Synthetic routes toward carborane-wheeled nanocars

Article abstract of DOI:10.1021/jo701400t

(Chemical Equation Presented) A new set of aryleneethynylene derivatives bearing three, four, and six p-carboranes as potential wheels attached to a semirigid chassis have been designed and synthesized. These molecules are expected to move in predetermined patterns on atomically smooth surfaces, depending on their specific configuration.

Full text of DOI:10.1021/jo701400t

Synthetic Routes toward Carborane-Wheeled Nanocars
Jean-Franc¸ois Morin, Takashi Sasaki, Yasuhiro Shirai, Jason M. Guerrero, and
James M. Tour*
Department of Chemistry, Department of Mechanical Engineering and Materials Science,
and the Smalley Institute for Nanoscale Science and Technology, Rice UniVersity, MS 222,
6100 Main Street, Houston, Texas 77005
tour@rice.edu
ReceiVed June 27, 2007
A new set of aryleneethynylene derivatives bearing three, four, and six p-carboranes as potential wheels
attached to a semirigid chassis have been designed and synthesized. These molecules are expected to
move in predetermined patterns on atomically smooth surfaces, depending on their specific configuration.
Introduction
molecules due to the lack of positive interactions with the
surface.4a Furthermore, other molecules have been imaged to
slide or migrate across surfaces.5 To improve directionality, four
fullerene-wheeled nanocars were recently synthesized by our
group.3a,6 Fullerene’s spherical and smooth shape, and most
importantly its affinity toward gold surfaces, have produced
directional rolling motions of the nanocars on the gold surface
upon heating or electrostatic control.6a However, synthetic
The development of microscopy tools, particularly scanning
tunneling microscopy (STM), has allowed scientists to study
biological1 and artificial2 molecular machines on surfaces. The
quickly expanding field of nanomachines has been reviewed.3
Because translational motion is the easiest to monitor by STM,
much effort has been devoted to observe single molecule
translational movement on flat metallic surfaces. For instance,
a family of molecular barrows with triptycene moieties as wheels
has been developed.4 Although these molecules can be imaged
and manipulated by STM in the direction perpendicular to the
axles, calculations suggest sliding rather than rolling of the
(4) (a) Joachim, C.; Tang, H.; Moresco, F.; Rapenne, G.; Meyer, G.
Nanotechnology 2002, 13, 330. (b) Jimenez-Bueno, G.; Rapenne, G.
Tetrahedron Lett. 2003, 44, 6261. (c) Rapenne, G. Org. Biomol. Chem.
2005, 7, 1165.
(5) (a) Kwon, K.-Y.; Wong, K. L.; Pawin, G.; Bartels, L.; Stolbov, S.;
Rahman, T. S. Phys. ReV. Lett. 2005, 95, 166101/1-166101/4. (b) Wong,
K. L.; Pawin, G.; Kwon, K. Y.; Lin, X.; Jiao, T.; Solanki, U.; Fawcett, R.
H. J.; Bartels, L.; Stolbov, S.; Rahman, T. S. Science 2007, 315, 1391-
1393.
(6) (a) Shirai, Y.; Osgood, A. J.; Zhao, Y.; Kelly, K. F.; Tour, J. M.
Nano Lett. 2005, 5, 2330. (b) Shirai, Y.; Osgood, A. J.; Zhao, Y.; Yao, Y.;
Saudan, L.; Yang, H.; Yu-Hung, C.; Sasaki, T.; Morin, J.-F.; Guerrero, J.
M.; Kelly, K. F.; Tour, J. T. J. Am. Chem. Soc. 2006, 128, 4854. (c) Sasaki,
T.; Tour, J. M. Tetrahedron Lett. 2007, 48, 5821-5824. (d) Sasaki, T.;
Morin, J.-F.; Lu, M.; Tour, J. M. Tetrahedron Lett. 2007, 48, 5817-5820.
(1) (a) Soong, R. K.; Bachand, G. D.; Neves, H. P.; Olkhovets, A. G.;
Craighead, H. G.; Montemagno, C. D. Science 2000, 290, 1555. (b) Bachand,
G. D.; Soong, R. K.; Neves, H. P.; Olkhovets, A. G.; Craighead, H. G.;
Montemagno, C. D. Nano Lett. 2001, 1, 42.
(2) For examples, see: Balzani, V.; Credi, A.; Venturi, M. Molecular
DeVices and Machines: A Journey Into the Nanoworld; Wiley-VCH:
Weinheim, Germany, 2003.
(3) (a) Shirai, Y.; Morin, J.-F.; Sasaki, T.; Guerrero, J.; Tour, J. M. Chem.
Soc. ReV. 2006, 35, 1043-1055. (b) Kay, E. R.; Leigh, D. A.; Zerbetto, F.
Angew. Chem., Int. Ed. 2006, 46, 72-191.
10.1021/jo701400t CCC: $37.00 © 2007 American Chemical Society
Published on Web 11/14/2007
J. Org. Chem. 2007, 72, 9481-9490
9481

Morin et al.
FIGURE 1. Molecular structures of 1-4 and the expected directionality of motion.
problems arise when using fullerenes. First, fullerenes and
fullerene-containing molecules have very poor solubility. Long
alkyl chains are incorporated to alleviate the solubility problems,
but in return this addition increases the number of synthetic
steps required to construct the nanocar. Second, because of their
tendency to deactivate metal-catalyzed reactions, fullerenes have
to be introduced onto the nanocar at the final step of the
synthesis, resulting in a low-yielding tetra-substitution reaction.
Additionally, the electronic nature of fullerene makes it unsuit-
able for the development of more complex nanomachines with
FIGURE 2. CPK models of planar conformations of 1 and 2.
use of light as the power input due to the rapid energy transfer
to the fullerenes.7
fewer synthetic steps. Last, carboranes can be introduced at any
stage of the synthesis because they do not inhibit organometallic
coupling reactions (unlike fullerenes). The work presented here
is an extension of our previous work on the fullerene-containing
nanocar3a,6 and the first step toward development of easily
accessible functional nanomachines to address molecular rolling
on surfaces. As shown in Figure 1, molecules 1-4 are designed
to move in specific patterns on the surface. Nanocar 1 and
nanocaterpillar 2 are expected to translate in a one-dimensional
fashion since the axles are parallel to each other. The difference
in the number of wheels will assist in the dimensional analysis
of the molecules sliding or rolling on the surface by STM.
