresponding electronic
configurations (vide infra).
d
in b. See Computational Procedures.
it was argued that 1-CCl has a singlet ground state (in
contrast to the parent 1-CH).5 On the other hand,
calculations gave conflicting results, with DFT favoring
Sheridan’s interpretation and MCSCF opposing it.6 It
was concluded that the lowest quintet and singlet states
of 1-CCl are essentially isoenergetic, with the former
being slightly favored.6 The apparent disagreement
between theory and experiment in this case is somewhat
disturbing. Furthermore, it is desirable to be able to
quantify, if possible, the relationship between the intrin-
sic preference of the carbene subunit for a singlet local
configuration and the overall configuration of the m-
phenylene bis(diradical). In this context, m-phenylene-
carbenonitrenes 2-X (X) F, Cl, and Br) seem to be
appropriate models for a systematic study of the halogen
effect on the carbene center. The ability to generate
appropriate photochemical precursors of these com-
pounds makes them attractive experimentally. In addi-
tion, the presence of just one carbene subunit implies that
halogen substitution introduces only “half” the chemical
perturbation introduced by Sheridan’s biscarbene, allow-
ing, perhaps, for subtler interactions to be revealed.
As expected, 5A′ is predicted (Table 1) to be the ground
state of 2-H at all levels of theory. Three other electronic
1
states lie rather close in energy, while A′′ is predicted
to be substantially higher in energy. 3A′′, which will turn
out to be the state of interest for the switching of
electronic configurations, lies 15-18 kcal/mol higher in
energy than the quintet (MCSCF and CASPT2 methods).
However, this energy gap is reduced to about 12 kcal/
mol at our highest level of theory (EQCI).10
We attempted to generate the desired carbenonitrene
by irradiating 3-H in an Ar matrix at 13 K (Scheme 1,
Figures 2 and S1). The process was followed by IR
spectroscopy, and the formation of diazonitrene 4-H at
the early stages of the reaction is suggested by the new
“diazo” absorption at 2076 cm-1. Continued irradiation
of 4-H resulted in the formation of a new product at the
expense of the diazo compound. However, the observed
peaks assigned to the final photoproduct were not what
was calculated for carbenonitrene 2-H. Instead, an excel-
lent match between experimental and computed vibra-
tional frequencies was found for substituted cyclopropene
Z-5-H (Figure 2a). Irradiation with shorter a wavelength
Resu lts
For 2-X (X ) H, F, Cl, and Br), five electronic states,
one quintet, two triplets (A′ and A′′), and two singlets
(A′ and A′′) were considered. Absolute and relative
energies are shown in Table 1 and in Supporting Infor-
mation (Tables S1 and S2). Optimized geometries were
obtained at the MCSCF and DFT levels of theory (Figure
1, Tables S3 and S4). Approximate valence-bond repre-
sentations of these states are shown in Scheme 4 and
discussed in more detail later on.
(7) There are two possible isomers with respect to the orientation
of the C-H bond relative to the nitrene center: anti or E and syn or
Z. As expected, their energies, geometries, and vibrationals are
essentially the same (Table S1, Figure S4). For simplicity, the anti
isomers are referred to throughout the main text for 2-X (X ) H, F,
Cl, and Br). Experimentally, both isomers are expected to form.
(8) This inability is reflected also in the high spin-contamination
related to these states (Figure S2).
(9) Bally, T.; Borden, W. T. Rev. Comput. Chem. 1999, 13, 1.
(10) The difference (of approximately 6 kcal/mol) between EQCI and
CASPT2 relative energies can be traced mainly to the inclusion of triple
excitations and to the larger basis set utilized in the former method
(Table S3). Both effects favor the energy of the 3A′′ state relative to
that of 5A′.
(5) Zuev, P. S.; Sheridan, R. S. J . Org. Chem. 1994, 59, 2267.
(6) Trindle, C.; Datta, S. N.; Mallik, B. J . Am. Chem. Soc. 1997,
119, 12947.
J . Org. Chem, Vol. 67, No. 16, 2002 5579

Enyo et al.
5
3
3
1
1
F IGURE 1. Selected geometrical parameters (distances in Å, angles in deg) for the A′, A′, A′′, A′, and A′′ states of 2-X (X )
H, F, Cl, and Br) at the MCSCF and UB3LYP (in italics) levels of theory with the 6-31G(d) basis set.
SCHEME 1
participate in the C-F bond. The C-N bond lengths in
2-F are similar to that in triplet phenylnitrene, except
1
1
for A′′-2-F , which resembles A2-9 in this respect.
