Synthesis and properties of carborane-appended C3-symmetrical extended π systems

Article abstract of DOI:10.1021/ja101845m

A series of C₃-symmetric π-conjugated compounds containing three to six o-carborane clusters have been synthesized by employing palladium-catalyzed Suzuki coupling reactions and palladium-catalyzed acetylation reactions, followed by silicon tet

Full text of DOI:10.1021/ja101845m

Published on Web 04/16/2010
Synthesis and Properties of Carborane-Appended
C3-Symmetrical Extended π Systems
Barada Prasanna Dash,† Rashmirekha Satapathy,† Elizabeth R. Gaillard,†
John A. Maguire,‡ and Narayan S. Hosmane*,†
Department of Chemistry and Biochemistry, Northern Illinois UniVersity, DeKalb, Illinois 60115-2862, and
Department of Chemistry, Southern Methodist UniVersity, Dallas, Texas 75275-0314
Received March 4, 2010; E-mail: hosmane@niu.edu
Abstract: A series of C3-symmetric π-conjugated compounds containing three to six o-carborane clusters
have been synthesized by employing palladium-catalyzed Suzuki coupling reactions and palladium-catalyzed
acetylation reactions, followed by silicon tetrachloride mediated trimerization reactions. Carborane-containing
extended trimers were found to emit blue light. Incorporation of o-carborane clusters into extended
π-conjugated systems led to 22-70% enhancement of their relative fluorescence quantum yields.
Decapitation of o-carborane clusters made these extended trimers water soluble, and their aqueous solutions
were also found to be fluorescent, but with a reduced fluorescence intensity. The carborane-appended
π-conjugated compounds are found to be extremely thermally stable, and for some of these compounds
only 10% mass loss occurred at temperatures close to 500 °C. The DSC thermograms of smaller Ccage
appended trimers indicate the occurrence of solid-solid phase transitions.
-
Introduction
and the aggregation leading to highly ordered nanostructured
materials.6 In a parallel development, it has been found that
icosahedral carborane clusters are versatile synthetic building
blocks and have been used in materials science, medicine, and
catalysis.7 Specifically, icosahedral carborane clusters are
thermally very stable and easy to functionalize.8 Current
investigations in our laboratory to incorporate readily available
o-carboranes into π-conjugated symmetrical structures have
resulted in hybrid compounds of dramatically high thermal
stability.9 Metalated carborane-containing macrocycles show
properties such as self-assembly which could make them useful
in the promising field of nanoscience.10 It has also been found
that o-carborane clusters can readily be functionalized at both
boron and carbon atoms of the cluster.11 The electron-rich B9
The synthesis of dendritic and symmetrical star-shaped
π-conjugated compounds and the study of their physical
properties are of intense current scientific interest. These classes
of compounds find a number of important applications,1
including use as organic light-emitting materials,2 liquid-
crystalline materials,3 effective drug delivery agents,4 and highly
efficient catalysts.5 Some of the very exciting applications of
such macromolecular compounds include their self-assembly
† Northern Illinois University.
‡ Southern Methodist University.
(1) For reviews see: (a) Northrop, B. H.; Zheng, Y.-R.; Chi, K.-W.; Stang,
P. J. Acc. Chem. Res. 2009, 42, 1554–1563. (b) Saragi, T. P. I.; Spehr,
T.; Siebert, A.; Fuhrman-Lieker, T.; Salbeck, J. Chem. ReV. 2007, 107,
1011–1065. (c) Lo, S.-C.; Burn, P. L. Chem. ReV. 2007, 107, 1097–
1116. (d) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning,
A. P. H. J. Chem. ReV. 2005, 105, 1491–1546. (e) Grayson, S. M.;
Frechet, J. M. J. Chem. ReV. 2001, 101, 3819–3867.
(5) (a) Astruc, D.; Orneals, C.; Ruiz, J. Acc. Chem. Res. 2008, 41, 841–
856. (b) Chi, Y.; Scroggins, S. T.; Boz, E.; Frechet, J. M. J. J. Am.
Chem. Soc. 2008, 130, 17287–117289. (c) Lemo, J.; Heuze, K.; Astruc,
D. Chem. Commun. 1999, 4351–4353.
(2) (a) Laquai, F.; Mishra, A. K.; Ribas, M. R.; Petrozza, A.; Jacob, J.;
Akcelrud, L.; Mullen, K.; Friend, R. H.; Wegner, G. AdV. Funct. Mater.
2007, 17, 3231–3240. (b) Watson, M. D.; Jackel, F.; Severin, N.; Rabe,
J. P.; Mullen, K. J. Am. Chem. Soc. 2004, 126, 1402–1407. (c)
Vijaylakshmi, N.; Maitra, U. Org. Lett. 2005, 7, 2727–2730. (d) Weil,
T.; Reuther, E.; Mullen, K. Angew. Chem., Int. Ed. 2002, 41, 1900–
1904.
(6) (a) Kraft, M. L.; Moore, J. F. Science 2008, 320, 620–621. (b) Zang,
L.; Che, Y.; Moore, J. F. Acc. Chem. Res. 2008, 41, 1596–1608. (c)
Li, S. S.; Northrop, B. H.; Yuan, Q.-H.; Wan, L.-J.; Stang, P. J. Acc.
Chem. Res. 2009, 42, 249–259. (d) Zheng, Y.-R.; Yang, H. B.; Ghosh,
K.; Zhao, L.; Stang, P. J. Chem. Eur. J. 2009, 15, 7203–7214. (e)
Northrop, B. H.; Yang, H.-B.; Stang, P. J. Chem. Commun. 2008,
5896–5908. (f) Ghosh, K.; Hu, J.; White, H. S.; Stang, P. J. J. Am.
Chem. Soc. 2009, 131, 6695–6697. (g) Lee, J.; Ghosh, K.; Stang, P. J.
J. Am. Chem. Soc. 2009, 131, 12028–12029.
(3) (a) Kumar, S. Chem. Soc. ReV. 2006, 35, 83–109. (b) Pisula, W.;
Kastler, M.; Wasserfallen, D.; Pakula, T.; Mullen, K. J. Am. Chem.
Soc. 2004, 126, 8074–8075. (c) Craats, A. M. V.; Warman, J. M.;
Fechtenkotter, A.; Brand, J. D.; Harbison, M. A.; Mullen, K. AdV.
Mater. 1999, 11, 1469–1472. (d) Muller, G. R. G.; Meiners, C.;
Enkelmann, V.; Geerts, Y.; Mullen, K. J. Mater. Chem. 1998, 1, 61–
64.
(7) For reviews see: (a) Zhu, Y.; Cheng, K. Y.; Maguire, J. A.; Hosmane,
N. S. Curr. Chem. Biol. 2007, 1, 141–149. (b) Grimes, R. N. J. Chem.
Educ. 2004, 81, 658–672. (c) Plesek, J. Chem. ReV. 1992, 92, 269–
278. (d) Hawthorne, M. F.; Maderna, A. Chem. ReV. 1999, 99, 3421–
3434. (e) Teixidor, F.; Vinas, C.; Demonceau, A.; Nunez, R. Pure
Appl. Chem. 2003, 75, 1305–1313.
(4) (a) Fox, M. E.; Szoka, F. C.; Frechet, J. M. J. Acc. Chem. Res. 2009,
42, 1141–1151. (b) Almutairi, A.; Rossin, R.; Shokeen, M.; Hangooly,
A.; Ananth, A.; Capoccia, B.; Guillaudeu, S.; Abendschein, D.; Welch,
M. J.; Frechet, J. M. J. Proc. Natl. Acad. Sci. U.S.A. 2009, 685–690.
(c) Nishimaya, N.; Iriyama, A.; Jang, W.-D.; Miyata, K.; Itaka, K.;
Inoue, Y.; Takahashi, H.; Yanagi, Y.; Tamaki, Y.; Koyama, H.;
Kataoka, K. Nat. Mater. 2005, 934–941.
(8) Valliant, J. F.; Guenther, K. J.; Arienne, S.; King, S.; Morel, P.;
Schaffer, P.; Sogbein, O. O.; Stephenson, K. A. Coord. Chem. ReV.
2002, 232, 173–230.
(9) Dash, B. P.; Satapathy, R.; Maguire, J. A.; Hosmane, N. S. Chem.
Commun. 2009, 3267–3269.
