Rigid molecular tripod with an adamantane framework and thiol legs. Synthesis and observation of an ordered monolayer on Au(111)

Article abstract of DOI:10.1021/jo051863j

Tripod-shaped trithiols 1-3, containing CH₂SH groups at the three bridgehead positions of the adamantane framework and a halogen-containing group [Br (1), p-BrC₆H₄ (2), or p-IC₆H ₄ (3)] at the fourth

Full text of DOI:10.1021/jo051863j

Rigid Molecular Tripod with an Adamantane Framework and Thiol
Legs. Synthesis and Observation of an Ordered Monolayer on
Au(111)
Toshikazu Kitagawa,*,† Yuichi Idomoto,† Hiroaki Matsubara,† Daisuke Hobara,‡
Takashi Kakiuchi,‡ Takao Okazaki,‡ and Koichi Komatsu*,†
Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan, Department of Energy and
Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto UniVersity, Kyoto 615-8510, Japan, and
CREST, Japan Science and Technology Agency (JST)
kitagawa@scl.kyoto-u.ac.jp
ReceiVed September 6, 2005
Tripod-shaped trithiols 1-3, containing CH2SH groups at the three bridgehead positions of the adamantane
framework and a halogen-containing group [Br (1), p-BrC6H4 (2), or p-IC6H4 (3)] at the fourth bridgehead,
were synthesized, and self-assembled monolayers (SAMs) were prepared on atomically flat Au(111)
surfaces. The three-point chemisorption of these tripods was confirmed by polarization modulation infrared
reflection absorption spectroscopy, which showed the absence of a S-H stretching band. Scanning
tunneling microscopy of the SAM of 1 exhibited a hexagonal arrangement of the adsorbed molecule
with a lattice constant of 8.7 Å. A unidirectionally oriented, head-to-tail array of 1, which allows the
close approach of neighboring molecules, is proposed as a reasonable model of the two-dimensional
crystal, where the adsorbed sulfur atoms form a quasi-(x3 × x3)R30° lattice. The charge of the
electrochemical reductive desorption of the SAM of 1 was in good agreement with the expected surface
coverage, while the SAMs of 2 and 3 showed somewhat less (ca. 70%) charge. The large negative reduction
peak potentials, observed for the SAM of 1, are taken to indicate a tight anchoring of this tripod by three
sulfur atoms.
Introduction
formation of tight S-Au bonds has attracted a great deal of
interest in recent years, since it is an effective approach for the
construction of a variety of functional molecular assemblies,
including unimolecular electronic devices2-9 and molecular
machines.10-13 One of the requirements for such applications
is the appropriate design of the anchoring point, which ensures
a fixed angle of the adsorbed molecule relative to the gold
surface and sufficient molecular separation to avoid possible
undesirable intermolecular interactions. Tripod-shaped mol-
Thiols and disulfides have a strong tendency to be chemi-
sorbed to a gold surface, forming self-assembled monolayers
(SAMs).1 The importance and utility of the spontaneous
† Institute for Chemical Research, Kyoto University and CREST, Japan
Science and Technology Agency.
‡ Graduate School of Engineering, Kyoto University.
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10.1021/jo051863j CCC: $33.50 © 2006 American Chemical Society
Published on Web 01/20/2006
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J. Org. Chem. 2006, 71, 1362-1369