Nanocar 3 was designed to make small circular motions on the
surface. This movement could be useful for monitoring surface
motion by using the STM.6a Trimer 4 was designed to pivot on
We report here the synthesis of four different nanovehicles:
nanocars 1 and 3, nanocaterpillar 2, and trimer molecule 4, all
bearing p-carboranes (Figure 1). p-Carborane was our choice
for the potential development of rolling molecules because of
its spherical shape and its aromatic nature8 that allow for easy
substitution reactions at one or both of the carbon atoms posi-
tioned para to each other.9 Moreover, they are robust, and unlike
fullerenes, are very soluble in common organic solvents, thus
allowing for the synthesis of smaller molecular machines in
(7) Morin, J.-F.; Shirai, Y.; Tour, J. M. Org. Lett. 2006, 8, 1713.
(8) (a) King, R. B. Chem. ReV. 2001, 101, 1119. (b) Chen, Z.; King, R.
B. Chem. ReV. 2005, 105, 3613.
(9) Valliant, J. F.; Guenter, K. J.; King, A. S.; Morel, P.; Schaffer, P.;
Sogbein, O. O.; Stephenson, K. A. Coord. Chem. ReV. 2002, 232, 173.
9482 J. Org. Chem., Vol. 72, No. 25, 2007

Synthetic Routes toward Carborane-Wheeled Nanocars
SCHEME 1. Synthesis of the Nanocar 1a
a Every vertex on the starting carborane is BH except the darkened sites that are CH. The product nanocar, 1, has the internal carborane carbon alkynyl-
substituted.
the surface with no translational movement, analogous to the
pattern observed for the related fullerene trimers.3a,6
bromoalkyne positions to give 1 in 68% yield. The nanocar 1
is quite soluble in common organic solvents such as chloroform,
acetone, THF, and toluene. Thermogravimetric analysis (TGA)
was performed on 1 to obtain information on its thermal stability.
At a scan rate of 20 deg/min (under N2), gradual decomposition
of 1 from its original mass was observed around 390 °C. This
result suggests that the alkyne-carborane bond is stronger and
more stable than the alkyne-fullerene bond, which showed
decomposition around 300 °C upon heating, and these data
will be essential as substrate heating6 is used to propel the
nanocars.
Results and Discussion
Synthesis. Two different strategies were used to synthesize
nanocar 1 and nanocaterpillar 2. For 1, the four wheels were
introduced at the end of the synthetic pathway on the tetra-
(bromoalkyne) moiety. On the other hand, the wheels on 2 were
coupled to the axle prior to the synthesis of the central part of
the chassis. This allowed us to compare both routes in terms of
synthetic feasibility and efficiency.
A CPK model of the planar conformation of 1 was generated
by using Spartan X. As shown in Figure 2, the wheel-to-wheel
distance for 1 is approximately 14 Å in both directions (parallel
and perpendicular to the axles), meaning that 1 is nearly a square
molecule. Although 1 is one of the simplest nanocars we can
synthesize, the square configuration limits its usefulness for
STM studies. Since we can only image the wheels and not the
inner chassis due to the relative differences in their density of
states, we will not be able to distinguish the orientation of 1 on
the surface. Thus addressing directionality will be even more
difficult.
In Scheme 1, the methoxy groups were installed to increase
the polarity of the molecules, which was necessary in order to
chromatographically separate the large excess of nonpolar
unreacted p-carborane from the desired product. The starting
compound 1,4-dimethoxy-2,5-bis(trimethylsilylacetylene)ben-
zene was synthesized in two steps following known proce-
dures.10,11 The alkynes were then deprotected. Due to the
unstable nature of the free alkyne intermediate, the deprotection
step was carried out immediately prior to the coupling with the
1-iodo-2,5-bis(trimethylsilyacetylene)benzene that was previ-
ously synthesized.7 The four TMS protecting groups were
removed by desilyl bromination12 to give 6 in good yield.
Finally, the p-carborane moieties were introduced at the four
To bypass this problem, nanocaterpillar 2, with three axles
rather than two, was synthesized. The CPK model (Figure 2)
of the planar conformation of 2 shows minimal spacing be-
tween the aligned carborane wheels. The UV data, explained
in the latter section, confirm the steric hindrance between
the wheels that might lead to problems for surface rolling.
The strategy used for the synthesis of 2 is depicted in
Scheme 2.
(10) Wariishi, K.; Morishima, S.; Inagaki, Y. Org. Process Res. DeV.
2003, 7, 98.
(11) Dirk, S. M.; Price, D. W.; Chanteau, S.; Kosynkin, D. V.; Tour, J.
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6, 3601.
J. Org. Chem, Vol. 72, No. 25, 2007 9483

Morin et al.
SCHEME 2. Synthesis of the Nanocaterpillar 2
SCHEME 3. Synthesis of the “Curved” Nanocar 3
Following known procedures,13,14 1,4-dibromo-2,5-bis(trim-
ethylsilylacetylene)benzene was synthesized in two steps
from 1,4-dibromobenzene. The bromides were replaced by
iodides by using tert-butyllithium followed by 1,2-diiodo-
(13) Hart, H.; Harada, K.; Du, C. J. F. J. Org. Chem. 1985, 50, 3104.
9484 J. Org. Chem., Vol. 72, No. 25, 2007
(14) Tovar, J. D.; Swager, T. J. Organomet. Chem. 2002, 653, 215.

Synthetic Routes toward Carborane-Wheeled Nanocars
SCHEME 4. Synthesis of the Trimer 4, Designed To Rotate about Its Center on the Surface
ethane. This step was necessary in order to perform the
Sonogashira coupling reaction between the iodide-containing
axle and the central chassis.15 The TIPS-acetylene groups were
introduced on 7 by standard Sonogashira coupling to provide 8
in good yield. Selective desilyl bromination of the TMS-
acetylene groups12b was achieved by using NBS and AgNO3 in
acetone to give 9 in almost quantitative yield. The p-carborane
wheels were introduced to give 10 followed by deprotection of
the TIPS-acetylene groups. The resulting 11 was then coupled
with wheel/axle (1-iodo-2,5-bis(p-carboraneacetylene)benzene)7
to give the nanocaterpillar 2. As with 1, 2 is also soluble in
common organic solvents.
hence Scheme 2 is preferable over Scheme 1. In this regard,
we synthesized nanocar 3 (Scheme 3), which is expected to
move in a circular motion due to its “curved” conformation
(Figure 1). The chassis 13 of nanocar 3 bearing two terminal
alkynes was synthesized in four straightforward steps.16 Com-
pound 13 was then coupled to the wheel/axle to give nanocar
3 in 67% yield.