9
6578 J. AM. CHEM. SOC. 2010, 132, 6578–6587
10.1021/ja101845m  2010 American Chemical Society

Carborane-Appended Symmetrical Extended π Systems
A R T I C L E S
Scheme 1. Synthesis of Ccage-Appended Trimers Containing Three
o-Carborane Clusters
Figure 1. Structure of the Bcage-appended trimer.
boron atom of the o-carborane clusters can undergo facile
Friedel-Craft type electrophilic substitution reactions to gener-
ate B9-substituted products. Carboranyl aromatic ketones at-
tached to this boron atom of the cluster were synthesized and
were found to undergo trimerization in the presence of a silicon
tetrachloride/ethanol mediated reaction to generate C3-sym-
metrical trimers having three carborane clusters in the periphery,
as shown in Figure 1.11 Because of the acidic nature of the cage
hydrogens, these o-carborane clusters can also undergo facile
functionalization at the cage carbon to produce Ccage-appended
extended trimers. Herein we describe the syntheses of a number
of Ccage-appended C3-symmetrical trimers, incorporating o-
carborane clusters. We also report on the relative thermal
stabilities of Bcage- and Ccage-appended trimers, as well as the
effect of carboranes on the photophysical properties of extended
trimers. The deboronation of the attached o-carborane clusters
led to the formation of the respective water-soluble com-
pounds,12 whose spectroscopic properties are also reported.
Scheme 2. Synthesis of Ccage-Appended Extended Trimers
Containing Three o-Carborane Clusters
Results and Discussion
The carborane Ccage-appended C3-symmetric trimers contain-
ing three to six o-carborane clusters were synthesized by
employing Suzuki coupling reactions and palladium-catalyzed
acetylation reactions,13 followed by SiCl4/ethanol mediated
trimerizations11,14 of the so formed carborane-appended aryl
ketones, as shown in Schemes 1-3. Suzuki coupling reactions
were generally carried out using the common palladium catalyst
PdCl2(PPh3)2 and K2CO3 as a base. Solubility difficulties of the
aryl ketones in the trimerization reactions were solved by using
as the solvent a mixture of toluene and ethanol in a 1:3
proportion. Smaller trimers could be obtained with room-
temperature reactions, but for the extended trimers an elevated
temperature of between 40-50 °C was required.
Synthesis of Trimers. The syntheses of the carborane Ccage
-
appended conjugated systems, containing three carborane
(10) (a) Jude, H.; Disteldorf, H.; Fischer, S.; Wedge, T.; Hawkridge, A. M.;
Arif, A. M.; Hawthorne, F.; Muddiman, D. C.; Stang, P. J. J. Am.
Chem. Soc. 2005, 127, 12131–12139. (b) Das, N.; Stang, P. J.; Arif,
A. M.; Campana, C. F. J. Org. Chem. 2005, 70, 10440–10446.
(11) Dash, B. P.; Satapathy, R.; Maguire, J. A.; Hosmane, N. S. Org. Lett.
2008, 10, 2247–2250, and references therein.
clusters, are outlined in Schemes 1 and 2. Compound 2 was
prepared from the lithiated 1-methyl-o-carborane and 4-iodo-
benzyl bromide in DME and was used as the starting com-
pound.15 A Suzuki coupling reaction of compound 2 with (4-
acetylphenyl)boronic acid in the presence of catalytic amounts
of water produced the desired carborane-appended aryl ketone
3 in good yield. Ketone 3 then underwent trimerization in the
presence of SiCl4/ethanol to produce the extended trimer 6, in
61% yield. Palladium-catalyzed acetylation13 of 2 worked well
to produce the desired carborane-appended aryl ketone 4, which
(12) (a) Vyakaranam, K.; Rana, G.; Ratanasuwan, A.; Hosmane, S. N.;
Maguire, J. A.; Hosmane, N. S. Organometallics 2002, 21, 3905–
3912. (b) Yoo, J.; Hwang, J.-W.; Do, Y. Inorg. Chem. 2001, 40, 568–
570. (c) Gonzalez-Campo, A.; Vinas, C.; Teixidor, F.; Nunez, R.;
Sillanpaa, R.; Kivekas, R. Macromolecules 2007, 40, 5644–5652.
(13) Cacchi, S.; Fabrizi, G.; Gavazza, F.; Goggiamani, A. Org. Lett. 2003,
5, 289–291.
(14) (a) Bao, C.; Lu, R.; Jin, M.; Xue, P.; Tan, C.; Xu, T.; Liu, G.; Zhao,
Y. Chem. Eur. J. 2006, 12, 3287–3294. (b) Yang, J.-X.; Tao, X.-T.;
Yuan, C. X.; Yan, Y. X.; Wang, L.; Liu, Z.; Ren, Y.; Jiang, M. H.
J. Am. Chem. Soc. 2005, 127, 3278–3279. (c) Cao, X.-Y.; Liu, X.-H.;
Zhou, X.-H.; Zhang, Y.; Jiang, Y.; Cao, Y.; Cui, Y.-X.; Pei, J. J. Org.
Chem. 2004, 69, 6050–6058.
(15) Vicente, H.; Edwards, F.; Shetty, S. J.; Hou, Y.; Boggan, J. E. Bioorg.
Med. Chem. 2002, 10, 481–492.
9
J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010 6579

A R T I C L E S
Dash et al.
Scheme 4. Synthesis of Substituted Ccage-Appended Trimers
Scheme 3. Synthesis of Ccage-Appended Trimers Containing Six
o-Carborane Clusters
was then trimerized with SiCl4/ethanol to produce trimer 5
(Scheme 1). A more extended version of the trimers, compound
10, was prepared from the precursor ketone 9 via another SiCl4/
ethanol mediated trimerization reaction in about 30% yield. The
intermediate ketone 9 was obtained via a Suzuki coupling
between carborane-appended boronic acid 716 and commercially
available 4′-(4-bromophenyl)acetophenone 8 (Scheme 2).
To further increase the number of carborane clusters around
the 1,3,5-phenylene core, we started from the commercially
available 3,5-dimethyl-1-iodobenzene (11), which underwent
radical bromination in the presence of N-bromosuccinimide and
the radical initiator AIBN to give compound 12 (Scheme 3).
Two equivalents of lithiated 1-methyl-o-carborane in the pres-
ence of a catalytic amount of lithium iodide produced the key
intermediate 13 in good yield, at room temperature. A Suzuki
coupling of 13 with (4-acetylphenyl)boronic acid or a palladium-
catalyzed acetylation produced the desired ketones 14 and 16,
respectively. Trimerizations of the ketones led to the respective
products 15 and 17, having six o-carborane clusters, as shown
in Scheme 3.
Scheme 5. Deboronation of o-Carborane Clusters in Trimers
compound, in this case 1-iodoheptane, to produce the Ccage
-
heptyl-substituted carborane 19. After deprotection of the
TBDMS group with TBAF, the resulting compound 20 was then
treated with 4-iodobenzyl bromide in the presence of BuLi in
DME to give 21, which upon palladium-catalyzed acetylation
led to the desired heptyl-substituted carboranyl ketone 22.
Trimerization of this ketone led to the heptyl-substituted
carboranyl trimer 23 in good yield (Scheme 4).
A general synthetic route to functionalized Ccage-appended
trimers is shown in Scheme 4. One of the acidic hydrogens on
the o-carborane was protected by the bulky TBDMS group,
following a literature procedure.17 The TBDMS-protected
o-carborane 18 could be lithiated and then treated with an iodo
Water-Soluble Trimers. The decapitation reaction of o-
carboranes by alcoholic NaOH leads to the removal of one of
the vertex boron atoms to give monoanionic nido-carborane
(16) Hao, E.; Fabre, B.; Fronczeka, F. R.; Vicente, M. G. H. Chem. Mater.
2007, 19, 6195–6205.
(17) Gomez, F. A.; Hawthorne, M. F. J. Org. Chem. 1992, 57, 1384–1390.
9
6580 J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010

Carborane-Appended Symmetrical Extended π Systems
A R T I C L E S
Figure 2. Water-soluble trimers.
Figure 3. C3-symmetric phenylene cores.
Figure 4. (A) Absorption and emission spectra of trimers in dichlo-
romethane: (red) 10; (blue) 15; (orange) 6; (green) control B. (B) Absorption
and emission spectra of water-soluble trimers in water: (red) 26; (orange)
24; (blue) 25. For compound 26 Raman scattering could not be completely
suppressed.
species. The monoanions can be further deprotonated to give
dianions, which can act as ligands for different metals.18 It was
found that all of the carboranes of the trimers 5, 6, 10, 15, and
17 could be decapitated to give the corresponding water-soluble
compounds. Scheme 5 shows a typical decapitation reaction of
6 to give 24. The two other extended trimers, 15 and 10,
underwent similar deboronation reactions that led to the
corresponding water-soluble trimers 25 and 26, respectively
(Figure 2). The spectroscopic properties of these trimers were
later evaluated in water.