Rigid Molecular Tripod with Adamantane Framework
ecules, having three legs and a binding group at each end of
the legs, are strong candidates for satisfying such a requirement.
It would, therefore, be of interest to determine if it is possible
for such a molecule, especially one with three SH groups, to
serve as a rigid anchor in the construction of highly ordered
SAMs. Several molecular tripods with three surface-binding legs
have been synthesized using the tetrahedral configuration of sp3-
hybridized carbon14-20 or silicon12,21,22 atom as a core structural
component. Other tripods have utilized adamantane, in which
the four bridgehead bonds have a tetrahedral orientation.16,17,23-26
Despite the well-defined geometry of these tripods, it appears
rather difficult to obtain a monolayer of such molecules with a
highly ordered, two-dimensional structure, due to insufficient
van der Waals interactions between neighboring molecules, an
important factor in self-assembly, and to the increased freedom
of orientation of each triangular molecule compared to rodlike
monothiols. To the best of our knowledge, the observation of
the ordered SAM of a tripod shaped molecule has not been
reported.
Here, we report on the synthesis of trithiol 1 and its haloaryl
derivatives 2 and 3, in which the C-Br or C-I bond, because
of the rigid adamantane framework, is fixed in a perpendicular
orientation to the surface of the gold substrate. Analyses of these
SAMs on Au(111) by means of infrared reflection absorption
spectroscopy, scanning tunneling microscopy (STM), and cyclic
voltammetry demonstrated that these molecules are indeed
adsorbed to the gold surface by three sulfur atoms per molecule
and that the SAM of 1, in particular, forms a two-dimensional
crystal, for which the lattice constants could be determined.
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Results and Discussion
Synthesis. Scheme 1 shows the synthetic route to tripod 1.
The photochemical carbonylation27 of the bridgehead C-H
bonds of 1-bromoadamantane gave triester 4, which was
converted to triol 5 by reduction with lithium aluminum hydride.
Conversion of this primary alcohol to a chloride or bromide by
treatment with SOCl2, SOBr2, or PBr3 was not successful
because these reactions involve nucleophilic substitution by
chloride or bromide ion at the sterically congested, neopentyl-
type carbon. On the other hand, the triol could be cleanly
converted to the tritriflate 6. With its excellent leaving group
characteristics, all of the three CF3SO3 groups in 6 could be
transformed to acetylthio groups by treatment with potassium
thioacetate in the presence of a crown ether, which was added
to enhance the nucleophilic reactivity of AcS-. Alkaline
hydrolysis of the trithioacetate 7 gave trithiol 1 as air-stable
crystals.
The introduction of a 4-bromophenyl group at C-1 was
achieved by the silver ion-promoted ionization of the C-Br
bond of tritriflate 6 and subsequent trapping of the carbocationic
intermediate with bromobenzene (Scheme 2). The three CF3SO3
groups of 8 were then converted to SH groups via trithioacetate
9 in a manner similar to that described above.
On the other hand, the reaction of tritriflate 6 with iodoben-
zene in the presence of AgSbF6 was sluggish, requiring 8 weeks
for a complete reaction in refluxing CH2Cl2. Alternatively, the
4-iodophenyl derivative 11 could be synthesized by a stepwise
conversion via the phenyl derivative 10 (Scheme 3). In the
iodination of 10 using PhI(OCOCF3)2/I2,28 a byproduct, 2,4-
diiodophenyl derivative 12, was formed, which could be
separated by recrystallization. The purified monoiodo derivative
11 was similarly converted to trithiol 3 via trithioacetate 13.
Structural Analysis. A single-crystal X-ray analysis of 1
(Figure 1a) indicated that one of the C-S bonds (C12-S2) is
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J. Org. Chem, Vol. 71, No. 4, 2006 1363

Kitagawa et al.
SCHEME 1. Synthesis of 1
SCHEME 2. Synthesis of 2
SCHEME 3. Synthesis of 3
nearly anti-periplanar to the C-Br bond, while the other two
sulfur atoms, S1 and S3, adopt gauche-like conformations with
respect to the C-Br bond. When a monolayer is formed on a
gold substrate, the latter sulfur atoms can be oriented anti, so
that the three sulfur atoms form an equilateral triangle. In such
a conformation, the S-S distance would be expected to be 5.0
Å (Figure 1b), which is close to the interatomic distance
observed between sulfur atoms in n-alkanethiol SAMs on Au-
(111), x3 × D ) 4.995 Å (D denotes the interatomic Au-Au
distance, 2.884 Å1f). Such features would ensure the secure
three-point chemisorption of 1 without an unfavorable change
in molecular geometry that could lead to increased molecular
strain.
The three-point adsorption of trithiols 1-3, with the elimina-
tion of the thiol hydrogen atoms, was confirmed by polarization
modulation infrared reflection absorption spectroscopy (PM-
IRRAS). S-H vibrations, observed at around 2560 cm-1 in the
transmission IR spectra of neat samples, were not detected in
PM-IRRAS spectra of monolayers on gold (Figure 2). The
absence of S-H absorption peaks supports the conclusion that
the molecules are attached via bonding with the three sulfur
atoms in a tripod-like, perpendicular orientation. This observa-
tion is consistent with the reported XPS data for the SAM of
tetrathiol 14, in which three sulfur atoms of the molecules were
bonded to the polycrystalline gold surface.23
Formation of SAMs on Au(111). The SAMs of tripods 1-3
were formed by immersing a substrate, prepared by depositing
gold on a mica sheet, in a 0.1 mM solution of one of trithiols
1-3 in EtOH or CH2Cl2 at ambient temperature. Immersion
for 24 h was required for the formation of monolayers that gave
reproducible voltammograms in electrochemical desorption
experiments.
1364 J. Org. Chem., Vol. 71, No. 4, 2006