The different computational methods all predict that
2-F has a triplet (3A′′) ground state in contrast to the
parent 2-H (Table 1). The energy difference between the
5
3A′′ and A′ states (which corresponds to the first excited
state) is 11.8 kcal/mol. As in the case of 2-H, MCSCF
and CASPT2 methods overestimate the relative stability
5
5
of the A′ state and the predicted A′-3A′′ energy gap is
(λ > 300 nm) caused further changes in the IR spectrum,
and the calculations suggest that this is due to a Z to E
isomerization of the cyano substituent (Scheme 1, Figure
2b,c).
found to be smaller (in absolute value) by a similar
1
3
amount. In addition, the A′ and A′ states are higher in
energy than the 5A′ state by amounts that are essentially
the same in 2-F as in 2-H.
m -P h en ylen e(flu or oca r ben o)n itr en e. As with the
parent carbenonitrene (2-H), the geometrical character-
istics of the A′ and A′′ states at the divalent carbon center
are similar to those of triplet and singlet phenylcarbene,
respectively (Figure 1). The bond angles at the carbene
center of the A′ states are systematically narrower (by
about 7°) in 2-F than those in 2-H. This can be attributed
to the higher electronegativity of F, which is expected to
decrease the s character of the carbon orbitals that
The initial photoproduct (Figure S5) of the irradiation
of 6-F is assigned to its diazo isomer 4-F on the basis of
the observed peak at 2029 cm-1 and in agreement with
our previous experience (Scheme 2, Figures 3 and S5).11
Further irradiation leads to a compound that shows no
azido or diazo absorptions in the IR and for which the
(11) Under similar experimental conditions, the para isomer of 4-F
also isomerizes to its corresponding diazo isomer.12
5580 J . Org. Chem., Vol. 67, No. 16, 2002

Halogen Derivatives of m-Phenylene(carbeno)nitrene
F IGURE 2. (a) Calculated (B3LYP/6-31G(d)) spectrum for
Z-5-H. (b) Difference spectrum between the photoproduct
formed after 505 min of irradiation with λ > 350 nm (Z-5-H,
upper part) and final photoproduct formed after an additional
215 min of irradiation with λ > 300 nm (E-5-H, lower part).
(c) Calculated (B3LYP/6-31G(d)) spectrum for E-5-H.
SCHEME 2
targeted carbenonitrene structure 2-F is proposed on the
basis of the good agreement with the computed spectra.
Actually, the computations suggest that the observed IR
spectrum should be assigned to the 3A′′ state of 2-F
5
rather than the A′ state implying a triplet ground state
(Figure S5d,e).
UV spectra of the secondary photoproduct (2-F ), taken
under the same experimental conditions, show a strong
absorption at 245 nm and a very weak and broad one at
420 nm. Although 2-F is observable, it decays upon
continued irradiation giving rise to new peaks in the IR
and UV spectra (Figure 3d,f). By comparison with
computed vibrational frequencies (Figure 3e), the struc-
ture 5-F is assigned to the final photoproduct.
F IGURE 3. Photolysis (λ > 300 nm) of 6-F in an Ar matrix
at 13 K. (a) Total IR spectrum of 6-F before irradiation. (b)
Total IR spectrum obtained after 6 min of irradiation. Bands
due to the secondary photoproduct (2-F ) are marked “f”. (c)
Calculated (B3LYP/6-31G(d)) spectrum of triplet (3A′′) 2-F . (d)
Difference spectrum of F2 (lower part) and the photoproduct
formed after 105 min of irradiation (5-F , “9”). (e) Calculated
(B3LYP/6-31G(d)) spectrum of 5-F . (f) UV spectra obtained
from 15 s to 23 min of irradiation time. (g) ESR spectra ob-
tained after 16 min of irradiation of 6-F in triacetin at 77 K.
Finally, irradiation of 6-F in a triacetin matrix at 77
K gave UV spectra similar to those obtained in the Ar
matrix. The peaks observed in triacetin that are at-
tributed to 2-F (Figure S5) are shifted by about 10 nm
with respect to the UV spectra taken in the Ar matrix.
At the same time, ESR signals attributable to a triplet
nitrene species were detected (Figure 3g), but no evidence
for the presence of a quintet species was found.
J . Org. Chem, Vol. 67, No. 16, 2002 5581

Enyo et al.