Controls. The control trimers A and B (Figure 3) without
any carboranes were also synthesized. Control A was prepared
by the SiCl4/ethanol mediated trimerization of acetophenone,
whereas the extended control B was prepared via a Suzuki
coupling reaction of commercially available 1,3,5-tris(4-io-
dophenyl)benzene with phenylboronic acid (see the Supporting
Information).19
Table 1. Spectroscopic Data of Carborane-Appended Trimers
absorption emission
maximum maximum
quantum
yield
(ΦF)
rel
area
λc E
(nm) (eV)
trimer
solvent
(nm)
(nm)
control B CH2Cl2
290
290
294
307
290
294
300
360
360
370
1.0
1.22
1.70
0.27
0.33
0.46
330 3.75
335 3.70
335 3.70
340 3.64
6
CH2Cl2
CH2Cl2
CH2Cl2
H2O
15
10
24
25
26
a
360, 380 2.38
0.86
390
380
370
0.042
0.030
0.09
0.011 335 3.70
0.008 340 3.64
0.024 340 3.64
H2O
H2O
a ΦF was calculated at OD290 ) 0.09.
Figure 4A and Table 1). The aqueous solution spectra of the
water-soluble trimers 24-26 are shown in Figure 4B. The UV
absorption of all the trimers, irrespective of the solvents used,
showed absorption maxima close to 290 nm, except for the
extended trimers 10 and 26, for which the absorbance maxima
were red-shifted to 307 and 300 nm, respectively, due to the
extended conjugation. Therefore, an excitation wavelength of
290 nm was chosen to measure emission intensities. A single
emission maximum was observed for all trimers between 360
and 390 nm, except the extended trimer 10, for which weak
vibrational splitting was observed at 360 and 380 nm. Trimers
6, 10, and 15 showed sharp emission intensities between 320
Photophysical Properties. Symmetrical 1,3,5-phenyleneben-
zene core based structures, shown in Figure 3, have been used
as photostable chromophores for light-emitting materials.20
Specifically, the extended versions such as 1,3,5-tris(p-oligophe-
nylene)benzenes (B) were found to be blue-violet light emitting
and therefore useful as hole-blocking molecular materials21a and
UV-laser dyes.21b However, the low solubility of such sym-
metrical polycyclic aromatic compounds in polar solvents was
considered as a primary drawback for their general use as laser
dyes.21b,22 The effect of carboranes on the photophysical
properties of polycyclic compounds was studied by comparing
the spectroscopic properties of dichloromethane solutions of the
carborane-appended extended trimers 6, 10, and 15 with B (see
(20) (a) Finke, A. D.; Moore, J. F. Org. Lett. 2008, 10, 4851–4854. (b)
He, Q.; Huang, H.; Yang, J.; Lin, H.; Bai, F. J. Mater. Chem. 2003,
13, 1085–1089. (c) Belser, P.; Zelewsky, A.; Frank, M.; Seel, C.;
Vogtle, F.; Cola, L. D.; Barigelletti, F.; Balzani, V. J. Am. Chem.
Soc. 1993, 115, 4076–4086. (d) Pang, J.; Marcotte, E. J.-P.; Seward,
C.; Brown, R. S.; Wang, S. Angew. Chem., Int. Ed. 2001, 40, 4042–
4045. (e) Miller, T. M.; Neenan, T. X.; Zayas, R.; Bair, H. E. J. Am.
Chem. Soc. 1992, 114, 1018–1025.
(18) (a) Deng, L.; Xie, Z. Organometallics 2007, 26, 1832–1845. (b) Xie,
Z. Coord. Chem. ReV. 2006, 250, 259–272. (c) Hosmane, N. S.;
Maguire, J. A. Organometallics 2005, 24, 1356–1389. (d) Hawthorne,
M. F.; Young, D. C.; Garrett, D. A.; Owen, D. A.; Schwerin, S. G.;
Tebbe, F. N.; Wegner, P. A. J. Am. Chem. Soc. 1968, 90, 862–868.
(e) Hawthorne, M. F.; Young, D. C.; Andrews, T. D.; Howe, D. V.;
Pilling, R. L.; Pitts, A. D.; Reintjes, M.; Warren, L. F.; Wegner, P. A.
J. Am. Chem. Soc. 1968, 90, 879–896.
(21) (a) Okumoto, K.; Shirota, Y. Chem. Mater. 2003, 15, 699–707. (b)
Schmidt, B.; Rinke, M.; Gusten, H. J. Photochem. Photobiol. A: Chem.
1989, 49, 131–135.
(19) Kotha, S.; Kasinath, D.; Lahiri, K.; Sunoj, R.B. Eur. J. Org. Chem.
2004, 4003–4013.
(22) Muller, M.; Kubel, C.; Mullen, K. Chem. Eur. J. 1998, 4, 2099–2109.
9
J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010 6581

A R T I C L E S
Dash et al.
and 470 nm (Figure 4A); less intense emissions were observed
for the water-soluble trimers 24-26, over a wider wavelength
range, between 320 and 550 nm (Figure 4B). However, all the
trimers emit in the blue-violet region of the spectrum, as seen
in Figure 4. In order to measure the relative fluorescence
quantum yields, the samples were excited at 290 nm and were
all prepared with OD290 ) 0.13. This allows for the emission
intensities to be directly compared.23
The aqueous solutions of the trimers 24 and 25, containing
three and six nido-carboranes, respectively, showed much lower
quantum yields, which were only about 3.4% and 1.8% of the
values for the corresponding o-carborane-containing trimers (6
and 15) in dichloromethane. Thus, we found that a small number
of monoanionic nido-carborane clusters induce good water
solubility to 1,3,5-oligophenylene chromophores. When we
increased the number of water solubility inducing nido-carborane
anions from three in trimer 24 to six in trimer 25, we observed
a decrease in ΦF of about 29%. Therefore, increasing the number
of water solubility inducing charged nido-carborane clusters
close to the chromophore diminishes the fluorescence quantum
yield. The ΦF values of water-soluble trimers have been
calculated by using the appropriate method, and only refractive
index terms are considered during the calculation.29
Water-soluble fluorescent materials are used as fluorescent
labels in biologically active probes.30 Thus, a number of other
water-soluble chromophores are also available. Such chro-
mophores carry hydrophilizing substituents such as sulfonic acid
moieties,31 quaternary amine groups,32 polyethylene glycols,33
benzoate groups,34 and peptide chains35 attached to the aromatic
chromophore scaffold. However, in most cases a very low
fluorescence is observed in water.36 A chromophore that meets
all the criteria, such as water solubility, high fluorescence
intensity, nontoxicity, and high photostability, is rather difficult
to find.36 The low fluorescence of chromophores in water may
be attributed to a number of factors, such as the photoinduced
electron transfer in polar solvents.29b,37,38 In addition, significant
hydrogen bonding in water reduces ΦF by enhancing the
nonradiative decay.29,37 However, in our case degassing the
solvents to get rid of the quenching due to dissolved oxygen
did not result in any noticeable change in the fluorescence
intensity.
Since the determination of absolute quantum yield is rather
difficult, it is a common practice to determine the quantum
yields of unknowns by comparing with a standard of known
fluorescence quantum yield.24 The quantum yield of control
B is reported to be 0.27.21b From the relative area under the
fluorescence band and the quantum yield of the control B,
the relative quantum yields (ΦF) of the carborane-appended
trimers have also been calculated and are given in Table 1.
The HOMO-LUMO gap energies were calculated using the
Planck equation from the wavelengths where the absorbance
and emission lines cross each other (λc) and are reported in
electronvolts (eV) in Table 1. The incorporation of multiple
o-carborane clusters in 6 and 15, containing three and six
o-carborane clusters, respectively, led to about a 22% and
70% increase in their quantum yields in comparison to the
carborane-unsubstituted control B. The combination of
extended conjugation and the inclusion of o-carborane, as
found in 10, resulted in a substantial enhancement (238%)
of the quantum yield. Incorporation of o-carborane clusters
also leads to a slight reduction of HOMO-LUMO gap
energies (Table 1).
Icosahedral carboranes are pseudoaromatic, and they also
possess electron-withdrawing properties.7c,e The increase in
quantum yields on the introduction of the o-carboranes into
control B (see Table 1) is in sharp contrast to the recent
observations of Kokado and co-workers, who found that the
introduction of m-carboranes into the backbones of the extended
π systems of p-phenyleneethynyl-like polymers resulted in
polymers having intense blue emission in solution, while the
introduction of o-carboranes in the extended π systems showed
no luminescence but only aggregation-induced emission (AIE).25,26
An intensely blue emitting polymer was produced when
p-carborane was introduced into the polymer backbone of a
polyfluorene polymer.27 However, in our studies the aromatic
conjugated core and o-carborane clusters are separated by a
methylene moiety which effectively blocks π interactions by
the carborane. Therefore, shifts in the absorption and emission
maxima due to the extension of the aromatic system on the
introduction of the o-carborane clusters are not observed. It
seems much more likely that the size and three-dimensional
structure of the o-carborane moieties serve to prevent π-π
stacking interactions and also help to rigidify the system,
minimizing nonradiative decay.28 Therefore, higher emission
intensity was observed in all o-carborane-appended extended
trimers.