Rigid Molecular Tripod with Adamantane Framework
FIGURE 1. (a) ORTEP drawing of 1 as determined by single-crystal
X-ray diffraction analysis at 100 K. Ellipsoids are drawn at the 50%
probability level for non-hydrogen atoms. Selected angles (deg): C5-
C11-S1, 117.06(16); C7-C12-S2, 116.12(18); C9-C13-S3, 117.13-
(17); C2-C5-C11-S1, -63.3(2); C3-C7-C12-S2, -177.30(16);
C4-C9-C13-S3, 59.6(2). (b) Top view of 1. Thiol hydrogens have
been omitted. C-C-C-S torsion angles and C-C-S angles have been
changed so that the C-S bonds align parallel to the C1-Br1 bond.
FIGURE 2. Infrared absorption spectra of trithiols 1-3. (a) Transmis-
sion spectra of neat samples on a KBr plate. The sample was prepared
by dropping a 5 mM solution of the compound in ethyl ether on a
plate and slowly evaporating the solvent. The dropping and evaporation
were repeated five times. (b) PM-IRRAS spectra of the monolayers on
Au(111).
The antisymmetric and symmetric CH2 stretching peaks,
appearing at 2910 and 2850 cm-1, respectively, in the spectra
of neat samples, were shifted by 8-17 cm-1 to higher
frequencies when the molecules were adsorbed on Au. These
shifts might indicate the tightening of the C-H bonds due to
the fixation of the three legs on the Au surface. The shoulders
observed on the high-wavenumber side of these peaks in the
SAM spectra can be ascribed to the CH2 groups adjacent to a
sulfur atom.
The model shown in Figure 4 represents a plausible structure.
Although the orientations of individual molecules, i.e., the twist
angles around the C3 axis, were not determined, a unidirection-
ally oriented, head-to-tail arrangement, which allows a close
approach, is considered to be a favorable model. In this model,
the adsorbed sulfur atoms occupy near-atop sites of the Au
lattice, forming a quasi-(x3 × x3)R30° lattice. Note that the
surface density of the adsorbed sulfur atom is identical to that
for n-alkanethiol SAMs, irrespective of the orientation of the
adsorbed molecules.
Reductive Desorption. The electrochemical behavior of thiol
SAMs on gold allows the state and surface coverage of the
adsorbed thiolate molecules to be examined. The cyclic volta-
mmetry of n-alkanethiol monolayers on a Au(111) electrode,
measured in alkaline solution, is known to exhibit a cathodic
wave due to the charge produced by reductive desorption (eq
1).30-34 The observed charge, typically 90-110 µC/cm2 for such
monolayers,31,32,34 is significantly greater than the theoretical
value, 73 µC/cm2, which corresponds to the coverage of 7.6 ×
10-10 mol/cm2, derived from a (x3 × x3)R30° packing
Scanning Tunneling Microscopy. The molecular arrange-
ment of 1 in a monolayer on a Au(111) surface was analyzed
by STM under water, using perchloric acid as the supporting
electrolyte. Figure 3a shows a typical image, which can be
interpreted as two overlapping periodical patterns. One consists
of bright spots, and the other contains dark spots with a larger
period. Each pattern could be individually visualized by
frequency deconvolution using the Fourier transform method
(Figure 3b).29 Both are hexagonal close-packed lattices with
nearest spacings of 2.9 and 8.7 Å, respectively. The former
corresponds to the Au(111) surface,1f while the latter indicates
that adsorbed molecules of 1 are separated by a distance equal
to three times the Au-Au interatomic distance. In the raw
image, the two patterns are overlapped so that the center of
each molecule of 1 lies on one of the underlying Au atoms.
(30) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B.
D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687.
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(b) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1997, 13, 243.
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(29) This technique was previously used to reduce the noise of the images
of adsorbed organic molecules: (a) Kushmerick, J. G.; Kandel, S. A.; Han,
P.; Johnson, J. A.; Weiss, P. S. J. Phys. Chem. B 2000, 104, 2980. (b)
Katsonis, N.; Marchenko, A.; Taillemite, S.; Fichou, D.; Chouraqui, G.;
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18, 5231.
J. Org. Chem, Vol. 71, No. 4, 2006 1365

