p-Phenylenebismethylene: Characterization, calculation, and conversion to a conjugated bis-carbonyloxide [10]

Full text of DOI:10.1021/ja9824324

11528
J. Am. Chem. Soc. 1998, 120, 11528-11529
biradical 5 (Figure 1).9,10 Irrespective of irradiation conditions,
all of the IR bands of 4 disappeared in concert, with simultaneous
growth of those of 5; no monodiazo compound could be observed.
The matrix also developed a richly structured UV/vis spectrum,
with strong bands at 365, 389, 418, and 460 nm, along with a
long regularly structured tail extending to 700 nm.
p-Phenylenebismethylene: Characterization,
Calculation, and Conversion to a Conjugated
Bis-Carbonyloxide
Waseem Subhan, Pawel Rempala, and Robert S. Sheridan*
In contrast to phenylcarbene itself, which is notoriously
photolabile,11 biscarbene 5 was surprisingly stable to photolysis.
Broadband irradiation (>200 nm) for hours had negligible effect
on the spectra of 5. The biscarbene could be trapped in situ,
however. Warming an HCl-containing nitrogen matrix (2% HCl
Department of Chemistry, UniVersity of NeVada
Reno, NeVada 89557
ReceiVed July 10, 1998
2
in N ) of 5 to 36 K caused the IR and UV/vis bands of 5 to
disappear and those of 6 to appear concurrently. Again, no
intermediates in this process were observed. Attempts to generate
The search for new molecular magnetic materials has spawned
numerous studies of high-spin systems built around triplet carbene
subunits linked across m-phenylenes.1 In these ferromagnetically
coupled systems, covalent interaction between the triplet centers
is topologically prohibited, but communication via shared π-space
maximizes multiplicity. Considerably less is known about their
5 in hydrocarbon matrices (CH
only product tentatively identified by IR as p-quinodimethane.
Warming oxygen-doped nitrogen matrices (0.5% O in N ) of
to 30 K instantly converted the biscarbene to bis(carbonyl
O-oxide) 7, which exhibited multiple very strong bands in the
4
or 3-methylpentane) failed, giving
2
2
1a
conjugated cousins, p-phenylenebiscarbenes, where spin-pairing
between the triplet carbene units leads to lower spin overall. For
example, 1 has an EPR silent singlet ground state, but a thermally
accessible triplet state ca. 1 kcal/mol higher in energy can be
detected.2 It has been suggested that this biscarbene system is
represented more accurately as 1b. Substituents can have a
profound influence on the electronic configurations in these
systems, however. For example, we have shown that the bischloro
5
-1
10
9
50-850 cm region. The matrix acquired a vivid orange hue,
with the appearance of a broad absorption from 400 to 500 nm
max 450 nm). Subsequent irradiation of the matrix with visible
-4
(λ
light (578 nm) rapidly destroyed the IR and visible spectra of 7,
10
producing weaker IR bands that we ascribe to bisdioxirane 8.
Finally, broad-band irradiation (>400 nm) converted 8 mainly
5
system 2 is best described as a quinonoidal biradical, but
12
to terephthalic acid (9). These results are summarized in Scheme
bisfluoro-3 is a closed shell bis-singlet carbene.6 We now report
the first spectroscopic characterization of the parent para system,
p-phenylenebismethylene (5), a preliminary examination of its
chemistry, and its conversion to a conjugated bis-carbonyl oxide.
,7
1.
Our spectral assignments were confirmed by DFT/ab initio
calculations.13 We have noted previously the electronic similar-
ity between bis-carbenes such as 5 and singlet p-benzyne
biradicals, where applicability of single determinantal density
5a
1
4
functional theory is currently under considerable discussion.
Bismethylene 5 likely also has a singlet-diradical ground state,
1
5
as does 1. We found, however, that UB3LYP/6-31G**
calculations on the corresponding triplet state of biradical 5 (syn
16
and anti conformations, frequencies scaled by 0.96 ) nicely fit
the experimental IR spectra (Figure 1). Such an approach is not
unreasonable given the expected small energy difference between
the singlet and triplet biradical states (i.e. interaction between the
Although bis-diazo compound 4 was first reported in 1964,8
little of its chemistry has been described. However, we found
(
9) For a description of the matrix isolation instrumentation and experi-
mental techniques, see: (a) Kesselmayer, M. A.; Sheridan, R. S. J. Am. Chem.
Soc. 1986, 108, 99. (b) Hayes, J. C.; Sheridan, R. S. J. Am. Chem. Soc. 1990,
2
that irradiation (334 nm) of 4 in a 10 K N matrix produced a
1
12, 5879. IR spectra were recorded on a Perkin-Elmer 2000 FTIR
new species in the IR and UV/vis spectra that we attribute to
spectrometer.