Thermal Properties. Incorporation of carboranes enhances
the thermal stability of the materials. Apart from a recent
investigation in our laboratory,9 there have also been a number
of literature reports on the enhancement of thermal stability due
to the presence of carboranes,39 which motivated us to conduct
a systematic thermal analysis of our trimers. TGA curves of
(29) (a) Bigger, S. W.; Ghiggino, K. P.; Meilak, G. A.; Verity, B. J. Chem.
Educ. 1992, 69, 675–677. (b) Barghouthi, S. A.; Perrault, J.; Holmes,
L. H., Jr. Chem. Educ. 1998, 3, 1–10.
(30) (a) Bastiaens, P. I. H.; Squire, A. Trends Cell Biol. 1999, 9, 48–52.
(b) Nguyen, D. C.; Keller, R. A.; Jett, J. H.; Martin, J. C. Anal. Chem.
1987, 59, 2158–2161. (c) Peck, K.; Stryer, L.; Glazer, A. N.; Mathies,
R. A. Proc. Natl. Acad. Sci. U.S.A. 1989, 86, 4087–4091.
(31) (a) Jose, J.; Burgess, K. J. Org. Chem. 2006, 71, 7835–7839. (b) Rivas,
B. L.; Maureira, A. Macromol. Symp. 2008, 270, 143–152.
(32) (a) Schnurpfeil, G.; Stark, J.; Wohrle, D. Dyes Pigm. 1995, 27, 339–
350. (b) Knaus, S.; Gruber, H. F. J. Macromol. Sci. A, Pure Appl.
Chem. 1996, 33, 869–881.
(33) Patrick, M. J.; Ernst, L. A.; Waggoner, A. S.; Thai, D.; Tai, D.; Salama,
G. Org. Biomol. Chem. 2007, 5, 3347–3353. Verma, S.; Padmawar,
P. A.; Hauck, T.; Canteenwala, T.; Chiang, L. Y.; Pritzker, K. P. H.
J. Macromol. Sci. A, Pure Appl. Chem. 2005, 42, 1497–1505.
(34) (a) Biosselier, E.; Orneals, Catia.; Pianet, I.; Aranzaes, R. J.; Astruc,
D. Chem. Eur. J. 2008, 14, 5577–5587.
(23) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings:
Menlo Park, CA, 1978.
(24) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London,
1970.
(35) (a) Klok, H.-A.; Hernandez, J. R.; Becker, S.; Mullen, K. J. Polym.
Sci., Part A: Polym. Chem. 2001, 39, 1572–1583. (b) Stafforst, T.;
Diederichsen, U. Eur. J. Org. Chem. 2007, 899–911. (c) Hempel, K.;
Heinze, K. Chem. Eur. J. 2009, 15, 1346–1358.
(25) (a) Kokado, K.; Tokoro, Y.; Chujo, Y. Macromolecules 2009, 42,
2925–2930. (b) Kokado, K.; Tokoro, Y.; Chujo, Y. Macromolecules
2009, 42, 9238–9242.
(36) (a) Kohl, C.; Weil, T.; Qu, J.; Mullen, K. Chem. Eur. J. 2004, 10,
5297–5310. (b) Quante, H.; Schlichting, P.; Rohr, U.; Geerts, Y.;
Mullen, K. Macromol. Chem. Phys. 1996, 197, 4029–4044.
(37) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum
Press: New York, 1986.
(26) Kokado, K.; Chujo, Y. Macromolecules 2009, 42, 1418–1420.
(27) Peterson, J. J.; Simon, Y. C.; Coughlin, B.; Carter, K. R. Chem.
Commun. 2009, 4950–4952.
(38) Lor, M.; Thielemans, J.; Viaene, L.; Cotlet, M.; Hofkens, J.; Weil,
T.; Hampel, C.; Mullen, K.; Verhoeven, J. W.; Auweraer, M. V.; De
Schryver, F. C. J. Am. Chem. Soc. 2002, 124, 9918–9925.
(28) (a) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun. 2009, 4332–
4353. (b) We are also thankful to reviewers for valuable suggestions.
9
6582 J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010

Carborane-Appended Symmetrical Extended π Systems
A R T I C L E S
Figure 5. (A) TGA curves of small trimers under nitrogen: (orange) 17;
(green) 5; (red) 1; (blue) control A. (B) TGA curves of extended trimers
under nitrogen: (green) 15; (red) 6; (blue) control B.
Table 2. 10% Mass Loss Temperatures from TG Analysis
trimer
temp (°C)
control A
1
5
17
control B
6
15
257
385
420
475
210
475
505
Figure 6. DSC analysis of small trimers at a heating rate of 5 K min-1
(A) control A; (T-5) 5; (T-17) 17; (T-23) 23.
:
small trimers are compared with that of control A (Figure 5A),
and those of the extended trimers are compared with that of
control B (Figure 5B). The temperatures at which 10% mass
loss of all the trimers occurred have been summarized in Table
2. In general, for both the Bcage -and Ccage-appended trimers there
was a dramatic enhancement of the thermal stability on
incorporating the carborane. By analyzing the TGA plot shown
in Figure 5A, we observe that the temperature at which 10%
mass loss occurs for the control A is 257 °C, and then rapid
decomposition of the compound was observed until complete
degradation of the compound happened at about 320 °C.
However, for 1, which contains three Bcage-appended o-carborane
clusters, the point where 10% mass loss occurred is found to
be 385 °C and finally about 47 wt % of the residue is maintained
at 600 °C, whereas for 5, which contains three Ccage-appended
o-carboranes, the 10% mass loss occurs at 420 °C and about
13 wt % of the residue is maintained at 600 °C. However, for
17, which has six Ccage-appended o-carboranes, the 10% mass
loss observed at 475 °C and about 30 wt % of the residue is
maintained at 600 °C. This was found to be the most stable
compound in the smaller trimer series. A similar trend was
observed for extended trimers, and these are even found to be
more thermally stable than the smaller ones. For the extended
control B the thermal degradation occurs in two successive steps.
The first step, which occurs between 155 and 220 °C, accounts
for about 10% of the mass loss. This could be due to the
decomposition of the peripheral phenyl ring. Then, in the second
step rapid decomposition of the molecule was observed in the
temperature range 380-500 °C and leads to almost 0 wt %
residue. Such stepwise degradation is commonly observed in
macromolecular and branched polymeric materials, where
degradation starts from the branching units and loss of small
molecular fragments also occurs.40 However, in the presence
of peripheral o-carboranes such stepwise mass loss is not
observed, and for both 6 and 15, about 51 wt % residue is
maintained at 600 °C. The 10% mass loss temperature for 6
and 15 is also found to be high, occurring at 475 and 505 °C,
respectively. Thus, we consistently found that increasing the
number of o-carborane clusters increases the thermal stability
of compounds.
Another property of small Ccage-appended trimers was seen
in their DSC thermograms (Figure 6). The phase transition
temperatures of those trimers that have shown such meso-
morphic behavior are summarized in Table 3. Only one
endothermic peak is seen for the control A at 176.6 °C,
whereas for 5, which contains three o-carborane clusters, we
observe three endothermic peaks at 113.9, 160.3, and 372.5
°C, respectively. Similarly for 17, which contains six
o-carborane clusters, endothermic peaks at 103.8, 146.3, and
388 °C were found. However, for the heptyl-substituted trimer
(39) (a) Hao, E.; Fabre, B.; Fronczeka, F. R.; Vicente, M. G. H. Chem.
Commun. 2007, 4387–4389. (b) Benhabbour, S. R.; Parrott, M. C.;
Gratton, S. E. A.; Adronov, A. Macromolecules 2007, 40, 5678–5688.
(c) Farha, O. K.; Spokoyny, A. M.; Mulfort, K. L.; Hawthorne, M. F.;
Mirkin, C. A.; Hupp, J. T. J. Am. Chem. Soc. 2007, 129, 12680–
12681.
(40) (a) Wriedt, M.; Sellmer, S.; Nather, C. Dalton Trans. 2009, 7975–
7984. (b) Kostler, S.; Zechner, B.; Trathnigg, B.; Fasl, H.; Kern, W.;
Ribitsch, V. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 1499–
1509. (c) Duquesne, S.; Bras, M. L.; Bourbogot, S.; Delobel, R.;
Camino, G.; Eling, B.; Lindsay, C.; Roels, T.; Vezin, H. J. Appl.