SCHEME 3
SCHEME 4
m-P h en ylen e(ch lor ocar ben o)n itr en e an d m-P h en -
ylen e(br om oca r ben o)n itr en e. The calculated geom-
etries of the various states of 2-Cl and 2-Br show general
characteristics similar to those discussed above for 2-H
and 2-F . Relative MCSCF and CASPT2 energies predict
that 5A′ should be the ground state for 2-Cl and 2-Br
3
and not A′′ as suggested by the IR spectra. B3LYP finds
ally transparent partition of the molecules in diradical
subunits (carbene or nitrene sites) and the linker (m-
phenylene). With the further approximation that the
“intrinsic” T-S splitting of a diradical site in 2-X is
modeled reasonably well by the appropriate T-S gap of
phenylcarbene 8-X or phenylnitrene (9), the relative
energies shown in Table 1 can be understood and the
preference of m-phenylene for high-spin states can be
quantified.
that the two states are essentially isoenergetic. At our
3
best level of theory (EQCI), both species have A′′ ground-
states, with the quintet state lying about 2-3 kcal/mol
higher in energy. As above, DFT and MCSCF methods
tend to overestimate the relative stability of the quintet.
The difference between EQCI and CASPT2 for the Q-T(A′′)
gap in 2-Cl and 2-Br is around 8 kcal/mol, similar to the
case of 2-H.
Irradiation of 6-Cl and 6-Br (Scheme 3, Figures 4, 5,
S6, and S7) gives initially the corresponding azido-
carbenes 7-X (X ) Cl and Br) as judged by the new azido
peaks observed in the IR. It seems, then, that the first
nitrogen molecule comes off from the diazirine ring and
not from the azido group. As before, the reaction proceeds
via the loss of a second N2 to give the targeted carbeno-
nitrenes 2-X (X ) Cl and Br). As with the fluorine
derivative, the computed vibrational frequencies suggest
The 5A′ states can be thought of as the high-spin
coupling of the triplet states of the two reactive centers,
while the 1A′ states can be thought of as the low-spin
coupling. Interestingly, the difference in energy between
these two states is about 10 kcal/mol independent of the
nature of X. This is the same amount by which singlet
1-CH2 (antiferromagnetic coupling of the radical centers)
lies higher in energy than triplet 1-CH2.3 It seems, then,
that 10 kcal/mol is the energy required to impose a low-
spin coupling in the π space of the m-phenylene linker
in order to overcome its preference for ferromagnetic
coupling.
3
that the A′′ states of the carbenonitrenes are observed.
Further irradiation causes both 2-Cl and 2-Br to
isomerize to substituted cyclopropenes (5-Cl and 5-Br ,
respectively). As in the case of 2-F , formation of the
carbenonitrenes 2-Cl and 2-Br is associated with the
observation of strong UV absorptions (257 nm for 2-Cl
and 241 and 262 nm for 2-Br ) and very weak and broad
absorptions at long wavelengths (395 and 404 nm,
respectively).
When 6-Cl and 6-Br were irradiated in triacetin at 77
K, the UV spectra were similar to those obtained in the
Ar matrix. The triacetin spectra are red-shifted by about
10 nm with respect to the Ar ones. This shift is similar
in magnitude but opposite in direction compared to that
in 2-F . ESR spectra obtained under these conditions
(Figures 4g and 5g) showed the presence of triplet nitrene
species (assigned to 2-Cl and 2-Br , respectively) and the
absence of significant amounts of quintet species.
Both A′′ states of 2-X can be thought of as containing
3
a singlet carbene subunit. Their difference is that the A′′
1
states have a triplet nitrene subunit, but the A′′ states
have a singlet (open-shell) nitrene site. As is known,13
the lowest singlet phenylnitrene (1A2) has open-shell
character and is approximately described as a biradical
with one odd electron localized in the σ orbital of the
nitrene and the other in the π space. Thus, the energy
difference 1A′′-3A′′ in 2-X should be approximately equal
to the singlet-triplet splitting of phenylnitrene (18-19
kcal/mol) and this is easily derived from the data in Table
1.
The 3A′ states lie about 5 kcal/mol higher in energy
than the corresponding quintet states. One may ap-
3
proximate the A′ states as the combination of a triplet
carbene with a nitrene in which its σ and π electrons have
opposite spins. It is tempting to identify the latter
configuration as being analogous to the lowest singlet
state of phenylnitrene (open-shell singlet, 1A2). However,
Discu ssion
The electronic configurations of the five electronic
states of 2-X considered can be approximated by the
valence pictures shown in Scheme 4. Support for these
descriptions comes from the geometrical characteristics
discussed in the previous section and also from inspection
of the leading configurations of the MCSCF wave func-
tions. Using these simplistic pictures allows a conceptu-
3
if that were the case, then the A′ state should be higher
(12) Nicolaides, A.; Enyo, T.; Miura, D.; Tomioka, H. J . Am. Chem.
Soc. 2001, 123, 2628.
(13) Hrovat, D. A.; Waali, E. E.; Borden, W. T J . Am. Chem. Soc.
1992, 114, 8698.