Kitagawa et al.
FIGURE 3. (a) STM image of the SAM of 1 on a Au(111) surface,
as measured in 0.1 M aq HClO4 at ambient temperature. A silver wire
was used as the reference electrode. Image area 12 nm × 12 nm,
setpoint current 400 pA, bias voltage 700 mV, Au electrode potential
-300 mV, tip potential 400 mV. (b) Computer images of the 8.7 Å
(left) and 2.9 Å (right) components of the lower left 4 nm × 4 nm area
of (a). These images were obtained by inverse Fourier transform of
each of the two intense frequency components obtained by two-
dimensional Fourier transform of the raw image. The unit cell, indicated
by hexagons, has a side length of 8.7 Å.
FIGURE 5. Cyclic voltammograms for the reductive desorption of
SAMs derived from n-dodecanethiol, monothiol 15, and trithiols 1-3
on Au(111) working electrode in 0.5 M KOH. Scan rate, 20 mV/s.
Geometric area of the working electrode, 0.152 cm2. The charge for
reductive desorption was calculated from the area below the dotted
line.
TABLE 1. Peak Potential, Full Width at Half-Maximum (fwhm),
and Charge for the Electrochemical Reductive Desorption of SAMs
Derived from Thiols on Au(111)
total reductive
fwhm
(mV)
thiol
Ep (V vs Ag/AgCl)
chargea (µC/cm2)
n-C12H25SH
1-AdCH2SH (15)
1
2
3
-1.084
-0.954
-1.088
-1.026
-1.032
100
72
100
70
20
47
91
129
128
70
a Calculated by integrating the cathodic wave. Uncertainty due to sample-
to-sample variations, (10%.
current resulting from the change in double layer charges upon
removal of the adsorbed molecules.34
RS-Au + e- f RS- + Au
(1)
Figure 5 shows cyclic voltammograms for the reductive
desorption of trithiols 1-3, monothiol 15,36 and n-dodecanethiol
adsorbed to Au(111). All exhibited an irreversible reduction
wave. The peak potential (Ep), full width at half-maximum
(fwhm), and reductive charge are listed in Table 1. The observed
reductive charge for n-dodecanethiol SAM, 100 µC/cm2, which
is 1.4 times greater than expected, is in agreement with the
FIGURE 4. Top view of a possible arrangement of 1 for the SAM on
Au(111). The unit cell is shown by a hexagon. All molecules are drawn
in the same orientation to minimize unfavorable intramolecular interac-
tions. Sulfur atoms are assumed to be located on the near-top sites.
structure. This difference can be attributed to the surface
roughness of the Au substrate, 1.05-1.15,34,35 and a nonfaradaic
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(36) 1-(Mercaptomethyl)adamantane (15) was prepared in a three-step
synthesis from 1-(hydroxymethyl)adamantane by similar transformations
employed for the synthesis of 1. See the Experimental Section.
1366 J. Org. Chem., Vol. 71, No. 4, 2006