Polym. Sci. 2001, 82, 3262–3274.
9
J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010 6583

A R T I C L E S
Dash et al.
and CDCl3 (13C, δ 77.0 ppm), and coupling constants are given in
Hz. All 13C spectra are proton-decoupled. 11B NMR spectra were
recorded at 64.2 MHz relative to BF3 ·Et2O. Infrared spectra were
recorded on an ATI Mattson Genesis series FT-IR spectrophotom-
eter. Elemental analyses were carried out with a CHN-Analyzer
2400 (Perkin-Elmer). Melting points were measured with a Mel-
Temp II apparatus (Laboratory Devices) and are uncorrected.
Differential scanning calorimetry (DSC) and thermal gravimetric
analysis (TGA) were performed on DSC 220C and TG/DTA 320
instrument (Seiko Instruments), respectively. UV absorption and
emission spectra were measured with a Perkin-Elmer Lambda 19
UV-vis spectrometer and Hitachi F-2500 fluorescence spectro-
photometer, respectively. Mass spectral analyses were performed
with a Applied Biosystems Voyager-DE STR-MALDI-TOF spec-
trometer and Waters Q-TOF Ultima ES spectrometer.
Table 3. DSC Data of Trimers
trimer
peak (°C)
control A
5
17
23
176.6
113.9, 160.3, 372.5
103.8, 146.3, 388
115.3, 166.7
23 only two peaks at 115.3 and 166.7 °C are observed. The
peaks observed at 372.5, 388, and 166.7 °C represent the
melting of the respective trimers; the other peaks reflect
solid-solid phase transitions. Substituted polycyclic aromatic
hydrocarbons are known to form liquid-crystalline phases3,41
and in some cases such thermally induced solid-state phase
transitions that are associated with the conformational
reorganization of the bulky side groups.42 Unique properties
such as molecular symmetry, conformational mobility, and
their role in stabilizing liquid-crystalline phases make closo-
carborane clusters ideal choices for use in the synthesis of
liquid-crystalline materials,43 and so the combination of
π-conjugated systems and closo-carboranes therefore could
be useful in making hybrid materials possessing such
characteristics.
Preparations and Analytical Data of Compounds. Ketone
3. Compound 2 (710 mg, 1.89 mmol) was dissolved in 20 mL of
solvent (THF/toluene, 1/1). Then 4-(acetylphenyl)boronic acid
(622.1 mg, 3.79 mmol), PdCl2(PPh3)2 (53.26 mg, 4 mol %), and a
solution of K2CO3 (785.35 mg, 5.69 mmol) in 5 mL of distilled
water were added to the solution. The mixture was refluxed at 100
°C for 24 h and then extracted three times with dichloromethane,
and the combined organic layers were dried over MgSO4. After
evaporation of the solvent, the residue was purified by silica gel
column chromatography with 5-25% ethyl acetate in hexanes as
the eluent to obtain 520 mg of pure compound 3 as a colorless
solid, yield 75%. Mp: 165 °C. 1H NMR (CDCl3, 500 MHz): δ 8.0
(d, 2H, J ) 7.96 Hz), 7.71 (d, 2H, J ) 7.98 Hz), 7.63 (d, 2H, J )
7.75 Hz), 7.31 (d, 2H, J ) 7.79 Hz), 3.54 (s, 2H), 2.67 (s, 3H),
2.21 (s, 3H, cluster-CH3). 13C NMR (CDCl3, 125 MHz): δ 197.5,
144.8, 139.6, 136.1, 135.0, 130.9, 128.9, 127.4, 127.1, 77.3 (Ccage),
74.8 (Ccage), 40.7, 26.6, 23.6. 11B NMR (proton decoupled): -4.08,
-5.47, -10.0. IR (KBr): 3034, 2589, 1678, 1602, 1365, 1266, 958,
817 cm-1. Anal. Calcd for C18H26B10O: C, 58.99; H, 7.15. Found:
C, 59.08; H, 7.03. ES-MS (m/z): calcd 366.5, found 365.1 (M - 1,
100%).
Conclusion
In summary, we have demonstrated a silicon tetrachloride
mediated facile synthetic route to symmetrical conjugated π
systems containing multiple o-carborane clusters. The properties
of carborane-containing π-conjugated compounds are found to
be unique and significantly different from those of carborane-
unsubstituted species. Presence of o-carborane clusters in
π-conjugated systems prevents π-π stacking interactions and
enhances the rigidity of such molecules. Thus, o-carborane-
appended extended trimers show enhanced fluorescent intensity.
Deboronation of o-carboranes led to the water-soluble com-
pounds that are also found to be fluorescent in water but with
a lower relative fluorescence quantum yield. The presence of
o-carborane clusters also made these compounds highly ther-
mally stable. An increase in the number of o-carborane clusters
led to more thermally stable products. Thermally induced
Ketone 4. Compound 2 (450 mg, 1.2 mmol) was dissolved in
10 mL of dry DMF. Then acetic anhydride (0.57 mL, 6 mmol),
i-Pr2NEt (0.42 mL, 2.4 mmol), LiCl (255 mg, 6 mmol), and
Pd2(dba)3 (22 mg, 2 mol %) were added to this solution. The
mixture was then heated at 100 °C for 20 h, quenched with water,
and extracted three times with diethyl ether. The combined organic
layer was dried over MgSO4. After evaporation of the solvent the
residue was purified by silica gel column chromatography with
5-15% ethyl acetate in hexane as eluent to obtain 250 mg of pure
compound 4 as a colorless solid, yield 72%. Mp: 90 °C. 1H NMR
(CDCl3, 500 MHz): δ 7.95 (d, 2H, J ) 6.85 Hz), 7.31 (d, 2H, J )
solid-solid phase transitions are observed in smaller Ccage
appended trimers.
-
Experimental Section
6.85 Hz), 3.53 (s, 2H), 2.62 (s, 3H), 2.19 (s, 3H, cluster-CH3). 13
C
General Methods. Reactions were generally performed under
argon in oven-dried flasks. Solvents and reagents were added by
syringes. Solvents were dried and distilled using standard proce-
dures. Reagents were purchased and were used as received without
further purification. All compounds were purified by column
chromatography on silica gel (70-230 mesh, Aldrich). Yields of
NMR (CDCl3, 125 MHz): δ 197.4, 139.9, 136.7, 130.6, 128.6, 76.6
(Ccage), 74.9 (Ccage), 40.9, 26.6, 23.6. 11B NMR (proton decoupled):
-3.89, -5.50, -10.0. IR (KBr): 3005, 2944, 2599, 1673, 1608,
1417, 1359, 1271, 1020 cm-1. Anal. Calcd for C12H22B10O: C,
49.63; H, 7.64. Found: C, 49.61; H, 7.58. ES-MS (m/z): calcd 290.2,
found 290.3 (M+).
the products refer to analytically pure samples. H and 13C NMR
1
Ketone 9. Commercially available 4-(4-bromophenyl)acetophe-
none (8; 370.7 mg, 1.34 mmol) was dissolved in 40 mL of solvent
(THF/toluene, 1/1). Then the carborane-appended boronic acid 716
(750 mg, 2.56 mmol), PdCl2(PPh3)2 (38 mg, 4 mol %), and a
solution of K2CO3 (557.6 mg, 4.04 mmol) in 4 mL of distilled water
were added to this solution. The reaction mixture was refluxed at
100 °C for 24 h and then quenched with water and extracted three
times with dichloromethane. The combined organic layer was then
dried over MgSO4. After evaporation of solvent the crude product
was purified by silica gel column chromatography with 5-40%
ethyl acetate in hexanes as the eluent to obtain 450 mg of the pure
spectra were recorded on a Bruker Fourier transform multinuclear
NMR spectrometer at 500.13 and 125.75 MHz, respectively.
Chemical shifts are reported relative to TMS (1H, δ 0.00 ppm),
(41) (a) Rohr, U.; Schlichting, P.; Bohm, A.; Gross, M.; Meerholz, K.;
Brauchhle, C.; Mullen, K. Angew. Chem., Int. Ed. 1998, 37, 1434–
1437. (b) Ito, S.; Wehmeier, M.; Brand, J. D.; Kubel, C.; Epsch, R.;
Rabe, J. P.; Mullen, K. Chem. Eur. J. 2000, 6, 4327–4342.
(42) Chen, J.; Anthony, J.; Martin, D. C. J. Phys. Chem. B 2006, 110,
16397–16403.
(43) (a) Kaszynski, P.; Pakhomov, S.; Tesh, K. F.; Young, V. G., Jr. Inorg.