The CPK models of 3 and 4 (Figure 3) were modeled by
Spartan X. Despite the “curved” feature of the inner chassis of
3, the model shows no overlapping of the inner carborane
wheels. Unlike 1, the distances perpendicular and parallel to
the axis are distinct. This will allow for an easier assessment of
molecular orientation and movement on the surface. Trimer 4
was designed to rotate on the surface with the center of rotation
coincident with the center of the molecule. A similar molecule,
bearing fullerene wheels instead of p-carborane, proved to be
The wheel/axle (1-iodo-2,5-bis(p-carboraneacetylene)ben-
zene)7 is a versatile tool and can be used in combination
with several different chassis to create nanovehicles having
specific conformations for accomplishing different tasks,
(15) Caution: It is known that 1,2-dialkynyl benzene derivatives can
decompose or explode violently upon heating.
(16) Wong, W.-Y.; Lu, G.-L.; Choi, K.-H.; Shi, J.-X. Macromolecules
2002, 35, 3506.
J. Org. Chem, Vol. 72, No. 25, 2007 9485

Morin et al.
FIGURE 3. CPK models of planar conformations of 3 and 4.
useful in studying the pivot-rolling mechanism with STM, since
it did not show translational movement on a gold surface.6a
To synthesize 4 (Scheme 4), we first tried a linear strategy
involving the successive addition of phenylacetylene moieties
onto a 1,3,5-trisubstituted benzene ring followed by addition
of the three carborane wheels. However, we found that desilyl
bromination of the long oligo(phenylene ethynylene) (OPE) gave
a low yield and led to various side products. Compound 4 was
instead synthesized by using a convergent strategy involving a
triangular central part and a carborane-containing substituted
phenylacetylene moiety. Thus, the triangular core was synthe-
sized in two steps from 1,3,5-dibromobenzene by using a
Sonogashira coupling reaction followed by a deprotection
reaction to provide 18 in 79% overall yield.17 Compound 18
was then coupled to 14, synthesized in three steps from
hydroquinone,18 using the Sonogashira coupling reaction to
afford 19 in 68% yield. The carborane-containing moiety was
synthesized in two steps from 1,4-bis(propyloxy)-2,5-diiodo-
benzene19 by desilyl bromination12 followed by reaction with
carborane-copper adduct as described above. In this case, the
latter reaction proceeded slowly and 48 h were necessary for
the reaction to be completed. This can be attributed to the
electron-donating nature of the propyloxy groups present on the
phenyl ring that partially deactivate the alkynyl bromide toward
oxidative addition. Compound 20 was then coupled to 16 to
give 4 in good yield.
Optical Properties. There are few reports on the optical
properties of carborane-containing conjugated molecules; there-
fore, the optical properties of molecules 1-4 were investigated
by using solution phase absorption and fluorescence spectros-
copy. As an alternative to STM, it is possible that the optical
properties could be exploited to image these molecules on a
nonmetallic surface by using fluorescence, hence these studies
are essential. As shown in Figure 4, 1-4 absorb light in the
regions λmax ) 375-410 nm, as commonly observed for OPEs
containing three phenyl rings.20 Compound 4 absorbs light at
the longest wavelength out of the four (λmax ) 410 nm), which
can be attributed to the electron-donating nature of the four
propyloxy groups and the increased conjugation length of the
FIGURE 4. UV-visible absorption spectra of 1-4 (1.0 × 10-5 M)
in chloroform.
OPE. Electron-donating substituents are known to increase the
energy of the HOMO level and, consequently, the band gap of
π-conjugated systems. A portion of this effect can be attributed
to the aromatic character of carborane,8 acting to extend the
conjugated length of the molecule. Next, compound 1 (λmax
)
385 nm) is further red-shifted than 2 and 3 due to again the
electron-donating methoxy groups. However, lower conjugation
of this molecule results in a shorter wavelength than that of
compound 4. Compound 3 contains no significant electron-
withdrawing groups, resulting in a shorter wavelength. The
interesting feature is that the spectrum of 3 shows a slight
absorbance at ∼362 nm, which may indicate a rigid conforma-
tion.21 On the other hand, the UV-visible spectrum of 2 is blue-
shifted compared to 1, 3, and 4. This blue shift can be attributed
to the steric hindrance between carborane wheels of different
axles, which leads to a high dihedral angle between phenyl rings
of the molecule’s core, thereby possibly limiting its usefulness
on a surface. However, the surface-carborane attraction might
be sufficient to make the system planar; therefore we are waiting
expectantly the surface analyses. Additionally, the UV-visible
spectrum of 2 shows two distinct maxima (353 and 375 nm)
indicating that 2 adopts a rigid conformation in its ground state.
This type of vibronic structure is frequently observed in cases
of rigid ladder-shaped molecules.21 The energy difference
between the two maxima (about 0.20 eV) is consistent with a
CdC stretching mode that would be expected to couple strongly
to the electronic structure.22 In comparison, 1, 3, and 4 do not
show significant vibronic structure, indicating that they have a
fairly flexible conformation in the ground state.
To determine the specific effect of the carboranes on the
optical properties of conjugated molecules, UV-visible spectra
of 1 and its TMS-substituted precursor 5 were compared. As
expected, the λmax of 1 (386 nm) is red-shifted compared to its
precursor 5 (379 nm). This can be attributed to an increase in
the conjugation length of the molecule containing carborane,
therefore underscoring the conjugation effect of the carboranes.
(17) Weber, E.; Hecker, M.; Koepp, E.; Orlia, W.; Czugler, M.; Csoregh,
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9486 J. Org. Chem., Vol. 72, No. 25, 2007

Synthetic Routes toward Carborane-Wheeled Nanocars
of these nanovehicles on a metallic surface. The synthesis of
more complex functional nanomachines is also underway.
Experimental Procedures
1
General Synthetic Methods. H NMR and 13C NMR spectra
were recorded at 400 and 100 MHz, respectively. Proton chemical
shifts (δ) are reported in ppm downfield from tetramethylsilane
(TMS). Mass spectrometry was performed at the Rice University
and University of South Carolina Mass Spectrometry Laboratory.
