Tri-9-anthrylborane and its derivatives: New boron- containing π-electron systems with divergently extended π- conjugation through boron [25]

Full text of DOI:10.1021/ja994522u

J. Am. Chem. Soc. 2000, 122, 6335-6336
Tri-9-anthrylborane and Its Derivatives: New
6335
Boron-Containing π-Electron Systems with
Divergently Extended π-Conjugation through Boron
Shigehiro Yamaguchi,* Seiji Akiyama, and Kohei Tamao*
Institute for Chemical Research, Kyoto UniVersity
Figure 1. Schematic representations of several extension modes of
π-conjugation: Trivalent hetero-elements are postulated as the diverging
point for the starburst extension mode.
Uji, Kyoto 611-0011, Japan
ReceiVed December 30, 1999
Several extension modes of π-conjugation have been realized
in the carbon π-electron systems, as shown in Figure 1. Those
involve (a) a linear chain-type, (b) a sheet-type, and (c) a sphere-
and tube-type extension, represented by polyacetylene, graphite,
and fullerene and carbon nanotube, respectively.1 In contrast to
these structures, (d) a starburst divergent-type extension of
π-conjugation seems difficult to achieve in the carbon π-electron
systems. Such π-conjugation may be readily realized using Group
13 or 15 elements at the diverging point. Triarylamine derivatives
represent one typical example in which π-conjugation through
the lone-pair electrons on nitrogen in the HOMO level causes
their low ionization potentials and makes them a useful class of
hole-transporting materials.2 As a new family of π-electron
systems with such π-conjugation, we now report the trianthryl-
borane derivatives, where three anthracene π-systems are intro-
duced on a boron atom. In contrast to the amine cases, the
π-conjugation would be divergently extended through the vacant
p-orbital on boron in the LUMO level,3-7 thus realizing the high
electron-accepting properties.8
Scheme 1a
Trianthrylborane 1 itself and three more extended derivatives
2-4 have been studied. Compound 2 is a dianthrylboryl-substitued
trianthrylborane and compounds 3 and 4 are the dimesitylboryl-
substituted derivatives of 1 and 2, respectively. These compounds
have been prepared from bromoanthracenes 5 and 6 as the key
precursors, as shown in Scheme 1. All the compounds thus
prepared have a substantial stability toward air and moisture and
(1) Handbook of Organic Conducting Molecules and Polymers; Nalwa,
H. S., Ed.; John Wiley & Sons: Chichester, 1997.
(2) For examples see: (a) Shirota, Y.; Kobata, T.; Noma, N. Chem. Lett.
1989, 1145. (b) Higuchi, A.; Inada, H.; Kobata, T.; Shirota, Y. AdV. Mater.
1991, 3, 549. (c) Inada, H.; Shirota, Y. J. Mater. Chem. 1993, 3, 319. (d)
Kuwabara, Y.; Ogawa, H.; Inada, H.; Noma, N.; Shirota, Y. AdV. Mater. 1994,
6, 677. (e) Shirota, Y.; Kuwabara, Y.; Inada, H.; Wakimoto, T.; Nakada, H.;
Yonemoto, Y.; Kawami, S.; Imai, K. Appl. Phys. Lett. 1994, 65, 807. (f)
Katsuma, K.; Shirota, Y. AdV. Mater. 1998, 10, 223. (g) Louie, J.; Hartwig,
J. F.; Fry, A. J. J. Am. Chem. Soc. 1997, 119, 11695. (h) Thelakkat, M.;
Schmidt, H.-W. AdV. Mater. 1998, 10, 219.
(3) π-Conjugation through the vacant p-orbital of boron; for examples
see: (a) Zweifel, G.; Clark, G. M.; Leung, T.; Whitney, C. C. J. Organomet.
Chem. 1976, 117, 303. (b) Eisch, J. J.; Galle, J. E.; Kozima, S. J. Am. Chem.