5582 J . Org. Chem., Vol. 67, No. 16, 2002

Halogen Derivatives of m-Phenylene(carbeno)nitrene
F IGURE 5. Photolysis (λ > 350 nm) of 6-Br in an Ar matrix
at 13 K. (a) Total IR spectrum of 6-Br before irradiation. (b)
Total IR spectrum obtained after 9 min of irradiation. Bands
due to the secondary photoproduct (2-Br ) are marked “f”.
Bands due to the final photoproduct (B3) are marked “9”. (c)
Calculated (B3LYP/6-31G(d)) spectrum of triplet (3A′′) 2-Br .
(d) Total IR spectrum obtained after 310 min of irradiation.
Bands due to the final photoproduct (5-Br ) are marked “9”.
(e) Calculated (B3LYP/6-31G(d)) spectrum of 5-Br . (f) UV
spectra obtained from 10 to 210 min of irradiation time. (g)
ESR spectrum obtained after 10 min of irradiation of 6-Br in
triacetin at 77 K.
F IGURE 4. Photolysis (λ > 350 nm) of 6-Cl in an Ar matrix
at 13 K. (a) Total IR spectrum of 6-Cl before irradiation. (b)
Total IR spectrum obtained after 10 min of irradiation. Bands
due to the secondary photoproduct (2-Cl) are marked “f”. (c)
Calculated (B3LYP/6-31G(d)) spectrum of triplet (3A′′) 2-Cl.
(d) Total IR spectrum obtained after 220 min of irradiation.
Bands due to the final photoproduct (5-Cl) are marked “9”.
(e) Calculated (B3LYP/6-31G(d)) spectrum of 5-Cl. (f) UV
spectra obtained from 4 to 90 min of irradiation time. (g) ESR
spectra obtained after 10 min of irradiation of 6-Cl in triacetin
at 77 K.
J . Org. Chem, Vol. 67, No. 16, 2002 5583

Enyo et al.
TABLE 2. Rela tive En er gies (k ca l/m ol) of 1A′ a n d 1A′′
Sta tes of 8-X (X ) H, F , Cl, a n d Br ) a n d 1A1 a n d 1A2 Sta tes
of 9 Rela tive to th e 3A′′ a n d 3A2 Sta tes, Resp ectively
are the states expected to be mostly affected by the
halogen substitution and are relevant for achieving
configuration switching. The implicit assumption is that
halogen substitution at the carbene center of 2-X stabi-
d
species
B3LYPa
MCSCFb
CASPT2c
EQCI
3
5
8-H (1A′′)
8-H (1A′)
8-H (3A′′)
8-F (1A′′)
8-F (1A′)
8-F (3A′′)
8-Cl (1A′′)
8-Cl (1A′)
23.2
10.6
22.1
14.1
lizes the A′′ states compared to the A′ ones, in a similar
way that the (closed-shell) singlet of the phenylcarbene
8-X is stabilized with respect to the triplet state (Table
2). In other words, it is expected that the Q-T(A′′)
splitting in 2-X (∆E(3A′′-5A′)) is proportional to the T-S
splitting in 8-X (∆E(1A′-3A′′)). Exactly this is found when
the computational data are plotted (Figure 6). From the
linear fitting of the EQCI energies (regression coefficient
) 0.998), the following equation linking the two quanti-
ties (in kcal/mol) is found:
7.3
4.4
-270.12902 -268.55494 -269.36123 -269.60053
24.6
-11.8
23.2
-9.1
-13.0
-17.0
-369.36822 -367.40263 -368.38390 -368.73644
24.1
-0.2
22.2
3.5
-4.3
-8.2
8-Cl (3A′′) -729.74144 -727.45881 -728.41184 -728.73818
8-Br (1A′′)
8-Br (1A′)
23.7
0.0
22.0
2.7
-4.4
-6.9
8-Br (3A′′) -2841.25159 -2837.86628 -2838.81882 -2841.5731
9 (1A1)
9 (1A2)
9 (3A2)
33.7
41.9
17.8
36.9
19.4
∆E(3A′′-5A′) ) 6.6 + 1.1 × ∆E(1A′-3A′′)
(1)
-286.21818 -284.59560 -285.42619
On the basis of these values, configuration switching is
expected to take place when ∆E(1A′-3A′′) is -6 kcal/mol
or less.
a
Includes zero-point energy corrections using B3LYP/6-31G(d)
b
frequencies scaled by 0.981. Active space includes 8 (9) MOs and
8 (10) electrons for 2-H and 9 (2-X, X ) F, Cl, and Br). The
Linear relationships were found with the other com-
putational methods as well.15 Thus, all four methods
predict that the stabilization effect of the halogen in 2-X
is linearly related to that in 8-X. On the other hand, there
is little agreement as far as the Q-T(A′′) splitting of 2-X
is concerned. While it is encouraging that the differences
between the methods appear to be systematic, this
splitting is important in establishing the ground-state
multiplicity of 2-Cl and 2-Br .