Infrared spectra (IR) assignments have 2 cm-1 resolution. Reagent
grade tetrahydrofuran (THF) and diethyl ether (Et2O) were distilled
from sodium and benzophenone under a N2 atmosphere. Triethy-
lamine (NEt2) and CH2Cl2 were distilled from CaH2 under N2
atmosphere unless otherwise stated. THF and NEt2 were degassed
with a stream of argon for 1 h before being used in the Castro-
Stephens-Sonogashira coupling. All other reagents were purchased
from commercial suppliers and used without further purification.
Trimethylsilylacetylene (TMSA) was donated by FAR Research
Inc. or Petra Research. n-BuLi 1.7 M in pentane, and t-BuLi 2.5
M in hexanes from Sigma-Aldrich Co. were used. Flash chroma-
tography was carried out with silica gel (grade 60, mesh size 230-
400, EM science). Thin layer chromatography (TLC) was performed
with use of glass silica gel plates (40 F254 0.25 mm layer thickness,
Merck). Melting points were measured on a Mel-Temp instrument
(uncorrected). All reactions were conducted under a dry oxygen-
free atmosphere with oven-dried glassware unless otherwise stated.
PdCl2(PPh3)2,1 1,4-dimethoxy-2,5-diiodobenzene,5 1,4-dimethoxy-
2,5-bis(trimethylsilylacetylene)benzene,6 1,4-dibromo-2,5-diiodo-
benzene,7 1,4-dibromo-2,5-bis(trimethylsilylacetylene)benzene,8 and
1,4-bis(propyloxy)-2,5-diiodobezene9 were prepared using literature
procedures.
FIGURE 5. Fluorescence spectra of 1-4 (1.0 × 10-7 M) in
chloroform.
TABLE 1. Optical Properties of Compounds 1-4
a
a
compd
Abs (λmax
)
Emi (λmax
)
Eg (eV)b
1
2
3
4
386
375 (353)
379
426
389 (411)
414
2.89
3.14
3.05
2.85
399
427 (451)
aSpectra in chloroform. bBand gap (optical), determined from the lower
energy onset of the absorption spectra.
General Procedure for the Palladium-Catalyzed Coupling
Reaction of Terminal Alkynes and Aryl Bromides or Aryl
Iodides (Castro-Stephens-Sonogashira Coupling). An oven-
dried round-bottomed flask equipped with a magnetic stir bar was
charged with the terminal alkyne (1 equiv), aryl halide (1 equiv),
PdCl2(PPh3)2 (3-5 mol % per halide), CuI (6-10 mol % per
halide), triethylamine (4 equiv per halide), and THF ([aryl halide]
) 0.1-0.3 M). If the halide was an aryl iodide, the mixture was
stirred at room temperature for 24 h. In the case of an aryl bromide,
PPh3 (6-10 mol % per halide) was added and the mixture was
stirred at 70 °C for the same period of time. After that period,
saturated NH4Cl was added and the mixture was extracted twice
with dichloromethane. The combined organic layers were washed
with water and dried over MgSO4. The mixture was filtered, the
solvent was removed, and the desire product was isolated with
column chromatography (silica gel as stationary phase) to provide
the product.
General Procedure for Deprotection of Trimethylsilyl-
Protected Alkynes. In a round-bottomed flask equipped with a
magnetic stir bar, the protected alkyne was dissolved in a mixture
of THF and MeOH ([protected alkyne] ) 0.05-0.1 M). Then, K2-
CO3 (2 equiv per alkyne) was added. The mixture was stirred at
room temperature for 2 h or until the reaction was complete
(monitored by TLC). After that period, brine was added and the
mixture was extracted twice with dichloromethane. The combined
organic layers were washed with water and dried over MgSO4. The
solvent was removed and the desired product was isolated with
column chromatography (silica gel as the stationary phase) to
provide the product.
The electron-withdrawing nature of carborane can be part of
an intramolecular charge-transfer complex with the 1,4-
dimethoxy moiety of the core, decreasing the band gap.
Furthermore, the introduction of carborane does not induce
significant steric hindrance between the two axles since both
spectra have a similar shape with no vibronic structure.
The fluorescence properties of 1-4 were investigated with
the results summarized in Figure 5 and Table 1. As expected,
all four compounds fluoresce in chloroform in the UV-blue
region. The wavelength of the emission maxima increases in
the order of 2 < 3 < 1 < 4 reflecting the extent of conjugation
plus the influence of electron-rich moieties of the propoxyl,
methoxyl, and butyl carbazole groups. A reversed result is seen
with the band gap (optical, Eg, from the lower energy onset of
the absorption spectra) in the order of 4 < 1 < 3 < 2, as
expected. Interestingly, 2 and 4 show fine vibronic structure,
generally associated with a rigid conformation in the excited
state. The small Stoke shift observed for compound 2 (14 nm)
is characteristic of a molecule having a rigid conformation.23
Table 1 summarized the optical data.
Conclusion
The design and the synthesis of four potential nanovehicles
bearing p-carborane as wheels is reported. The use of p-
carboranes overcomes several synthetic problems observed in
fullerene-wheel nanovehicles: no long-chain alkyl groups are
necessary to obtain soluble structures and addition of the wheels
at different stages in the synthesis is possible. STM imaging
analysis of the 1-4 is underway to investigate the movement
1,4-Dimethoxy-2,5-bis[(2′,5′-trimethylsilylacetylenebenzene)-
acetylenebenzene] (5). A 100 mL round-bottomed flask equipped
with a magnetic stirrer was charged with 1,4-dimethoxy-2,5-
(trimethylsilylacetylene)benzene10,11 (0.75 g, 2.27 mmol), K2CO3
(1.26 g, 9.07 mmol), THF (25 mL), and MeOH (25 mL). The
mixture was stirred at room temperature for 2 h and poured into
water. The mixture was extracted three times with dichloromethane
and the combined organic layers were washed twice with water
(23) Michels, J. J.; O’Connell, M. J.; Taylor, P. N.; Wilson, J. S.; Cacialli,
F.; Anderson, H. L. Chem. Eur. J. 2003, 9, 6167.