Chem. 2001, 40, 6622–6631. (b) Nagamine, T.; Januszko, A.;
Kaszynski, P.; Ohta, K.; Yasuyuki, E. J. Mater. Chem. 2006, 6, 3836–
3843. (c) Januszko, A.; Kaszynski, P.; Gruner, B. Inorg. Chem. 2007,
46, 6078–6082. (d) Kaszynski, P.; Douglas, A. G. J. Organomet. Chem.
1999, 581, 28–38.
1
product 9 as a colorless solid, yield 75%. Mp: 240 °C. H NMR
(CDCl3, 500 MHz): δ 8.0 (d, 2H, J ) 8.24 Hz), 7.77-7.72 (m,
6H), 7.65 (d, 2H, J ) 8.0 Hz), 7.31 (d, 2H, J ) 8.0 Hz), 3.54 (s,
2H), 2.67 (s, 3H), 2.22 (s, 3H, cluster-CH3). 13C NMR (CDCl3,
9
6584 J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010

Carborane-Appended Symmetrical Extended π Systems
A R T I C L E S
125 MHz): δ 197.6, 145.1, 140.2,140.1, 138.9, 136.0, 134.3, 130.8,
128.9, 127.7, 127.6, 127.1, 127.0, 77.5 (Ccage), 74.8 (Ccage), 40.9,
26.6, 23.7. 11B NMR (proton decoupled): -5.37, -9.99. IR (KBr):
3191, 2588, 1683, 1600, 1260, 1128, 809 cm-1. Anal. Calcd for
C24H30B10O: C, 65.13; H, 6.83. Found: C, 65.0; H, 7.01. ES-MS
(m/z): calcd 442.6, found 441.3 (M - 1, 100%).
temperature for 2 h and refluxed for 3 h. After removal of solvent
the product was purified by silica gel column chromatography with
5-10% ethyl acetate in hexanes as the eluent to obtain a quantitative
5.5 g of pure compound 19 as an oil. 1H NMR (CDCl3, 500 MHz):
δ 2.20-2.17 (m, 2H), 1.56-1.53 (m, 2H), 1.27 (br, s, 8H), 1.08
(s, 9H), 0.90 (br, s, 3H), 0.34 (s, 6H). 13C NMR (CDCl3, 125 MHz):
δ 81.8, 76.2, 38.0, 31.5, 30.17, 29.16, 28.8, 27.57, 22.5, 20.3, 13.9,
-2.4. Compound 19 (5.5 g, 15.42 mmol) was then solubilized in
40 mL of THF. It was cooled to -78 °C, and a 1 M solution of
tetrabutylammonium fluoride in THF (32 mL, 32 mmol) was added
to it at the same temperature over 10 min; this mixture was then
warmed to room temperature and stirred at room temperature for
30 min. Water was added, and the reaction mixture was extracted
with diethyl ether. Removal of the solvent and purification by
column chromatography using hexane gave 3.5 g of the pure product
20 as a colorless oil, yield 94%. Compound 20 (600 mg, 2.475
mmol) was dissolved in 25 mL of DME, and to this solution was
added n-BuLi (1.6 M solution in hexane, 1.7 mL, 2.72 mmol) at 0
°C. This mixture was then stirred at 0 °C for 1 h, and a solution of
4-iodobenzyl bromide (598 mg, 1.98 mmol) in 5 mL of dry DME
was added to it at 0 °C; then it was refluxed at 90 °C for 24 h,
after which the reaction mixture was cooled to room temperature
and filtered over a silica gel pad. Removal of solvent and
purification of the compound by silica gel column chromatography
using hexane as the eluent resulted in 705 mg of pure compound
Carboranyl Adduct 13. 1-Methyl-o-carborane (2.49, 15.80
mmol) was dissolved in 120 mL of dry THF. This solution was
cooled to 0 °C, and BuLi (10.36 mL, 1.6 M solution in hexane)
was added dropwise to it. Then it was stirred at 0 °C for 1 h and
then cooled to -10 °C. At -10 °C, LiI (307.5 mg, 2.29 mmol)
and compound 12 (2.8 g, 7.18 mmol) in 10 mL of dry THF were
added dropwise to the reaction mixture. Then the reaction mixture
was stirred for 30 min at same temperature and warmed to room
temperature for 24 h. After completion of the reaction, the reaction
mixture was filtered off through a silica gel pad and concentrated.
After evaporation of the solvent, the residue was purified by silica
gel column chromatography with 5-10% ethyl acetate in hexanes
as the eluent to obtain 2.5 mg of pure compound 13 as a colorless
solid, yield 64%. Mp: 165 °C. 1H NMR (CDCl3, 500 MHz): δ 7.53
(s, 2H), 7.01 (s, 1H), 3.42 (s, 4H), 2.18 (s, 6H, cluster-CH3). 13C
NMR (CDCl3, 125 MHz): δ 138.8, 137.3, 131.4, 94.1, 76.5 (Ccage),
74.8 (Ccage), 40.3, 23.6. 11B NMR (proton decoupled): -5.39, -9.96.
IR (KBr): 2944, 2584, 1567, 1437, 1020 cm-1. Anal. Calcd for
C14H33B20I: C, 30.88; H, 6.11. Found: C, 30.69; H, 6.08.
1
Ketone 14. Compound 13 (300 mg, 0.55 mmol) was dissolved
in 12 mL of solvent (THF/toluene, 1/1). Then 4-(acetylphenyl)bo-
ronic acid (180.6 mg, 1.101 mmol), PdCl2(PPh3)2 (15.4 mg, 4 mol
%), and a solution of K2CO3 (228 mg, 1.65 mmol) in 2 mL of
distilled water were added to the solution. The mixture was refluxed
at 100 °C for 24 h and then extracted three times with dichlo-
romethane, and the combined organic layer was dried over MgSO4.
After evaporation of the solvent, the residue was purified by silica
gel column chromatography with 40% ethyl acetate in hexanes as
the eluent to obtain 270 mg of pure compound 14 as a colorless
solid, yield 91%. Mp: 240 °C. 1H NMR (CDCl3, 500 MHz): δ 8.0
(d, 2H, J ) 7.8 Hz), 7.67 (d, 2H, J ) 7.8 Hz), 7.41 (s, 2H), 7.06
(s, 1H), 3.56 (s, 4H), 2.68 (s, 3H), 2.23 (s, 6H, cluster-CH3). 13C
NMR (CDCl3, 125 MHz): δ 197.3, 144.3, 140.9, 136.5, 136.2,
131.6, 129.0, 128.9, 127.3, 74.9 (Ccage), 60.3 (Ccage), 41.1, 26.6,
23.6. 11B NMR (proton decoupled): -5.40, -9.94. IR (KBr): 2941,
21 as an oily liquid, yield 62%. H NMR (CDCl3, 200 MHz): δ
7.72 (d, 2H, J ) 8.17), 6.97 (d, 2H, J ) 8.17), 3.41 (s, 2H),
2.29-2.19 (m, 2H), 1.59-1.48 (m, 2H), 1.29 (br, s, 8H), 0.94 (t,
3H). Compound 21 (130 mg, 0.282 mmol), Pd2(dba)3 (5.16 mg, 2
mol %), and LiCl (60 mg) were dissolved in 2 mL of dry DMF.
Then acetic anhydride (0.13 mL, 1.414 mmol) and i-Pr2NEt (0.098
mL, 0.564 mmol) were added to this solution. The mixture was
then stirred at 100 °C for 20 h. After this mixture was cooled to
room temperature, it was filtered over a silica gel pad and then
water was added. This mixture was then extracted with diethyl ether.
The combined organic layer was dried with MgSO4. After evapora-
tion of the solvent the residue was purified by column chromatog-
raphy with 30% ethyl acetate in hexane to obtain 80 mg of pure
1
product 22 as a colorless oil, yield 76%. H NMR (CDCl3, 500
MHz): δ 7.96 (d, 2H, J ) 8.15 Hz), 7.31 (d, 2H, J ) 8.15 Hz),
3.51 (s, 2H), 2.63 (s, 3H), 2.36-2.32 (m, 2H), 1.64-1.62 (m, 2H),
1.36-1.32 (m, 8H), 0.92 (t, 3H, J ) 7.0 Hz). 13C NMR (CDCl3,
125 MHz): δ 197.3, 140.0, 136.7, 130.6, 128.5, 80.1, 78.1, 40.6,
35.6, 31.6, 29.8, 29.2, 28.8, 26.6, 22.5, 14.0. 11B NMR (proton
decoupled): -4.49, -10.25. IR (neat): 2957, 2928, 2856, 2580,
1685, 1609, 1264, 1012 cm-1. Anal. Calcd for C18H34B10O: C,
57.52; H, 9.15. Found: C, 57.50; H, 9.11. ES-MS (m/z): calcd
374.57, found 375.37 (M+ + 1).