Soc. 1986, 108, 379. (c) Budzelaar, P. H. M.; van der Kerk, S. M.; Krogh-
Jespersen, K.; Schleyer, P. v. R. J. Am. Chem. Soc. 1986, 108, 3960. (d) Eisch,
J. J.; Shafii, B.; Odom, J. D.; Rheingold, A. L. J. Am. Chem. Soc. 1990, 112,
1847. (e) Byun, Y.-G.; Saebo. S.; Pittman, C. U., Jr. J. Am. Chem. Soc. 1991,
113, 3689. (f) Sugihara, Y.; Yagi, T.; Murata, I.; Imamura, A. J. Am. Chem.
Soc. 1992, 114, 1479. (g) Salzner, U.; Lagowski, J. B.; Pickup, P. G.; Poirier,
R. A. Synth. Met. 1998, 96, 177. Recent reports on the boron-based π-electron
systems directed toward materials, see refs 4-7.
(4) (a) Yuan, Z.; Taylor, N. J.; Marder, T. B.; Williams, I. D.; Kurtz, S.
K.; Cheng, L.-T. J. Chem. Soc., Chem. Commun. 1990, 1489. (b) Yuan, Z.;
Taylor, N. J.; Sun, Y.; Marder, T. B.; Williams, I. D.; Cheng, L.-T. J.
Organomet. Chem. 1993, 449, 27. (c) Yuan, Z.; Taylor, N. J.; Ramachandran,
R.; Marder, T. B. Appl. Organomet. Chem. 1996, 10, 305.
(5) (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.
(6) Corriu, R. J.-P.; Deforth, T.; Douglas, W. E.; Guerrero, G.; Siebert,
W. S. Chem. Commun. 1998, 963.
a Reagents and conditions: (i) n-BuLi (1.0-1.2 mol. amt.), Et2O; (ii)
BF3‚OEt2 (0.3 mol. amt.); (iii) BF3‚OEt2 (1.5 mol. amt.); (iv) 9,10-
dilithioanthracene (0.25 mol. amt.), generated from 5b with n-BuLi (2.2
mol. amt.) in Et2O; (v) Mes2BF (1.1 mol. amt.). Mol. amt. ) molar
amount.
can be handled without special care due to the protection of the
central boron atom by the bulky anthryl groups.
Among these compounds, the structure of 1 (see the Supporting
Information) has been determined by X-ray crystallography, which
reveals a completely planar geometry at the boron atom and a
propeller-like arrangement of the three anthryl groups with the
dihedral angles of about 53° between the central borane plane
and anthracene planes. Despite these relatively large dihedral
angles, π-conjugation through the central boron seems to still work
well in this skeleton, and the color of compound 1 is bright orange
and that of compounds 2-4 is bright red.
In solution, compounds 1-4 actually show unique UV-vis
absorption spectra.9 Their spectra are shown in Figure 2 and
the data are summarized in Table 1 together with those for
related compounds, di(9-anthryl)mesitylborane (7) and (9-anthryl)-
(7) (a) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714. (b) Noda,
T.; Ogawa, H.; Shirota, Y. AdV. Mater. 1999, 11, 283.
(8) (a) Leffler, J. E.; Watts, G. B.; Tanigaki, T.; Dolan, E.; Miller, D. S. J.
Am. Chem. Soc. 1970, 92, 6825. (b) DuPont, T. J.; Mills, J. L. J. Am. Chem.
Soc. 1975, 97, 6375. (c) Kaim, W.; Schulz, A. Angew. Chem., Int. Ed. Engl.
1984, 23, 615. (d) Schulz, A.; Kaim, W. Chem. Ber. 1989, 122, 1863. (e)
Okada, K.; Sugawa, T.; Oda, M. J. Chem. Soc., Chem. Commun. 1992, 74.
(9) UV absorption spectra of triarylboranes: (a) Ramsey, B. G.; Leffler, J.
E. J. Phys. Chem. 1963, 67, 2242. (b) Ramsey, B. G. J. Phys. Chem. 1966,
70, 611. (c) Miller, D. S.; Leffler, J. E. J. Phys. Chem. 1970, 74, 2571.