J. Org. Chem, Vol. 72, No. 25, 2007 9487

Morin et al.
and dried over MgSO4. The solvent was removed under reduced
pressure to provide a yellow solid that was used for the Sonogashira
coupling without further purification. See general procedure for
Castro-Stephens-Sonogashira coupling. The compounds used
were 1,4-dimethoxy-2,5-diacetylenebenzene (100 mg, 0.54 mmol),
5 (639 mg, 1.61 mmol), CuI (10.2 mg, 0.05 mmol), PdCl2(PPh3)2
(18.8 mg, 26.8 µmol), well-degassed dry triethylamine (0.6 mL),
and THF (5 mL) at room temperature for 16 h. The resulting orange
brown solid was purified by column chromatography (silica gel,
25% dichloromethane in hexanes as eluent) to provide 300 mg of
the desired product as a white amorphous solid (77%): mp 199-
201 °C; IR (KBr) 2955, 2155, 1503, 1248, 843, 858, 759 cm-1; 1H
NMR (400 MHz, CDCl3, ppm) δ 7.67 (dd, J ) 1.6 and 0.4 Hz,
2H), 7.43 (dd, J ) 8.1 and 0.4 Hz, 2H), 7.33 (dd, J ) 8.1 and 1.7
Hz, 2H), 7.03 (s, 2H), 3.88 (s, 6H), 0.253 (s, 18H), 0.251 (s, 18H);
13C NMR (100 MHz, CDCl3, ppm) δ 154.2, 135.5, 132.3, 131.3,
126.3, 125.3, 123.3, 116.1, 113.8, 103.9, 103.3, 100.8, 97.1, 93.0,
90.2, 56.7, 0.1, 0.0; HRMS calcd for C44H50O2Si4 722.2888, found
722.2884.
sodium bisulfite and water and dried over MgSO4. The solvent was
removed under reduced pressure and the resulting orange solid was
purified by column chromatography (silica gel, hexanes as eluent)
to provide 1.93 g of the title product as a white waxy solid (61%):
mp 129-131 °C; IR (KBr) 1453, 1248, 1048, 887, 860, 837, 796,
761 cm-1; 1H NMR (400 MHz, CDCl3, ppm) δ 7.87 (s, 2H), 0.27
(s, 18H); 13C NMR (100 MHz, CDCl3, ppm) δ 141.8, 131.0, 104.8,
102.3, 99.7, -0.1; HRMS calcd for C16H20I2Si2 521.9193, found
521.9191.
1,4-Bis(triisopropylsilylacetylene)-2,5-bis(trimethylsilylacety-
lene)benzene (8). See the general procedure for Castro-Stephens-
Sonogashira coupling. The compounds used were 7 (1.20 g, 2.30
mmol), triisopropylsilylacetylene (1.24 mL, 5.51 mmol), CuI (43.8
mg, 0.23 mmol), PdCl2(PPh3)2 (80.6 mg, 0.11 mmol), well-degassed
dry triethylamine (2.6 mL), and THF (25 mL) at room temperature
for 16 h. The resulting brown oil was purified by column
chromatography (silica gel, hexanes as eluent) to provide 1.25 g
of the title product as a pale yellow solid (86%): mp 152-154 °C;
IR (KBr) 2959, 2943, 2865, 1481, 1249, 1186, 876, 842, 769, 759
1
cm-1; H NMR (400 MHz, CDCl3, ppm) δ 7.53 (s, 2H), 1.15 (s,
1,4-Dimethoxy-2,5-bis[(2′,5′-bromoacetylenebenzene)acetyle-
nebenzene] (6). A 25 mL round-bottomed flask equipped with a
magnetic stirrer was charged with 5 (100 mg, 0.14 mmol) and
acetone (10 mL). Then, freshly purified and dried N-bromosuccin-
imide (113 mg, 0.64 mmol) and AgNO3 (2.3 mg, 13.8 µmol) were
added in order. The mixture was stirred in the dark at room
temperature for 2 h and poured into methanol (100 mL). After 5
min of vigorous stirring, the yellow precipitate formed was collected
by filtration, rinsed thoroughly with water followed by MeOH, and
dried under reduced pressure for 24 h to provide 101 mg of 6 as a
bright yellow solid (97%): mp >250 °C; IR (KBr) 1509, 1482,
42H), 0.24 (s, 18H); 13C NMR (100 MHz, CDCl3, ppm) δ 137.2,
125.4, 125.3, 104.2, 102.4, 100.6, 97.5, 19.0, 11.5, 0.1; HRMS calcd
for C38H62Si4 630.3929, found 630.3940.
1,4-Bis(triisopropylsilylacetylene)-2,5-bis(bromoacetylene)-
benzene (9). A 25 mL round-bottomed flask equipped with a
magnetic stirrer was charged with 8 (675 mg, 1.07 mmol) and
acetone (10 mL). Freshly purified and dried N-bromosuccinimide
(438 mg, 2.46 mmol) and silver(I) nitrate (27.2 mg, 0.16 mmol)
were added, in that order. The mixture was stirred in the dark at
room temperature for 2 h and poured into water (100 mL). After 5
min of vigorous stirring, the white precipitate formed was collected
by filtration, rinsed thoroughly with water, and dried under reduced
pressure for 24 h to provide 679 mg of 9 as a white solid (98%):
mp 170-175 °C; IR (KBr) 2955, 2942, 2863, 1481, 1460, 1016,
1
1458, 1381, 1281, 1222, 1208, 1037, 830 cm-1; H NMR (400
MHz, CDCl3, ppm) δ 7.65 (d, J ) 1.4 Hz, 2H), 7.43 (d, J ) 8.4
Hz, 2H), 7.32 (dd, J ) 8.1 and 1.6 Hz, 2H), 7.07 (s, 2H), 3.95 (s,
6H). This compound was not soluble enough for 13C NMR analysis;
HRMS calcd for C32H14Br4O2 750.12, found both electrospray and
direct exposure probe failed.
1
902, 875, 779, 680, 655 cm-1; H NMR (400 MHz, CDCl3, ppm)
δ 7.5 (s, 2H), 1.1 (s, 42H); 13C NMR (100 MHz, CDCl3, ppm) δ
136.1, 126.1, 125.5, 103.6, 98.3, 78.0, 56.5, 18.8, 11.4; HRMS calcd
for C32H44Br2Si2 644.1331, found 644.1329.