(
10) For tabulations of experimental and calculated IR spectra of species
(
1) (a) Zuev, P. S.; Sheridan, R. S. Tetrahedron 1995, 51, 11337 and
reported in this work, see the Supporting Information.
references therein. (b) Iwamura, H. AdV. Phys. Org. Chem. 1990, 26, 179
(11) (a) West, P. R.; Chapman, O. L.; LeRoux, J.-P. J. Am. Chem. Soc.
1982, 104, 1779. (b) McMahon, R. J.; Abelt, C. J.; Chapman, O. L.; Johnson,
J. W.; Kreil, C. L.; LeRoux, J.-P.; Mooring, A. M.; West, P. R. J. Am. Chem.
Soc. 1987, 109, 2456.
and references therein.
(
2) (a) Trozzolo, A. M.; Murray, R. W.; Smolinsky, G.; Yager, W. A.;
Wasserman, E. J. Am. Chem. Soc. 1963, 85, 2526. (b) Itoh, K. Pure Appl.
Chem. 1978, 50, 1251. (c) Sixl, H.; Mathes, R.; Schaupp, A.; Ulrich, K. Chem.
Phys. 1986, 107, 105. (d) Teki, Y.; Sato, K.; Okamoto, M.; Yamashita, A.;
Yamaguchi, Y.; Takkui, T.; Kinoshita, T.; Itoh, K. Bull. Magn. Reson. 1992,
(12) A minor amount of an additional carbonyl-containing product was
also observed, whose identity is still under investigation.
(13) Gaussian 94, Revision D.4, M. J. Frisch, G. W. Trucks, H. B. Schlegel,
P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A.
Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G.
Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A.
Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W.
Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J.
S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C.
Gonzalez, and J. A. Pople, Gaussian, Inc.: Pittsburgh, PA, 1995. Geometric
parameters are given in the Supporting Information.
1
4, 24. (e) Yamaguchi, Y.; Sato, K.; Teki, Y.; Kinoshita, T.; Takui, T.; Itoh,
K. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1995, 271, 67.
(3) Nicolaides, Tomioka, and Murata have recently characterized p-
phenylenebis(nitrene) by IR. Nicolaides, A.; Tomioka, H.; Murata, S. J. Am.
Chem. Soc. 1998, 120, 11530.
(4) The mixed p-phenylenecarbenonitrene has also been observed, although
experimental details were vague. Koseki, S.; Tomioka, H.; Toyota, A. J. Phys.
Chem. 1994, 98, 13203.
(5) (a) Zuev, P. S.; Sheridan, R. S. J. Am. Chem. Soc. 1993, 115, 3788. (b)
(14) (a) Cramer, C.; Nash, J. J.; Squires, R. R. Chem. Phys. Lett. 1997,
277, 311. (b) Schreiner, P. R. J. Am. Chem. Soc. 1998, 120, 4184. (c) For an
in-depth discussion of similarly successful use of broken-spin symmetry
unrestricted DFT methods with singlet diradicals see: Cramer, C. J. Am. Chem.
Soc. 1998, 120, 6261.
Tomioka, H.; Komatsu, K.; Nakayama, T.; Shimizu, M. Chem. Lett. 1993,
291. These workers also reported the reaction of 2 with O , although
1
2
intermediates were not characterized.
(
6) Zuev, P. S.; Sheridan, R. S. J. Am. Chem. Soc. 1994, 116, 9381.
7) Trindle and co-workers have recently described calculations on 2 and
(
(15) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b)
5
. Trindle, C.; Datta, S. N.; Mallik, B. J. Am. Chem. Soc. 1997, 119, 12947.
Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(8) Murray, R. W.; Trozzolo, A. M. J. Org. Chem. 1964, 29, 1268.
(16) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
1
0.1021/ja9824324 CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/22/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 44, 1998 11529
the triplet, but 1.2 kcal/mol lower in energy.13 Davidson has
suggested recently that CAS calculations predict the ground state
of 5 to be a quintet bis-carbene.19 Our UB3LYP calculations,
however, place the quintet 24 kcal/mol higher in energy than
triplet 5. The predicted IR spectrum of quintet biscarbene also
bears little resemblance to experiment.10
20,21
B3LYP/6-31G** calculations on bis-carbonyloxide 7 gave a
less satisfactory fit to the experimental spectra. Following the
lead of Warner,22 however, we found that addition of diffuse
functions (B3LYP/6-31+G**) significantly improved the agree-
ment between experiment and theory.10 The simpler 6-31G**
basis set sufficed to confirm the assignment of bis-dioxirane 8
based on IR spectra.10
The lack of photochemical reactivity of 5 is striking. Phenyl-
carbene readily ring expands on irradiation under similar condi-
tions.11 Nicolaides, Tomioka, and Murata also have found
recently that the corresponding p-bisnitrene undergoes photo-
3
chemical rearrangement. The UV/vis spectra of 5 and phenyl-
carbene11 are superficially similar, but that of 5 is substantially
broader and more intense. The very different electronic nature
of the excited states of the mono- and bis-carbenes may account
for the photolability contrast. The difference between the
3
biscarbene and bisnitrene is even less clear. Although it has
been suggested that carbonyl O-oxides possess considerable
biradical character,23 the bis-O
adduct 7 shows little indication
2
in its spectra for interaction between the methylene centers. The
IR spectra for 7 show numerous, strong O-O stretching bands,
18
identified by O labeling and DFT calculations, in the region
-
1 10
9
50-850 cm , analogous to those reported for benzaldehyde
-
1 24
O-oxide (915 and 890 cm ). The complexity of the IR in this
band group likely reflects the multiple conformational isomers
of 7, supporting its assignment. The visible spectrum observed
for 7 is unremarkable, and is in a wavelength region similar to
Figure 1. (a) Difference of IR spectra of bis-diazo compound 4 matrix
isolated in N at 10 K, before and after irradiation at 313 nm for 12 h.