10.1021/ja994522u CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/16/2000

6336 J. Am. Chem. Soc., Vol. 122, No. 26, 2000
Communications to the Editor
Figure 3. Representation of (a) HOMO and (b) LUMO of trianthryl-
borane 1 based on the HF/6-31G(d) calculation.
Figure 2. UV-vis absorption spectra of trianthrylborane derivatives in
indicates that the new absorption band observed for 1 consists of
two excitations with comparable oscillator strengths: Excitation
1, λcalc 382 nm, f ) 0.392 (HOMO f LUMO 63%); Excitation
2, λcalc 377 nm, f ) 0.360 (HOMO-1 f LUMO 55%).
THF.
Table 1. UV-Vis Absorption and Electrochemical Data for
Anthrylborane Derivatives
To evaluate the electron-accepting properties, cyclic voltam-
metry measurements have been carried out for compounds 1-4,
7, and 8, the data being included in Table 1. While a series of
mono-boron compounds 1, 7, and 8 show an irreversible reduction
like other triarylboranes,8c compounds 2, 3, and 4, containing two,
four, and six boron atoms, respectively, show two reversible
reductions. In the mono-boron compound series, the reduction
peak potential (Epc) steadily becomes more positive (smaller
number in negative) from 8 to 7 to 1, as the number of anthryl
groups increases. From a comparison of the trianthrylborane
derivatives 1-4, the extension by the additional dianthrylboryl
or dimesitylboryl groups also causes to shift the reduction
potentials in the positive direction. Thus, compound 2 has an about
0.2 V more positive Epc value relative to that of 1. Moreover,
about 0.1 V positive shifts of the first Epc are observed from 1
and 2 to their dimesithylboryl-substituted derivatives 3 and 4,
respectively. These results are consistent with the UV-vis
absorption data, and strongly suggest that the π-conjugation in
the LUMO is extended over the molecule in the trianthrylborane
derivatives. It should be also noted that their reduction peak
potentials are comparable to or lower than those for the π-electron
deficient polymeric materials,14 such as poly(2,2′-bithiazole-5,5′-
diyl)14a and poly(quinoxaline-2,6-diyl).14c
UV-visa
CVb
compd
λ
max/nm (log ꢀ)
Epc (E1/2)/V vs Fc/Fc+
1
2
3
4
7
8
470 (4.35)
485 (4.45)c
524 (4.54)
535 (4.59)
448 (4.14)
420 (4.03)
-1.86
-1.64 (-1.61), -1.82 (-1.76)d
-1.71 (-1.66), -2.08 (-2.01)
-1.56 (-1.52), -1.86 (-1.78)
-2.00
-2.14
a In THF. b Measured under the following condition; sample con-
centrated 1 mM in 0.1 M Bu4NClO4 CH3CN/CH2Cl2 1/1 solution, scan
rate 100 mV/s, unless otherwise stated. c A shoulder peak is observed
at 512 nm (log ꢀ ) 4.37). d CH2Cl2 was used as a solvent due to the
low solubility.
dimesitylborane (8),10 for comparison. Trianthrylborane 1 has
absorption bands around 350-390 nm attributed to the π-π*
transitions of the anthracene moieties and, in addition, an intense
band at 470 nm. A comparison of 1 with 7 and 8 shows
bathochromic shifts of the longest absorption maximum wave-
lengths as the number of anthryl groups increases, suggesting that
π-conjugation is divergently extended to the three anthryl groups
in 1.11 Furthermore, compounds 2-4 have longer absorption
maxima than that of 1. Notably, about 50 nm bathchromic shifts
are observed for 3 and 4 in comparison with 1 and 2, respectively,
indicative of the effective extension of the π-conjugation to the
external boron moieties in 3 and 4.