Nanocar 1. An oven-dried 25 mL round-bottomed flask equipped
with a magnetic stirrer was charged with p-carborane (144 mg, 1.00
mmol) and dry THF (3 mL). The solution was cooled to -78 °C
and n-BuLi (0.41 mL, 1.02 mmol) was added dropwise. The
solution was allowed to warm to room temperature and stirred for
30 min before it was cooled again at -78 °C. Copper(I) bromide
(179 mg, 1.25 mmol) was then added and the mixture was allowed
to warm at room temperature for 30 min. A solution of 6 (75 mg,
1.00 µmol) in dry THF (2 mL) was then added and the resulting
mixture was allowed to stir at room temperature for 16 h. A few
drops of water were added and the mixture was filtered on a silica
gel pad with dichloromethane as eluent. The resulting greenish solid
was purified by column chromatography (silica gel, 15% dichlo-
romethane in hexanes as eluent) to provide 68 mg of the title product
as a white powder (68%): mp >250 °C; IR (KBr) 2613, 1504,
1,4-Bis(triisopropylsilylacetylene)-2,5-bis(1′,12′-dicarba-closo-
dodecaborane)benzene (10). An oven-dried 50 mL round-bot-
tomed flask equipped with a magnetic stirrer was charged with
p-carborane (246 mg, 1.71 mmol) and dry THF (20 mL). The
solution was cooled to -78 °C and n-BuLi (0.68 mL, 1.71 mmol,
2.5 M in hexanes) was added dropwise. The solution was allowed
to warm to room temperature and stirred for 30 min before it was
cooled again at -78 °C. Copper(I) bromide (267 mg, 1.86 mmol)
was then added and the mixture was allowed to warm to room
temperature for 30 min. A solution of compound 9 (500 mg, 0.78
mmol) in THF (5 mL) was then added and the resulting mixture
was allowed to stir at room temperature for 16 h. A few drops of
water were added and the mixture was filtered on silica gel pad
with dichloromethane as eluent. The resulting greenish solid was
purified by column chromatography (silica gel, hexanes as eluent)
to provide 425 mg of 10 as a white powder (71%): mp >250 °C;
1
1410, 1222, 1063, 820, 726 cm-1; H NMR (400 MHz, CDCl3,
ppm) δ 7.48 (d, J ) 1.5 Hz, 2H), 7.23 (d, J ) 8.1 Hz, 2H), 7.15
(dd, J ) 8.1 and 1.6 Hz, 2H), 7.07 (s, 2H), 4.00 (s, 6H), 3.34-
1.47 (br m, 44H); 13C NMR (100 MHz, CDCl3, ppm) δ 154.2,
135.5, 132.3, 131.1, 126.7, 124.0, 122.1, 115.8, 113.5, 92.1, 91.2,
90.9, 88.0, 78.1, 78.0, 69.6, 69.3, 60.5 (2C), 56.6; HRMS calcd
for C40H58B40O2 1003.8445, found 1003.9116.
1,4-Diiodo-2,5-bis(trimethylsilylacetylene)benzene (7). An oven-
dried 250 mL round-bottomed flask equipped with a magnetic stirrer
was charged with 1,4-dibromo-2,5-bis(trimethylsilylacetylene)-
benzene8 (2.59 g, 6.05 mmol), diethyl ether (60 mL), and THF (60
mL). The mixture was cooled -78 °C and t-BuLi (16.0 mL, 27.2
mmol) was added over 15 min. The resulting deep red solution
was stirred for 1 h at -78 °C and 1,2-diiodoethane (5.45 g, 19.4
mmol) was added quickly. The solution was stirred for 1 h at
-78 °C and an additional 16 h at room temperature. The red
solution was poured into water and extracted twice with dichlo-
romethane. The combined organic layers were washed with 0.1 M
IR (KBr) 2943, 2865, 2614, 1488, 1064, 882, 841, 708, 675 cm-1
;
1H NMR (400 MHz, CDCl3, ppm) δ 7.28 (s, 2H), 3.33-1.42 (m,
22H), 1.14 (s, 42H); 13C NMR (100 MHz, CDCl3, ppm) δ 137.5,
125.4, 123.8, 103.3, 98.6, 91.0, 69.4, 60.6, 19.0, 11.5; HRMS calcd
for C36H66B20Si2 771.6703, found 771.6713.
1,4-Bis(acetylene)-2,5-bis(1′,12′-dicarba-closo-dodecaborane)-
benzene (11). An oven-dried 50 mL round-bottomed flask equipped
with a magnetic stirrer was charged with 10 (300 mg, 0.39 mmol),
THF (40 mL), and tetrabutylammonium fluoride (TBAF) (1.94 mL,
1.94 mmol, 1.0 M in THF, Aldrich Co.). The resulting solution
was stirred at room temperature for 4 h and poured into water (ca.
100 mL). The mixture was extracted twice with dichloromethane
and the combined organic layers were washed with water. The
organic layer was dried over MgSO4 and the solvent was removed
under reduced pressure leaving an off-white powder (11) that was
9488 J. Org. Chem., Vol. 72, No. 25, 2007

Synthetic Routes toward Carborane-Wheeled Nanocars
used for the next step without further purification (73%): mp
1262, 1214, 1060, 1048, 1009, 974, 853, 837, 731; 1H NMR (400
MHz, CDCl3, ppm) δ 7.27 (s, 1H), 6.82 (s, 1H), 3.90 (m, 4H),
1.82 (m, 4H), 1.06 (q, J ) 7.5 Hz, 6H); 13C NMR (100 MHz,
CDCl3, ppm) δ 155.1, 151.8, 124.1, 116.6, 113.1, 88.3, 76.4, 71.69,
71.63, 54.1, 22.7 (2C), 10.9, 10.6; HRMS calcd for C14H16BrIO2
421.9378, found 421.9374.
>250 °C; IR (KBr) 2617, 1488, 1062, 906, 727, 655, 626 cm-1
;
1H NMR (400 MHz, CDCl3, ppm) δ 7.36 (s, 2H), 3.34 (s, 2H),
3.25-1.40 (m, 22H); 13C NMR (100 MHz, CDCl3, ppm). This
compound is not soluble enough for 13C NMR analysis; HRMS
calcd for C18H26B20 458.4052, found 458.4040.
