J. Am. Chem. Soc. 2000, 122, 3969-3970
3969
Scheme 1
Boratastilbene: Synthesis, Structural
Characterization, and Photophysics
Bun Yeoul Lee, Shujun Wang, Markus Putzer,
Glenn P. Bartholomew, Xianhui Bu, and Guillermo C. Bazan*
Department of Chemistry, UniVersity of California
Santa Barbara, California 93106
ReceiVed NoVember 8, 1999
ReVised Manuscript ReceiVed February 28, 2000
Interest in the chemistry of boracycloalkenes, boraarenes, and
borataarenes stems from the concepts of π-electron aromaticity
and conjugation across sp2-hybridized boron.1 Boratabenzene, the
most representative example of this class of compounds, was
initially observed as a ligand coordinated to transition metals,
and several catalysts have been designed that are based on this
fragment.2 Less attention has been focused on how the B--for-C
substitution influences the photophysics and photochemistry of
organic chromophores. This isoelectronic replacement maintains
the total number of electrons constant and the overall structural
features intact within a given molecular fragment. The study of
boron-containing chromophores should therefore give insight into
the effect of an additional negative charge and an asymmetric
charge distribution on the properties of photoexcited organic
compounds. Additionally, boron-containing conjugated polymers
are of current interest because of their potentially useful opto-
electronic properties.3 Understanding the behavior of the indi-
vidual monomeric units provides a useful knowledge base for
designing more complex conjugated structures.
Within the context of this idea, we chose boratastilbene as an
attractive synthetic and study candidate since the photophysics
of stilbene are well understood.4 Singlet and triplet energy surfaces
have been determined which account for photoisomerization
efficiency and fluorescence quantum yield. The importance of
the photochemistry of stilbenoid compounds in materials science
is also well documented.5 This communication reports the
synthesis, characterization, and optical properties of sodium and
lithium boratastilbene.
similar volatility and solubility properties, and we have been
unable to isolate pure 1 by this method.
Deprotonation of 1 with NaH or LDA in THF affords the
sodium and lithium salts of boratastilbene (Na-2 and Li-2). Single
crystals of Na-2‚(Et2O) suitable for X-ray diffraction studies were
obtained by allowing an Et2O/benzene (10:1) solution to stand at
-35 °C for 15 h. The metrical parameters within the boratastilbene
anion are in agreement with those of previously characterized
boratabenzene lithium salts (average distances in Å, B-CR
)
1.514(2), CR-Câ ) 1.393(2), Câ-Cγ ) 1.396(2));9 however, the
intermolecular organization exhibits interesting features. The
molecules arrange along the c axis as a polymeric contact ion
pair chain. Within the chains each sodium ion is coordinated to
two boratabenzene fragments and to an ether molecule (Figure
1). The boratastilbene anions are tilted away from each other at
an angle of ∼49°. Li-2 is obtained as microcrystals that are not
suitable for X-ray diffraction.
As shown in Figure 2a, the absorption spectrum of Li-2 in THF
displays a λmax at 348 nm (ꢀ348 ) 1.7 × 105 L mol-1 cm-1). Also
noticeable is a “shoulder” in the 400-440 nm region, which
becomes less pronounced with increasing concentration and is
absent when the spectra are measured in Et2O or toluene. The
emission in THF (Figure 2b) lacks vibronic structure and has a
maximum at ∼500 nm. The excitation spectrum (Figure 2c) is
considerably different from the absorption spectrum (λmax ) 403
nm). Negligible emission is observed in Et2O or toluene. These
data suggest that in THF there is more than one species in solution
and that most of the emission arises from the species absorbing
in the “shoulder” region of the absorption spectrum. The
dependence on solvent and concentration suggests that Li-2 in
THF is in equilibrium between an aggregated species, most likely
a sandwich structure in which two boratastilbene units are
coordinated to a lithium cation ([Li(THF)x(22Li)], in eq 1), and a
solvent-separated ion pair (Li(THF)x//2).10 Furthermore, we
propose that Li(THF)x//2 is responsible for the absorption band
centered at 403 nm and accounts for most of the emission.