2
Positive peaks correspond to biradical 5 and negative peaks to 4. (b)
UB3LYP/6-31G** calculated IR spectra for triplet anti-5 (solid) and syn-5
2
4
1
6
that described for the parent mono-carbonyl oxide. Finally,
(open). Freqencies have been scaled by a factor of 0.96.
photochemical rearrangement of 7 to bisdioxirane 8, and thence
2
3
Scheme 1
to 9, is in line with the behavior of other carbonyl oxides.
In summary, our results confirm that p-phenylenebismethylene
5) is a quinoidal biradical. Although photochemically stable,
(
2
the bis-methylene shows carbene-like reactivity with HCl and O ,
2
5
giving in the latter case a novel bis-carbonyl O-oxide.
Acknowledgment. This paper is dedicated to the memory of Robert
Squires. We thank the National Science Foundation and the donors of
the Petroleum Research Fund, administered by the American Chemical
Society, for support. We are also grateful to Wes Borden and David
Hrovat for insightful assistance with diradical calculations, as well as
sharing the results of their own studies. Finally, we thank Hideo Tomioka
and Athanassios Nicolaides for communication of unpublished results
and helpful discussions.
Supporting Information Available: Tabulated experimental/calcu-
lated IR spectra and geometric parameters for 5, 7, and 8, and UV/vis
spectrum and calculated energies for 5 (14 pages, print/PDF). See any
current masthead page for ordering information and Web access
instructions.
“
σ-radical” centers in 5 is small).17 Indeed, CAS[10,10]/6-31G**
calculations on singlet 5 predicted a quinonoidal geometry very
similar to that from the triplet DFT calculations (see Supporting
JA9824324
1
8
Information).
Moreover, broken spin-symmetry UB3LYP
(
19) Davidson, E. R. J. Am. Chem. Soc. 1997, 119, 1449.
calculations1 on the singlet biradical state of 5 (with an 〈S 〉
value of 1.17 indicating an approximate 50:50 singlet/triplet mixed
state) gave nearly the same geometry and IR predictions as for
4c
2
(20) The calculational results reported by Trindle and co-workers in ref 7
also do not support a quintet ground state for 5.
(
21) For successful applications of DFT to aryl carbene energetics, see:
(
9
a) Cramer, C. J.; Dulles., F. J.; Falvey, D. E. J. Am. Chem. Soc. 1994, 116,
787. (b) Matzinger, S.; Bally, T.; Patterson, E. V.; McMahon, R. J. J. Am.
(17) D. A. Hrovat and W. T. Borden (personal communication) have found
Chem. Soc. 1996, 118, 1535. (c) Schreiner, P. R.; Karney, W. L.; Schleyer,
P. v.-R.; Borden, W. T.; Hamilton, T. P.; Schaefer, H. F. J. Org. Chem. 1996,
61, 7030. (d) Wong, M. W.; Wentrup, C. J. Org. Chem. 1996, 61, 7022.
(22) Warner, P. M. J. Org. Chem. 1996, 61, 7192.
(23) Sander, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 344.
(24) Sander, W. J. Org. Chem. 1989, 54, 333.
that CASPT2 calculations place the triplet states 2.0 kcal/mol above the singlets
of syn and anti 5. They also see evidence for dynamic spin polarization in the
π system incited by the σ nonbonding orbitals, accounting for the singlet
stabilization.
(18) The IR spectrum predicted for 5 by CAS[10,10]/6-31G** calculations
fits the experiment less well than did DFT (on C2h isomer, see Supporting
Information). This discrepancy may be due to lack of σ-electron correlation
in the CAS calculations or subtle complications with numeric frequency
calculations.
(25) Attempts to observe a low-temperature EPR spectrum of the excited
triplet state of 5 have thus far been unsuccessful (Paul Lahti, personal
communication). The extreme reactivity of the diradical may explain why its
EPR has not been reported previously.