Nanocaterpillar 2. See general procedure for Castro-Stephens-
Sonogashira coupling. The compounds used were 11 (65.0 mg, 0.14
mmol), 1-iodo-2,5-bis(p-carboraneacetylene)benzene7 (232 mg, 0.43
mmol), CuI (2.74 mg, 14.4 µmol), PdCl2(PPh3)2 (5.1 mg, 7.21
µmol), well-degassed dry triethylamine (0.2 mL), and THF (2 mL)
at room temperature for 16 h. The resulting brown oil was purified
by column chromatography (silica gel, 5% dichloromethane in
hexanes as eluent) to provide 98 mg of the title product as a yellow
solid (54%): mp >250 °C; IR (KBr) 2612, 1492, 1062, 901, 831,
727, 705 cm-1; 1H NMR (400 MHz, CDCl3, ppm) δ 7.50 (s, 2H),
7.47 (d, J ) 1.1 Hz, 2H), 7.23 (d, J ) 8.2 Hz, 2H), 7.18 (dd, J )
8.2 and 1.5 Hz, 2H), 3.45-1.43 (m, 66H); 13C NMR (100 MHz,
CDCl3, ppm) δ 135.6, 135.4, 132.1, 131.6, 125.9, 125.6, 124.3,
124.2, 122.2, 92.8, 91.7, 91.6, 91.1, 88.0, 77.9, 77.5, 77.3, 69.3
(2C), 60.7 (2C), 60.4 (2C); MALDI-TIF MS m/z (no natrix; positive
ion mode) calcd for C46H74B60 1287.24, found 1287.
1,4-Bis(propyloxy)-2-iodo-5-(1′,12′-dicarba-closo-dodecabo-
rane)benzene (16). An oven-dried 50 mL round-bottomed flask
equipped with a magnetic stir bar was charged with p-carborane
(135 mg, 0.94 mmol) and THF (20 mL). The solution was cooled
to -78 °C and n-BuLi (0.39 mL, 0.98 mmol, 2.5 M in hexanes)
was added dropwise. The solution was allowed to warm to room
temperature and stirred for 30 min before it was cooled again to
-78 °C. Copper(I) bromide (146 mg, 1.02 mmol) was then added
and the mixture was allowed to stir at room temperature for 30
min. A solution of 15 (330 mg, 0.78 mmol) in THF (10 mL) was
then added and the resulting mixture was allowed to stir at room
temperature for 48 h. A few drops of water were added and the
mixture was filtered through a silica gel pad with dichloromethane
as the eluent. The resulting greenish solid was purified by column
chromatography (silica gel, 20% dichloromethane in hexanes as
eluent) to provide 288 mg of 16 as a white powder (76%): mp
96-98 °C; IR (KBr) 2614, 1496, 1486, 1463, 1383, 1212, 1064,
1026, 979 cm-1; 1H NMR (400 MHz, CDCl3, ppm) δ 7.19 (s, 1H),
6.63 (s, 1H), 3.83 (m, 4H), 3.39-1.39 (br m, 15H), 1.05 (t, J )
7.4 Hz, 6H); 13C NMR (100 MHz, CDCl3, ppm) δ 155.2, 151.8,
124.0, 116.1, 111.8, 90.3, 88.9, 76.0, 71.9, 71.4, 60.18, 60.15, 22.94,
22.85, 11.0, 10.8. HRMS calcd for C16H27B10IO2 488.1986, found
488.1984.
1,3,5-Tris[1′,4′-bis(propyloxy)-2′-(trimethylsilylacetylene)-5′-
acetylenebenzene] (19). See general procedure for Castro-
Stephens-Sonogashira coupling. The compounds used were 1817
(100 mg, 0.68 mmol), 14 (1.13 g, 2.72 mmol), CuI (16 mg, 84
µmol), PdCl2(PPh3)2 (29 mg, 41 µmol), well-degassed triethylamine
(2 mL), and THF (7 mL) at room temperature for 16 h. The resulting
orange oil was purified by column chromatography (silica gel, 30%
dichloromethane in hexanes as eluent) to provide 471 mg of 19
as a yellow solid (68%): mp 158-160 °C; IR (KBr) 2962, 2151,
1500, 1387, 1240, 1214, 860, 840, 758 cm-1; 1H NMR (400 MHz,
CDCl3, ppm) δ 7.62 (s, 3H), 6.97 (s, 3H), 6.96 (s, 3H), 3.96 (m,
12H), 1.85 (m, 12H), 1.09 (td, J ) 7.4 and 2.3 Hz, 18H), 0.27 (s,
27H); 13C NMR (100 MHz, CDCl3, ppm) δ 154.6, 154.0, 134.4,
124.6, 117.7, 117.6, 114.68, 114.67, 114.2, 101.5, 100.8, 93.6, 87.5,
71.53, 71.49, 23.1, 11.01, 11.00, 0.4; MALDI-TIF MS m/z (no
matrix; positive ion mode) calcd for C63H78O6Si3 1014.51, found
1015.
Nanocar 3. See general procedure for Castro-Stephens-
Sonogashira coupling. The compounds used were 1316(75 mg, 0.28
mmol), 1-iodo-2,5-bis(p-carboraneacetylene)benzene7 (408 mg, 0.76
mmol), CuI (4.2 mg, 22.1 µmol), PdCl2(PPh3)2 (7.8 mg, 11.0 µmol),
well-degassed triethylamine (0.5 mL), and THF (3.0 mL) at room
temperature for 16 h. The white precipitate formed during the course
of the reaction was filtered, rinsed thoroughly with methanol, and
recrystallized twice from ethyl acetate to give 202 mg of 3 as a
white solid (67%): mp >250 °C; IR (KBr) 2613, 2209, 1592, 1492,
1
1212, 1063, 804, 727 cm-1; H NMR (400 MHz, CDCl3, ppm)
δ 8.40 (s, 2H), 7.67 (d, J ) 7.6 Hz, 2H), 7.43 (d, J ) 8.5 Hz, 2H),
7.39 (s, 2H), 7.24 (d, J ) 8.2 Hz, 2H), 7.11 (dd, J ) 8.0 and
1.3 Hz, 2H), 4.35 (t, J ) 7.0 Hz, 2H), 3.45-1.37 (br m, 48H),
1.00 (t, J ) 7.3 Hz, 3H); 13C NMR (100 MHz, CDCl3, ppm) δ
141.2, 135.0, 132.60, 132.58, 129.9, 127.5, 125.4, 124.0, 123.0,
122.2, 113.8, 110.0, 109.5, 96.5, 91.2, 88.0, 85.6, 78.44, 78.39,
77.6, 68.4, 60.7 (4C), 43.7, 31.5, 26.0, 21.0, 14.3; MALDI-TIF
MS m/z (no matrix; positive ion mode) calcd for C48H65B40N
1095.88, found 1096.