6-31G(d) basis set was used. c Using the MCSCF wave function
d
described in b. See Computational Procedures.
5
in energy than the corresponding A′ state by the same
amount that 1A′′ is higher in energy than 3A′′ (18-19
kcal/mol, vide supra). To resolve the issue, we propose
that, although the σ and π electrons on the nitrene center
have opposite spins, the overall configuration is some-
what different than that of an open-shell singlet (1A2)
phenylnitrene. In the latter molecule, the nitrene center
can be described (qualitatively) by two determinants:
|...πN(R)σN(â)| - |...σN(R)πN(â)|, whereas, in the former
case, only the first determinant would be important
because the m-phenylene linker promotes the ferromag-
netic coupling between the nitrene and carbene subunits.
(15) The following equations were obtained for the other methods
(regression coefficients in parentheses): ∆E(3A′′-5A′) ) 4.8 + 1.0 ×
∆E(1A′-3A′′) B3LYP (1.000); ∆E(3A′′-5A′) ) 4.2 + 1.0 × ∆E(1A′-3A′′)
MCSCF (1.000); ∆E(3A′′-5A′) ) 3.3 + 1.0 × ∆E(1A′-3A′′) CASPT2
(0.997).
(16) Tomioka, H.; Kitagawa, H.; Izawa, Y. J . Org. Chem. 1979, 44,
3072.
3
(17) C¸ elebi, S.; Levya, S.; Modarelli, D. A.; Platz, M. S. J . Am. Chem.
Soc. 1993, 115, 8613.
In other words, in the A′ states of 2-X, if the carbene is
locally in a triplet configuration (|...πC(R)σC(R)|), a reason-
ably good qualitative description of these states is
|...πC(R)σC(R)πN(R)σN(â)|, whereas the contribution of
|...πC(R)σC(R)σN(R)πN(â)| is not as important. This is also
supported by examining the leading configurations of the
5A′ MCSCF wave functions, which show that two π
electrons are triplet coupled.
(18) The intensity of the strongest absorption of 3-H is computed
to be ca. 820 km/mol, while that of quintet 2-H is about 32 km/mol
(Figure S1h). Since, in the former case, the strongest measured
absorbance was 1.45, those of 2-H should be around 0.06(well within
the limits of experimental detection). This approximation assumes that
the 3-H f 2-H transformation is completed before the latter starts
reacting further. However, this is not the case, since 5-H appears
during the early stages of the irradiation, when about 10% of 3-H has
reacted. Thus, the strongest absorbances of 5A′-2-H are an order of
magnitude less than “normally” expected, and we expect the corre-
sponding signals to lie within the noise level of our instrument.
(19) Using the strongest observed peak for the 3A′′ state of 2-F (1115
cm-1, absorbance 0.126) and computed peaks (1111 cm-1 (240 km/mol)
for the 3A′′ state and 1380 cm-1 (105 km/mol) for the 5A′) and assuming
a threshold absorbance of 0.005 finds the maximum possible amount
of quintet present to be less than 10% of the triplet A′′ state.
(20) Assuming a threshold absorbance of 0.005, the estimation is
based on the observed peak at 1139 cm-1 (absorbance ) 0.081) and
the computed peaks at 1126 cm-1 (178 km/mol) for 3A′′-2-Cl and at
837 cm-1 (71 km/mol) for 5A′-2-Cl.
It can be shown that, for two electrons, a determinant
of the type |π(R)σ(â)| corresponds to neither a pure singlet
nor a pure triplet wave function. It represents rather a
“mixed” wave function made of 50% singlet and 50%
3
triplet character.14 By extending this analogy to the A′
states of 2-X, the “local” nitrene subunit is expected to
lie energetically “halfway” between triplet and (open-
shell) singlet phenylnitrene. This means that these states
should be roughly 9 kcal/mol (i.e., half of the gap in
phenylnitrene) higher in energy than the corresponding
5A′ states. If one takes into account our simplistic
assumptions, this expectation is essentially fulfilled by
the computationally derived 5A′-3A′ gap of 5 kcal/mol.
Thus, one may argue that the m-phenylene linker, by
promoting ferromagnetic coupling in its π space, allows
for favorable electron correlation at the nitrene center,
(21) From the observed peak at 1138 cm-1 (absorbance ) 0.145) and
the computed peaks at 1128 cm-1 (188 km/mol) for 3A′′-2-Br and at
803 cm-1 (19 km/mol) for 5A′-2-Br , the maximum possible amount of
quintet present is about 35% of that of the observed triplet. A smaller
percentage (15%) is found if the peak at 655 cm-1 (42 km/mol) is used.