General Procedure for Trimerization of Ketones. Carborane-
appended aryl ketones were dissolved in the required amounts of
solvent (dry absolute ethanol/toluene, 1/1). Then SiCl4 was added
to the solution and the reaction mixture was stirred at room
temperature to 50 °C for 10-24 h. The reaction mixture was
quenched with water and extracted three times with dichlo-
romethane. The combined organic layer was dried with MgSO4.
After evaporation of the solvent the crude product was purified by
silica gel column chromatography.
Trimer 5. Ketone 4 (170 mg, 0.58 mmol) was solubilized in 5
mL of solvent (absolute ethanol/toluene 3/1). Then SiCl4 (0.67 mL,
5.8 mmol) of was added to the solution at room temperature, and
this mixture was then stirred at room temperature for 10 h.
Purification of the compound with silica gel column chromatography
with 5 -15% ethyl acetate in hexanes as eluent resulted in 120 mg
of the pure product 5 as a colorless solid, yield 75%. Mp: >260
°C. 1H NMR (CDCl3, 500 MHz): δ 7.81 (s, 3H), 7.70 (d, 6H, J )
8.0 Hz), 7.33 (d, 6H, J ) 8.0 Hz), 3.56 (s, 6H), 2.23 (s, 9H, cluster-
CH3). 13C NMR (CDCl3, 125 MHz): δ 141.6, 140.7, 134.4, 130.8,
127.5, 125.2, 77.5 (Ccage), 74.8 (Ccage), 40.9, 23.7. 11B NMR (proton
decoupled): -5.43, -10.0. IR (KBr): 3033, 2925, 2853, 2581, 1598,
2586, 1733, 1675, 1603, 1435, 1359, 1270, 1025, 957, 840 cm-1
.
Anal. Calcd for C22H40B20O: C, 49.23; H, 7.51. Found: C, 49.31;
H, 7.54. ES-MS (m/z): calcd. 536.7, found: 536.6 (M+, 100%).
Ketone 16. Compound 13 (490 mg, 0.9 mmol) was dissolved
in 10 mL of dry DMF. Then acetic anhydride (0.43 mL, 4.5 mmol),
i-Pr2NEt (0.32 mL, 1.8 mmol), LiCl (191 mg, 4.5 mmol), and
Pd2(dba)3 (16.5 mg, 2 mol %) were added to the solution. It was
heated at 100 °C for 20 h and then quenched with water, and the
reaction mixture was extracted three times with diethyl ether. The
combined organic layer was dried over MgSO4. After evaporation
of the solvent the residue was purified by silica gel column
chromatography with 30% ethyl acetate in hexane as eluent to obtain
400 mg of pure compound 16 as a colorless solid, yield 96%. Mp:
185 °C. 1H NMR (CDCl3, 500 MHz): δ 7.76 (s, 2H), 7.25 (s, 1H),
3.56 (s, 4H), 2.66 (s, 3H), 2.12 (s, 6H, cluster-CH3). 13C NMR
(CDCl3, 125 MHz): δ 196.8, 137.8, 136.3, 136.2, 129.7, 76.5 (Ccage),
74.9 (Ccage), 40.8, 26.6, 23.7. 11B NMR (proton decoupled): -4.34,
-8.93, -11.10. IR (KBr): 2944, 2585, 2351, 1703, 1603, 1448,
1360, 1293, 1210 cm-1. Anal. Calcd for C16H36B20O: C, 41.72; H,
7.88. Found: C, 41.67; H, 7.89. ES-MS (m/z): calcd 460.6764, found
461.5 (M+ + 1).
Ketone 22. The TBDMS-protected o-carborane 18 was prepared
as per the reported procedure.17 A solution of 18 (4 g, 15.48 mmol)
in 60 mL of dry THF at 0 °C was treated with n-BuLi (1.6 M
solution in hexane, 10.2 mL, 16.25 mmol). The solution was stirred
at 0 °C for 30 min at room temperature. Then a solution of
1-iodoheptane (3.67 g, 16.25 mmol) in 5 mL of THF was added at
0 °C and the resulting reaction mixture was stirred at room
9
J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010 6585

A R T I C L E S
Dash et al.
1512, 1261, 1020, 810 cm-1. Anal. Calcd for C36H60B30: C, 52.91;
H, 7.40. Found: C, 53.03; H, 7.33. MALDI-TOF-MS (m/z): calcd
817.19, found 817.78 (M+, 100%).
and then SiCl4 (0.49 mL, 4.27 mmol) was added to it. This
mixture was then stirred at room temperature for 5 h. Then the
reaction was quenched with water and the mixture extracted three
times with dichloromethane. The combined organic layer was
dried with MgSO4. After evaporation of the solvent the residue
was purified by silica gel column chromatography using 20-25%
ethyl acetate in hexane to obtain 40 mg of the pure product 23
Trimer 6. Ketone 3 (480 mg, 1.31 mmol) was dissolved in 24
mL of solvent (absolute ethanol/toluene, 3/1). Then SiCl4 (4.5 mL,
39.29 mmol) was added to the solution at room temperature, and
this mixture was then stirred at 40 °C for 24 h. Purification of the
compound with silica gel column chromatography with 5 -20%
ethyl acetate in hexanes as eluent resulted in 280 mg of the pure
1
as a colorless solid, yield 50%. H NMR (CDCl3, 500 MHz): δ
7.80 (s, 3H), 7.70 (d, 6H, J ) 8.0 Hz), 7.32 (d, 6H, J ) 8.0
Hz), 3.53 (s, 6H), 2.39-2.36 (m, 6H), 1.65-1.56 (m, 6H),
1.38-1.28 (m, 24H), 0.92 (t, 9H, J ) 7.0 Hz). 13C NMR (CDCl3,
125 MHz): δ 141.6, 140.6, 134.4, 130.8, 127.4, 125.2, 80.1, 79.0,
40.6, 35.6, 31.6, 29.8, 29.3, 28.9, 22.5, 14.0. 11B NMR (proton
decoupled): -4.76, -10.0. IR (KBr): 2925, 2859, 2591, 1511,
1397, 1023, 808 cm-1. Anal. Calcd for C54H96B30: C, 60.63; H,
9.05. Found: C, 60.58; H, 9.11. MALDI-TOF-MS (m/z): calcd
1069.67, found 1069.68 (M+, 100%).
1
product 6 as a colorless solid, yield 61%. Mp: >260 °C. H NMR
(CDCl3, 500 MHz): δ 7.92 (s, 3H), 7.85 (d, 6H, J ) 8.10 Hz),
7.76 (d, 6H, J ) 8.10 Hz), 7.68 (d, 6H, J ) 8.0 Hz), 7.32 (d, 6H,
J ) 8.0 Hz), 3.55 (s, 6H), 2.23 (s, 9H, cluster-CH3). 13C NMR
(CDCl3, 125 MHz): δ 141.9, 140.3, 140.2, 139.6, 134.2, 130.8,
127.8, 127.5, 127.1, 125.0, 77.5 (Ccage), 74.8 (Ccage), 40.9, 23.7.
11B NMR (proton decoupled): -5.45, -10.1. IR (KBr): 3029, 2580,
1589, 1504, 1387, 812 cm-1. Anal. Calcd for C54H72B30: C, 62.04;
H, 6.94. Found: C, 62.11; H, 7.03. MALDI-TOF-MS (m/z): calcd
1045.47, found 1044.48 (M - 1, 100%).
General Procedure for Deboronation (Decapitation) of o-
Carborane Clusters Present in Trimers. NaOH was solubilized
by stirring it in 5-10 mL of dry methanol. Then a solution of
trimers in about 3-5 mL of THF was added to the solution and
the reaction mixture was refluxed at 90 °C for 20 h. After that
time the excess NaOH was neutralized by adding a few pieces of
dry ice, and then the precipitate was filtered off over a frit.
Evaporation of the solvent led to a white solid which was
characterized without further purification.
Trimer 10. Ketone 9 (410 mg, 0.926 mmol) was dissolved in
50 mL of solvent (absolute ethanol/toluene 3/1). Then SiCl4 (10.6
mL, 92.6 mmol) was added to the solution at room temperature,
and this mixture was then stirred at 50 °C for 24 h. Purification
of the compound with silica gel column chromatography with 5
-30% ethyl acetate in hexanes as eluent resulted in 118 mg of
the pure product 10 as a colorless solid, yield 30%. Mp: >260
1
°C. H NMR (CDCl3, 500 MHz): δ 7.94 (s, 3H), 7.86 (d, 6H, J
) 8.0 Hz), 7.83-7.78 (m, 12H), 7.74 (d, 6H, J ) 8.0 Hz), 7.66
(d, 6H, J ) 7.7 Hz), 7.31 (d, 6H, J ) 7.7 Hz), 3.55 (s, 6H),
2.22 (s, 9H, cluster-CH3). 13C NMR (CDCl3, 125 MHz): δ 141.9,
140.3, 140.1, 139.8, 139.7, 139.3, 134.1, 130.8, 127.8, 127.5,
127.1, 125.0, 77.5 (Ccage), 74.8 (Ccage), 40.9, 23.7. 11B NMR
(proton decoupled): -9.0, -11.25. IR (KBr): 3028, 2924, 2583,
1906, 1595, 1492, 819 cm-1. Anal. Calcd for C72H84B30: C,
67.89; H, 6.65. Found: C, 67.92; H, 6.63. MALDI-TOF-MS (m/
z): calcd 1273.76, found 1273.48 (M+, 100%).