1,4-Bis(propyloxy)-2-iodo-5-trimethylsilylacetylenebenzene (14).
See general procedure for Castro-Stephens-Sonogashira coupling.
The compounds used were 1,4-bis(propyloxy)-2,5-diiodobenzene9
(3.35 g, 7.51 mmol), TMSA (1.06 mL, 7.51 mmol), CuI (86 mg,
0.45 mmol), PdCl2(PPh3)2 (158 mg, 0.23 mmol), well-degassed
triethylamine (5 mL), and THF (75 mL) at room temperature for
16 h. The resulting brown oil was purified by column chromatog-
raphy (silica gel, 8% dichloromethane in hexanes as eluent) to
provide 1.77 g of 14 as a yellow solid (56%): mp 34-36 °C; IR
1,3,5-Tris[1′,4′-bis(propyloxy)-2′-(acetylene)-5′-acetyleneben-
zene] (20). See general procedure for deprotection of TMS-protected
alkyne. The compounds used were 19 (350 mg, 0.35 mmol),
K2CO3 (573 mg, 4.15 mmol), THF (5 mL), and methanol (5 mL)
at room temperature for 2 h. The resulting crude yellowish
solid was dissolved in a minimum amount of dichloromethane and
poured into 100 mL of cold methanol. The bright yellow solid
was collected by filtration and dried under reduced pressure to
provide 225 mg of 20 as a bright yellow solid (82%): mp 140-
142 °C; IR (KBr) 2963, 1578, 1500, 1420, 1388, 1275, 1218, 851
1
(KBr) 2149, 1495, 1464, 1376, 1248; 1216, 860, 845 cm-1; H
NMR (400 MHz, CDCl3, ppm) δ 7.26 (s, 1H), 6.84 (s, 1H), 3.90
(m, 4H), 1.81 (m, 4H), 1.07 (t, 6H, J ) 7.4 Hz), 0.25 (9H); 13C
NMR (100 MHz, CDCl3, ppm) δ 155.3, 152.1, 124.4, 116.7, 113.9,
101.2, 99.9, 88.3, 72.0, 71.8, 23.1, 23.0, 11.2, 10.9, 0.4; HRMS
calcd for C17H25IO2Si 416.0669, found 416.0671.
1
cm-1; H NMR (400 MHz, CDCl3, ppm) δ 7.63 (s, 3H), 6.99 (s,
1,4-Bis(propyloxy)-2-iodo-5-bromoacetylenebenzene (15). A
25 mL round-bottomed flask equipped with a magnetic stir bar was
charged with 14 (400 mg, 0.96 mmol) and acetone (12 mL). Then,
freshly purified and dried N-bromosuccinimide (205 mg, 1.15
mmol) and silver(I) nitrate (16 mg, 96 µmol) were added. The
mixture was stirred in the dark at room temperature for 2 h and
poured into water (100 mL). The resulting slurry was extracted
twice with dichloromethane and the combined organic layers were
dried with MgSO4. The solvent was removed under reduced
pressure and the resulting orange-red oil solid was purified by
column chromatography (silica gel, 10% dichloromethane in
hexanes as eluent) to provide 330 mg of 15 as an orange oil
(81%): IR (KBr) 2961, 2933, 2872, 2200, 1586, 1487, 1461, 1375,
6H), 3.98 (m, 12H), 3.36 (s, 3H), 1.86 (m, 12H), 1.08 (m, 18H);
13C NMR (100 MHz, CDCl3, ppm) δ 154.3, 153.7, 134.2, 124.3,
118.1, 117.3, 114.3, 113.3, 93.4, 87.1, 82.8, 80.1, 71.4, 71.3, 22.9,
22.8, 10.8, 10.7; HRMS calcd for C54H54O6 799.3998, found
799.3990.
4. See general procedure for Castro-Stephens-Sonogashira
coupling. The compounds used were 20 (95 mg, 0.12 mmol), 16
(233 mg, 0.48 mmol), CuI (2.2 mg, 12 µmol), PdCl2(PPh3)2 (5.0
mg, 7.2 µmol), well-degassed triethylamine (0.2 mL), and THF
(1.5 mL) at room temperature for 16 h. The resulting yellow solid
was purified by column chromatography (silica gel, 40% dichlo-
romethane in hexanes as eluent) to provide 129 mg of 4 as a yellow
J. Org. Chem, Vol. 72, No. 25, 2007 9489

Morin et al.
solid (58%): mp 166-168 °C; IR (KBr) 2963, 2611, 1505, 1423,
Acknowledgment. We thank the NSF (Penn State MRSEC),
NSF NIRT (ECCS-070-8765), the Welch Foundation, and
Honda for funding, and FAR Research (Dr. I. Chester) and Petra
Research (Dr. R. Awartani) for TMSA. J.-F. Morin thanks
NSERC (Canada) and FQRNT (Que´bec) for a postdoctoral
fellowship.
Supporting Information Available: 1H and 13C NMR spectra
for compounds 1-11, 14-16, 19, and 20. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
1385, 1274, 1215, 1062 cm-1; H NMR (400 MHz, CDCl3, ppm)
δ 7.63 (s, 3H), 7.011 (s, 3H), 7.006 (s, 3H), 6.92 (s, 3H), 6.76 (s,
3H), 3.99 (m, 12H), 3.94 (t, J ) 6.5 Hz, 6H), 3.86 (t, J ) 6.2 Hz,
6H), 3.30-1.51 (br m, 68H), 1.08 (m, 36H); 13C NMR (100 MHz,
CDCl3, ppm) δ 154.6, 154.0, 153.7, 153.5, 134.2, 124.5, 117.60,
117.56, 117.4, 117.3, 115.4, 114.9, 113.9, 112.2, 93.6, 92.0, 91.7,
91.0, 87.4, 76.5, 71.5, 71.4, 71.2, 60.3, 60.2, 23.04, 23.01, 22.98,
10.9; MALDI-TIF MS m/z (no matrix; positive ion mode) calcd
for C102H132B30O12 1879.25, found 1879.
JO701400T
9490 J. Org. Chem., Vol. 72, No. 25, 2007