However, if these peaks overlap with those observed for the triplet
(794 and 663 m-1, respectively), then the presence of larger amounts
of quintet could be masked. Estimations based on ratios of computed
intensities show that, depending on the peaks chosen, the presence of
the quintet could vary between 10% and 110% of the triplet.
(22) One can think of several possible isomers depending on the
positions of the halogen and CN substituents and the relative
conformation of the two exocyclic double bonds. It is not possible to
discriminate among all the possibilities only from the IR data, and
further studies are required before the exact structures are assigned
unambiguously.
3
reducing the energy of the A′ state.
3
Finally, the A′′ states result from the combination of
triplet nitrene and a closed-shell singlet carbene. These
(14) See, for example: Szabo, A.; Ostlund, N. In Modern Quantum
Chemistry; Dover Publications, Inc.: Mineola, NY, 1996; p 102.
(23) Nicolaides, A.; Tomioka, H.; Murata, S. J . Am. Chem. Soc. 1998,
120, 11530.
5584 J . Org. Chem., Vol. 67, No. 16, 2002

Halogen Derivatives of m-Phenylene(carbeno)nitrene
this failure is that 4-H transforms directly to Z-5-H,
without the intermediate formation of 2-H. Indeed, it has
been demonstrated that the photochemical decomposition
of diazo compounds often leads directly to products
derived from rearrangements of the nitrogen-containing
precursors themselves.16,17 However, in the particular
case of 4-H, it seems less likely that this may be the case,
since the concomitant elimination of two nitrogen mol-
ecules is required. An alternative explanation is that 2-H
is formed but its concentration remains below the thresh-
old required for its detection under our experimental
conditions. This can happen if 2-H is formed vibrationally
excited and/or is photosensitive under the irradiation
conditions. We favor this explanation, since 2-H has been
detected by ESR2c under similar conditions and since we
are able to observe its halogen analogues by IR.18
In the case of 2-F , the computed 5A′-3A′′ gap is
sufficiently large that the quintet state is not expected
to be thermally populated. In addition, the strongest IR
absorption of the quintet state is predicted to be well
separated from peaks attributable to 3A′′ 2-F and there-
fore should have been observed if present in significant
amounts.19
F IGURE 6. EQCI relative energies of 3A′′ and 5A′ states
(∆E(3A′′-5A′), kcal/mol) of 2-X (X ) H, F, Cl, Br) plotted
against the EQCI singlet-triplet splittings (∆E(1A′-3A′′), kcal/
mol) of the corresponding phenylcarbenes (8-X).
Computing relative energies of entities of different
multiplicities is a very difficult task, and in this respect,
the EQCI energies are the best computational results
reported thus far in the study of such bis(diradicals).
These results are unlikely to be changed significantly by
higher-level calculations and can be used as benchmarks
for B3LYP- and MCSCF-based methods. According to this
method, all three halocarbenes 2-X (X ) F, Cl, and Br)
are predicted to have triplet ground states. From Tables
1 and 2, B3LYP tends to overestimate the stability of the
high-spin state (Q in the case of 2-X, T in the case of 8-X)
compared to the low-spin state (T(A′′) and S, respectively)
on average by about 3 kcal/mol. At the MCSCF and
CASPT2 levels, the overestimation is about 6 and 8 kcal/
mol, respectively. It should be noted that the B3LYP
values (the least expensive computationally) are in better
agreement with the EQCI results than are the MCSCF
and CASPT2 values.
To the extent that the differences in the methods are
systematic, the above overestimations are likely to be
transferable. If this is correct, then in the case of 1-CCl,
where the quintet and singlet states were found to be
nearly isoenergetic at the MCSCF level of theory,6 in
reality the singlet is likely to be the ground state. On
the basis of the previous approximation and taking into
account that there are two Cl substituents in 1-CCl, the
singlet is expected to be about 12 kcal/mol more stable
than the quintet. In agreement with this expectation, the
In the case of 2-Cl, the quintet state can be present in
up to about 20% of the amount of the observed triplet
(A′′) and still evade detection under our experimental
conditions.20 The strongest peaks of the quintet state of
2-Br are predicted to be close to absorptions of its triplet
state. Possible overlap between the peaks of the two
states could result in masking of significant amounts of
quintet.21
The ESR spectra, although obtained under different
conditions (triacetin matrix, 77 K), support the results
of the IR experiments. This is particularly important for
2-Br (and to a lesser extent for 2-Cl and 2-F ). Compound
2-Br has the smallest predicted Q-T(A′′) splitting among
the three halocarbenes and is the one for which IR has
the greatest uncertainty as far as the presence of the
quintet state is observed. Thus, the observation of ESR
signals attributable to triplet nitrene species under
conditions where UV spectroscopy suggests the presence
of 3A′′-2-Br provides strong evidence that this is the
observed species. The absence of signals attributable to
quintet 2-Br strongly suggests that if this state is
present, then its concentration must be significantly less
than that of the observed triplet.