To obtain deeper insights into the electronic structures of the
trianthrylborane π-conjugated systems, molecular orbital calcula-
tions have been performed for compound 1.12 Based on the HF/
6-31G(d) level calculation, while the HOMO and HOMO-1 are
degenerate to each other and are respectively primarily localized
on one anthryl group with significant density on a second
anthracene, the LUMO is delocalized over three anthracene
moieties via the boron vacant p-orbital, as shown in Figure 3. As
a consequence, the LUMO energy level of 1 is about 0.75 eV
lower than that of anthracene itself.12 The ZINDO calculation13
The key point of the present molecular design is the introduc-
tion of three identical π-electron systems onto a planar boron
atom, which makes it possible to extend the LUMO over the three
π-systems across the vacant p-orbital of boron.15 The results
demonstrate a new direction for the construction of electron-
accepting organic molecules susceptible to n-doping and electron-
transporting, which is one of the most important topics for the
development of organic electronic devices such as organic light
emitting diodes (OLEDs).
Acknowledgment. This work was partly supported by a Grant-in-
Aid (No. 11740349) from the Ministry of Education, Science, Sports and
Culture of Japan.
Supporting Information Available: Experimental procedures and
data for 1-4, 6, and 7, and crystal structural data for 1 (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
(10) Blount, J. F.; Finocchiaro, P.; Gust, D.; Mislow, K. J. Am. Chem.
Soc. 1973, 95, 7019.
(11) As the related compounds, trianthrylsilanes have their absorption
maxima at around 400 nm: Yamaguchi, S.; Akiyama, S.; Tamao, K.
Organometallics 1998, 17, 4347.
JA994522U
(13) Bacon, A. D.; Zerner, M. C. Theor. Chim. Acta 1979, 53, 21.
(14) For examples see: (a) Curtis, M. D.; Cheng, H.; Nanos, J. I.
Macromolecules 1998, 31, 205. (b) Yamamoto, T.; Maruyama, T.; Zhou, Z.-
h.; Ito, T.; Fukuda, T.; Yoneda, Y.; Begum, F.; Ikeda, T.; Sasaki, S.; Takezoe,
H.; Fukuda, A.; Kubota, K. J. Am. Chem. Soc. 1994, 116, 4832. (c) Yamamoto,
T.; Sugiyama, K.; Kushida, T.; Inoue, T.; Kanbara, T. J. Am. Chem. Soc.
1996, 118, 3930. (d) Jenekhe, S. A.; Zhang, X.; Chen, X. L.; Choong, V. E.;
Gao, Y.; Hsieh, B. R. Chem. Mater. 1997, 9, 409. (e) Jandke, M.; Strohriegl,
P.; Berleb, S.; Werner, E.; Bru¨tting, W. Macromolecules 1998, 31, 6434. (f)
Wang, Y. Z.; Gebler, D. D.; Fu, D. K.; Swager, T. M.; MacDiarmid, A. G.;
Epstein, A. J. Synth. Met. 1997, 85, 1179.
(12) The calculations were performed using the Gaussian 98 program.
Single point calculation of 1 was carried out using the geometry derived from
its crystal structure, whereas the geometry of anthracene was fully optimized
at the HF/6-31G(d) level. The LUMO energy levels of 1 and anthracene are
1.175 and 1.924 eV, respectively. Notably, the HOMO energy level of 1
(-6.593 eV) is also higher than that of anthracene (-7.008 eV) by 0.42 eV,
mainly due to the substituent effect of the electron-positive boron atom. It
should be also noted that the LUMO of 1 is about 0.75 eV lower than that of
Ph3B (1.921 eV, HF/6-31G(d)), indicating the significant contribution of the
anthryl moieties to the LUMO. However, one of the reviewers suggested the
LUMO of 1 is mostly localized on the vacant p-orbital of the boron atom.
Further detailed analysis would be necessary for a full understanding of the
electronic structure.
(15) In mixed triarylboranes, the LUMO may not be delocalized over the
molecule but localized on the aryl-boryl moieties having more π-extended
aryl group(s).