Transmetalation of the styryl fragment from Cp2ZrCl(CHd
CHPh)6 (Cp ) C5H5) to 1-chloro-1-boracyclohexa-2,5-diene7
gives trans-1-styryl-1-boracyclohexa-2,5-diene (1, in Scheme 1).
Extraction of 1 from Cp2ZrCl2 using pentane, followed by
sublimation, affords 1 in 35% overall yield.8 Alternatively, 1 can
be prepared by reaction of 1,1-dibutyl-1-stannacyclohexa-2,5-
diene with trans-styrylboron dichloride in essentially quantitative
yield by 1H NMR spectroscopy. However, 1 and Cl2SnBu2 have
(1) Elschenbroich, C.; Salzer, A. Organometallics; VCH: New York, 1989.
(2) (a) Herberich, G. E.; Greiss, G.; Heil, H. F. Angew. Chem., Int. Ed.
Engl. 1970, 9, 805. Review: (b) Herberich, G. E. In ComprehensiVe
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Pergamon Press: Oxford, 1995; Vol. 1, p 197. (c) Ashe A. J.; Al-Ahmad
S.; Fang X. G. J. Organomet. Chem. 1999, 581, 92. For recent applications
of boratabenzene complexes in catalysis, see: (d) Bazan, G. C.; Rodriguez,
G.; Ashe, A. J., III; Al-Ahmad, S.; Mu¨ller, C. J. Am. Chem. Soc. 1996, 118,
2291. (e) Bazan, G. C.; Rodriguez, G.; Ashe, A. J., III; Al-Ahmad, S.; Kampf,
J. W. Organometallics 1997, 16, 2492. (f) Rogers, J. S.; Bazan, G. C.; Sperry,
C. K. J. Am. Chem. Soc. 1997 119, 9305. (g) Rogers, J. S.; Lachicotte, R. J.;
Bazan, G. C. J. Am. Chem. Soc. 1999, 121, 1288.
(3) (a) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998, 120,
5112. (b) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998, 120,
10776. (c) Corriu, R. J.-P.; Daforth, T.; Douglas, W. E.; Guerrero, G.; Siebert,
W. S. Chem. Commun. 1998, 963.
To test our hypothesis, we added an excess of 12-crown-4 to
THF solutions of Li-2. Under these conditions the majority of
the Li cations will be coordinated by the 12-crown-4, thereby
(4) Saltiel, J., Waller, A. S.; Sears, D. F., Jr.; Gareett, C. J. J. Phys. Chem.
1993, 97, 2516.
(8) Approximately 5% of the conjugated isomer, trans-1-styryl-1-boracy-
clohexa-2,4-diene, is observed at this stage.
(9) For comparison against [Li(Me2NCH2CH2NMe2)][C5H5B-NMe2], see:
Herberich, G. E.; Schmidt, B.; Unglert, U.; Wagner, T. Organometallics 1993,
12, 2891. Na-2 is the first sodium salt of a substituted boratabenzene to be
structurally characterized.
(10) (a) Hoic, D. A.; Davis, W. M.; Fu, G. C. J. Am. Chem. Soc. 1995,
117, 8480. (b) Ashe, A. J., III; Kampf, J. W.; Mu¨ller, C.; Schneider, M.
Organometallics 1996, 15, 386.
(5) (a) Meier, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 1399. (b) Whitten,
D. G.; Chen, L. H.; Geiger, H. C.; Perlstein, J.; Song, X. D. J. Phys. Chem.
B 1998, 102, 10098. (c) Bazan, G. C.; Oldham, W. J., Jr.; Lachicotte, R. J.;
Treticak, S.; Chernyak, V.; Mukamel, S. J. Am. Chem. Soc. 1998, 120, 9188.
(6) Harada, S.; Taguchi, T.; Tabuch, N.; Narita, K.; Hanzawa, Y. Angew.
Chem., Int. Ed. Engl. 1998, 37, 1698.
(7) Maier, G.; Hankelmann, J.; Reisenauer, H. P. Angew. Chem., Int. Ed.
Engl. 1985, 24, 1065.
10.1021/ja993930v CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/07/2000