Overall the experimental data suggest that the ob-
served halocarbenes 2-X (X) F, Cl, and Br) are mainly
(if not totally) in their triplet states. Taking into account
the high-level computational results, the best interpreta-
tion seems to be that these are the ground states of the
species under consideration.
E
QCI level of theory predicts a splitting of -11.5 kcal/mol,
6
supporting Sheridan’s interpretation.
The above shows that special caution is needed when
estimating relative energies of different electronic states
in these kinds of systems. At the same time, it shows
the importance of generating these species experimen-
tally, since small energy differences, even when computed
at high levels of theory, can be sources of ambiguity and
controversy.
Compounds 3-H and 4-X (X ) F, Cl, and Br) were
synthesized as precursors for the computed m-phenylene-
linked carbenonitrenes. In all cases, the final photo-
products are substituted cyclopropenes (5-X, X ) H, F,
Cl, and Br). In the case of the halogen derivatives, the
intermediate formation of the corresponding carbeno-
nitrenes 2-X was also observed. On the other hand, we
were not able to detect 2-H. A possible explanation for
Under our experimental conditions, the carbeno-
nitrenes 2-X (X ) F, Cl, and Br), although observable by
IR (in contrast to 2-H), react photochemically giving rise
to new compounds as monitored both by IR and UV
spectroscopy. The computational data strongly suggest
that the final photoproducts are substituted cyclopro-
penes 5-X (X ) F, Cl, and Br).22 This remarkable bond
reorganization reaction, while surprising at first, is
reminiscent of the photoreactivity of p-phenylenebis-
nitrene, which also isomerizes to a cyclopropene deriva-
tive.23 The formation of a strong carbon-nitrogen triple
J . Org. Chem, Vol. 67, No. 16, 2002 5585

Enyo et al.
theory were scaled by 0.961 and zero-point energies (ZPE) by
0.981.28 Better relative energies were obtained at a modified
G2(MP2,SVP)29 level (denoted by EQCI) of theory. More specif-
ically, the MP2 calculations for the open-shell species were
carried out on ROHF wave functions25 to avoid problems
arising from spin contamination.30a The MCSCF-optimized
geometries were used for the required QCISD(T) and MP2
single-point calculations. These modifications and omitting the
so-called higher-level correction allowed a T-S gap of 4.4 kcal/
mol (Table 2) to be predicted for phenylcarbene (8-H) in very
good agreement with the available experimental and compu-
tational data.30
The active space of the MCSCF wave functions consisted of
10 (2-H) or 12 (2-X, X ) F, Cl, Br) electrons and 10 or 11
molecular orbitals, respectively. The active MOs were the six
MOs contributed by the m-phenylene linker (three π and three
π*), one σ (A′), and one π (A′′) per reactive center and (for the
halogen derivatives) a MO corresponding to the A′′ lone pair
of the halogen. In total, five different electronic states were
considered for each carbenonitrene 2-X; one quintet (5A′), two
triplets (3A′ and 3A′′), and two singlets (1A′ and 1A′′).
For phenylcarbene and phenylnitrene, the active space
included the six π MOs related to the carbons of the benzene
ring and one σ MO and one π MO of the active center
(MCSCF(8,8)). In the halogen-substituted phenylcarbenes, the
active space included, in addition, the π lone pair (A′′) of the
halogen (MCSCF(10,9)).
bond seems to be part of the driving force behind this
transformation.
On the other hand, the para isomers of 2-X are
reasonably inert under similar irradiation conditions.12
This difference in reactivity between meta and para
topologies is difficult to explain without a knowledge of
the mechanism. While this is under investigation, a
plausible explanation can be found by looking at the
geometries and electronic structures of the two kinds of
species. In the para isomers of 2-X, there is strong
conjugative coupling between the reactive centers, trans-
forming them to vinyl and iminyl moieties, and the whole
molecule is better described as a biradical.12 On the other
hand, in the meta carbenonitrenes 2-X (X ) H, F, Cl,
and Br), the two reactive centers retain much of their
carbenic and nitrenic identities and the whole molecule
can be thought of as resulting from a rather weak
coupling of a local (triplet) phenylnitrene and a local
phenylcarbene (triplet for X ) H or singlet for X ) F, Cl,
and Br). Since phenylnitrene and phenylcarbene are
known to be photoreactive, the higher reactivity of the
meta compounds may be understood at least in this
qualitative manner.