Trimer 15. Ketone 14 (260 mg, 0.484 mmol) was solubilized
in 15 mL of solvent (absolute ethanol/toluene, 3/1). Then SiCl4
(2.8 mL, 24.2 mmol) was added to the solution at room temperature,
and this mixture was then stirred at 45 °C for 24 h. Purification of
the compound with silica gel column chromatography with 50%
ethyl acetate in hexanes as eluent resulted in 100 mg of the pure
product 15 as a colorless solid, yield 40%. Mp: >260 °C. 1H NMR
(CDCl3, 500 MHz): δ 7.93 (s, 3H), 7.87 (d, 6H, J ) 7.2 Hz), 7.73
(d, 6H, J ) 7.2 Hz), 7.47 (s, 6H), 7.0 (s, 3H), 3.59 (s, 12H), 2.24
(s, 18H, cluster-CH3). 13C NMR (CDCl3, 125 MHz): δ 141.9, 141.6,
140.6, 139.3, 136.1, 130.9, 128.8, 127.9, 127.7, 125.2, 77.1 (Ccage),
74.9 (Ccage), 41.2, 23.7. 11B NMR (proton decoupled): -5.59, -9.95.
IR (KBr): 3026, 2942, 2585, 1601, 1518, 1455, 1386, 1035, 830
cm-1. Anal. Calcd for C66H114B60: C, 50.94; H, 7.38. Found: C,
50.82; H, 7.20. MALDI-TOF-MS (m/z): calcd 1556.27, found
1555.46 (M+ - 1, 100%).
Trimer 17. Ketone 16 (470 mg, 1.02 mmol) was dissolved in
40 mL of solvent (absolute ethanol/toluene, 3/1). Then SiCl4 (9.3
mL, 81.6 mmol) was added to the solution at room temperature,
and this mixture was then stirred at 40 °C for 24 h. Purification of
the compound with silica gel column chromatography with 20-25%
ethyl acetate in hexanes as eluent resulted in 250 mg of the pure
product 17 as a colorless solid, yield 55%. Mp: >260 °C. 1H NMR
(CDCl3, 500 MHz): δ 7.74 (s, 3H), 7.52 (s, 6H), 7.11 (s, 3H), 3.60
(s, 12H), 2.24 (s, 18H, cluster-CH3). 13C NMR (CDCl3, 125 MHz):
δ 141.4, 136.4, 131.4, 128.8, 125.5, 76.4 (Ccage), 75.0 (Ccage), 41.1,
23.7. 11B NMR (proton decoupled): -0.81, -5.21. IR (KBr): 2994,
2942, 2857, 2593 1593, 1449, 1387, 1328, 1027, 867 cm-1. Anal.
Calcd for C48H102B60: C, 43.41; H, 7.74. Found: C, 43.46; H, 7.61.
MALDI-TOF-MS (m/z): calcd 1327.98, found 1328.5 (M+ + 1,
100%).
Decapitated Ttrimer 24. Trimer 6 (150 mg, 0.143 mmol) and
NaOH (274 mg, 6.86 mmol) gave 140 mg of the colorless solid
24, which was water-soluble. Yield: 90%. Mp: > 260 °C. 1H NMR
(CD3CN, 500 MHz): δ 8.05 (s, 3H), 7.98 (d, 6H, J ) 8.0 Hz),
7.85 (d, 6H, J ) 8.1 Hz), 7.7 (d, 6H, J ) 8.0 Hz), 7.45 (d, 6H, J
) 7.85 Hz), 3.31 (s, 6H), 2.18 (s, 9H, cluster-CH3), -2.4 (br,
cluster-bridge 3H). 13C NMR (CD3CN, 125 MHz): δ 142.5, 142.0,
140.5, 139.4, 137.2, 129.2, 127.2, 126.9, 125.7, 124.0, 62.0 (br,
C
cage), 54.6 (br, Ccage), 41.7, 21.8.11B NMR (proton decoupled):
-9.4, -16.4, -19.8, -34.6, -36.7. IR (KBr): 2984, 2950, 2862,
2516 (B-H), 2014 (B-H-B), 1627, 1473, 1371, 1099, 825 cm-1
.
Anal. Calcd for C54H72B27Na3: C, 59.94; H, 6.71. Found: C, 59.89;
H, 6.70. MALDI-TOF-MS (m/z): calcd 1082.01, found 1010.76
(M+ - 3Na - 3H).
Decapitated Trimer 25. Trimer 15 (100 mg, 0.064 mmol) and
NaOH (139 mg, 3.47 mmol) gave 90 mg of the colorless solid 25,
1
which was water-soluble. Yield: 86%. Mp: > 260 °C. H NMR
(CD3CN, 200 MHz): δ 8.13 (s, 3H), 8.07 (d, 2H, J ) 8.21), 7.91
(d, 2H, J ) 8.16), 7.59 (s, 3H), 7.17(s, 2H), 3.21 (s, 12H), 2.22 (s,
18H, cluster-CH3), -2.28 (br, cluster-bridge 6H). 13C NMR
(CD3CN, 125 MHz): δ 143.5, 141.8, 141.2, 139.5, 139.1, 129.0,
127.9, 127.3, 124.7, 62.3 (br, Ccage), 54.8 (br, Ccage), 41.9, 22.3.
11B NMR (proton decoupled): -9.4, -16.4, -19.7, -34.5, -36.7.
IR (KBr): 3015, 2951, 2862, 2517 (B-H), 2013 (B-H-B), 1631,
1473, 1376, 1099 cm-1. Anal. Calcd for C66H114B54Na6: C, 48.65;
H, 7.05. Found: C, 48.43; H, 7.22. MALDI-TOF-MS (m/z): calcd
1629.34, found 1537.62 (M+ - 4Na).
Decapitated Trimer 26. Trimer 10 (140 mg, 0.11 mmol) and
NaOH (211 mg, 5.2 mmol) gave 120 mg of the colorless solid 26,
1
which was water-soluble. Yield: 83%. Mp: > 260 °C. H NMR
(CD3CN, 500 MHz): δ 8.08 (s, 3H), 8.03-7.99 (m, 6H), 7.92-7.82
(m, 12H), 7.78-7.76 (m, 6H), 7.69-7.64 (m, 6H), 7.45-7.44 (m,
6H), 3.19 (s, 6H), 2.20 (s, 9H, cluster-CH3), -2.4 (br, cluster-bridge
3H). 13C NMR (CD3CN, 125 MHz): δ 142.8, 141.7, 140.1, 139.6,
138.6, 137.0, 129.3, 129.2, 127.3, 126.8, 125.6, 124.7, 61.4 (br,
Ccage), 54.6 (br, Ccage), 41.7, 21.8. 11B NMR (proton decoupled):
-9.7, -16.5, -19.8, -34.4, -36.4. IR (KBr): 2984, 2952, 2863,
2520 (B-H), 2002 (B-H-B), 1632, 1472, 1376, 1192, 1099 cm-1
.
Anal. Calcd for C72H84B27Na3: C, 66.00; H, 6.46. Found: C, 65.94;
H, 6.42. MALDI-TOF-MS (m/z): calcd 1310.30, found 1216.62
(M+ - 3Na - 3BH).
Trimer 23. Ketone 22 (80 mg, 0.213 mmol) was dissolved in
4 mL of dry absolute ethanol. The solution was cooled to 0 °C,
9
6586 J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010

Carborane-Appended Symmetrical Extended π Systems
A R T I C L E S
Acknowledgment. We thank Michael Recchia for assistance
during TGA and DSC studies. This work was supported by grants
from the National Science Foundation (Nos. CHE-0906179 and
CHE-0840504), the Robert A. Welch Foundation (No. N-1322),
the Alexander von Humboldt Foundation and an NIU Inaugural
Board of Trustees Professorship Award.
Supporting Information Available: Text and figures giving
synthetic procedures and characterization data for controls A
and B and NMR spectra of the compounds prepared in this
paper. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA101845M
9
J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010 6587