In conclusion, 3-H and 4-X (X ) F, Cl, and Br) react
photochemically to give, as final photoproducts, substi-
tuted cyclopropenes. Our computational and experimen-
tal data support the existence of the corresponding
carbenonitrenes 2-X (X ) H, F, Cl, and Br) as intermedi-
ates. The ground-state electronic configuration in 2-X
depends on the intrinsic preference of the carbene
subunit for the singlet or triplet state (approximated as
the T-S gap of the corresponding phenylcarbene 8-X) and
on the preference of the m-phenylene linker to promote
high-spin coupling in its π space (estimated at 6-10 kcal/
mol). On the basis of this simple model, the quintet
ground state of the parent 2-H and the triplet (A′′) state
of 2-F on one hand and the small Q-T(A′′) splitting of
2-Cl and 2-Br on the other are easily rationalized. The
finding that 2-Cl has a A′′ ground state instead of a A′
one supports Sheridan’s assignment of 1-CCl as having
a singlet ground state.5 It further suggests that just one
Cl substituent at a carbene center suffices to cause
configuration switching in m-phenylene-linked bis-
(diradicals) of this type.
Our data justify the hypothesis that chemical substitu-
tion at the carbene center can cause switching from a
high-spin ground state to a lower multiplicity and also
provide quantitative insight as to when such switching
is likely to take place.
Exp er im en ta l Section
Ma ter ia l a n d Gen er a l Meth od s. 1H and 13C NMR spectra
were recorded using CDCl3 as a solvent. ESR spectra were
measured on a spectrometer with an X-band microwave unit
and 100 kHz field modulation.
Synthetic procedures and spectroscopic data are given in
Supporting Information.
Ma tr ix-Isola tion Sp ectr oscop y. Matrix experiments were
performed by means of standard techniques31,32 using a closed-
cycle helium cryostat. For IR experiments, a CsI window was
attached to the copper holder at the bottom of the cold head.
Two opposing ports of a vacuum shroud surrounding the cold
3
5
(25) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J . A. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998.
(26) Schmidt, M. W.; Baldridge, K. K.; Boats, J . A.; Elbert, S. T.;
Gordon, M. S.; J ensen, J . H.; Koseki, S.; Matsunaga, N.; Nguyen, K.
A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J . A., J r. J . Comput.
Chem. 1993, 14, 1347.
(27) Andersson, K.; Blomberg, M. R. A.; Fu¨lscher, M. P.; Karlstro¨m,
G.; Lindh, R.; Malmqvist, P.-Å.; Neogra´dy, P.; Olsen, J .; Roos, B. O.;
Sadlej, A. J .; Schu¨tz, M.; Seijo, L.; Serrano-Andre´s, L.; Siegbahn, P.
E. M.; Widmark, P.-O. Molcas, version 4; Lund University: Lund,
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(28) Scott, A. P.; Radom, L. J . Phys. Chem. 1996, 100, 16502.
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All four carbenonitrenes are photolabile irrespective of
their ground-state multiplicity isomerizing to substituted
cyclopropenes upon continued irradiation.
Com p u ta tion a l P r oced u r es
B3LYP/6-31G(d)24-optimized geometries and vibrational
frequencies were obtained with GAUSSIAN 98;25 MCSCF/
6-31G(d) geometry optimizations were carried out with
GAMESS,26 and single-point CASPT2 energies (at MCSCF/6-
31G(d)-optimized geometries) were computed with MOLCAS.27
Vibrational frequencies obtained at the B3LYP level of
(30) (a) Wong, M. W.; Wentrup, C. J . Org. Chem. 1996, 61, 7022.
(b) Matzinger, S.; Bally, T.; Patterson, E. V.; McMahon, R. J . J . Am.
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5586 J . Org. Chem., Vol. 67, No. 16, 2002

Halogen Derivatives of m-Phenylene(carbeno)nitrene
head were fit with KBr, with a quartz plate for UV irradiation
and a deposition plate for admitting the sample and matrix
gas. For UV experiments, a sapphire cold window and quartz
outer window were used. The temperature of the matrix was
controlled by a gold vs chromel thermocouple.
Irradiations were carried out with a 500 W xenon high-
pressure arc lamp. For broadband irradiation, cutoff filters
were used (50% transmittance at the wavelength specified).
(No. 12002007). We also acknowledge the National
Center of Computations and Simulations in Greece for
a generous allocation of computing time and Professor
H. Panagopoulos at the University of Cyprus for access
to computational facilities.
Su p p or tin g In for m a tion Ava ila ble: Details of experi-
mental methods and data, total energies and geometries of 2-X,
and IR, UV, and ESR spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. The authors are grateful to the
Ministry of Education, Science, Sports and Culture of
J apan for support of this work through a Grant-in-Aid
for Scientific Research for Specially Promoted Research
J O025877Q
J . Org. Chem, Vol. 67, No. 16, 2002 5587