Rigid Molecular Tripod with Adamantane Framework
reported values and serves as a reference for charge determi-
nation. Thus, with the assumption that the total contribution
from surface roughness37 and the nonfaradaic current is assumed
to be constant at ca. 30% of the total reductive charge for all
the SAMs studied, the faradaic component determined for each
SAM can be taken as reflecting the surface density of the
adsorbed sulfur atoms. The very small fwhm, such as that
observed for n-dodecanethiol, has been interpreted to be the
result of interchain attractive interactions between van der
Waals-contacting molecules.34
It has been reported that, in the SAM prepared from disulfide
16, the 1-AdCH2S groups are hexagonally packed on Au(111)
with a lattice constant of 6.65 Å.38 The reductive charge
observed for the SAM of 15, 72 µC/cm2, can be interpreted as
the sum of the theoretical reductive charge (41 µC/cm2) for the
1-AdCH2S groups adsorbed in the same surface density, plus a
nonfaradaic contribution. The fwhm is significantly larger than
that for the n-dodecanethiol SAM, indicating a reduced lateral
interactions between neighboring adamantane frameworks.
The SAM of trithiol 1 showed essentially the same reductive
charge (100 µC/cm2) as that of n-dodecanethiol SAM, which
would be expected from the arrangement shown in Figure 4.
The observation of a large fwhm (91 mV) indicates poor
attractive interactions between neighboring molecules of 1,
which is in complete agreement with the extended (8.7 Å)
intermolecular distance. Despite this result, the Ep is as negative
as n-dodecanethiol. In general, the large negative Ep values
reported for long n-alkanethiol SAMs have been explained by
low solubility of the detached thiol as well as by the resistance
of the adsorbed molecules to detachment due to strong attractive
intermolecular interactions. The present results indicate that 1
is not readily desorbed despite the lack of such interactions,
and are better attributed to the tight binding of 1 to the gold
substrate by the three S-Au bonds.
approximately equal to the van der Waals diameter of the
sphere-like adamantane framework, the SAM of 1 showed a
significantly greater separation between adsorbed molecules due
to the spatial requirements of the three legs. The present result
is to our knowledge the first example of the observation of a
two-dimensional crystal composed of tripod-shaped molecules
that are chemisorbed to a solid surface. The spontaneous
formation of the highly ordered SAM of 1 would provide a
useful end of larger functional molecules, thus enabling the
vertical fixing of rod-shaped electro- and photoactive molecular
components to metal surfaces with a well defined two-
dimensional structure. More detailed studies of the SAMs of
adamantane tripods by means of STM under ultrahigh vacuum
conditions are currently underway.
Experimental Section
1-Bromo-3,5,7-tris(methoxycarbonyl)adamantane (4). A solu-
tion of 1-bromoadamantane41 (5.00 g, 23.2 mmol) in oxalyl chloride
(20 mL) was irradiated for 50 h with a 450-W high-pressure
mercury lamp at room temperature using a Pyrex flask, and the
oxalyl chloride was removed under reduced pressure. The tricar-
boxylic acid trichloride thus obtained was treated under stirring
with dry methanol (50 mL) at room temperature for 3 h. Evaporation
of the methanol followed by medium-pressure liquid chromatog-
raphy (MPLC) (SiO2, hexane-ether 1:1) afforded 4 (2.17 g, 24%)
1
as colorless crystals: mp 123-124 °C; H NMR (CDCl3) δ 2.00
(d, J ) 12.9 Hz, 3H), 2.06 (d, J ) 12.9 Hz, 3H), 2.40 (s, 6H), 3.71
(s, 9H); 13C NMR (CDCl3) δ 37.9, 44.5, 47.9, 52.3, 59.0, 174.4.
Anal. Calcd for C16H21BrO6: C, 49.37; H, 5.44. Found: C, 49.51;
H, 5.45.
1-Bromo-3,5,7-tris(hydroxymethyl)adamantane (5). A solution
of triester 4 (0.773 g, 1.99 mmol) in ether (20 mL) was added to
a stirred suspension of lithium aluminum hydride (0.274 g, 7.22
mmol) in ether (20 mL) over a period of 10 min. The reaction
mixture was stirred at room temperature for 3.5 h, cooled on ice,
and quenched by the addition of water (2 mL). The solvent was
evaporated under reduced pressure, and the residue was extracted
seven times with 20 mL portions of ethyl acetate. The combined
extracts were evaporated to give 5 (0.574 g, 95%) as colorless
crystals: mp 136-137 °C; 1H NMR (CD3OD) δ 1.19 (d, J ) 12.0
Hz, 3H), 1.35 (d, J ) 12.0 Hz, 3H), 2.04 (s, 6H), 3.25 (s, 6H), the
hydroxyl protons were not observed due to rapid H-D exchange;
13C NMR (CD3OD) δ 40.3, 41.4, 51.5, 66.5, 72.3. Anal. Calcd for
C13H21BrO3: C, 51.16; H, 6.94. Found: C, 51.21; H, 7.05.
1-Bromo-3,5,7-tris[(trifluoromethylsulfonyloxy)methyl]-
adamantane (6). A solution of trifluoromethanesulfonic anhydride
(2.85 g, 10.1 mmol) in CH2Cl2 (20 mL) was added dropwise to a
solution of triol 5 (0.564 g, 1.85 mmol) and pyridine (3.0 mL) in
CH2Cl2 (20 mL) with stirring at 0 °C over a period of 10 min, and
the reaction mixture was then further stirred at 0 °C for 2 h. The
mixture was diluted with CH2Cl2 (30 mL), washed with 10% HCl
and 5% NaHCO3, and dried (MgSO4). The solvent was removed
under reduced pressure to give essentially pure 6 (1.14 g, 88%) as
colorless crystals: mp 116-117 °C after purification by GPC (Japan
The diminished reductive charges and large fwhms of the
SAMs of 2 and 3 suggests the existence of disorder as a result
of incomplete self-assembly. The degree of disorder has been
reported to vary with several different factors, such as the solvent
used in the preparation of the SAM39 and the time after
preparation.40 The presence of an aromatic group in 2 and 3
may also affect the rate of two-dimensional crystallization. Note
that the aromatic ring would not be expected to cause any
significant intramolecular repulsion when the molecules of 2
and 3 stand upright on the Au surface. Thus, modification of
the conditions for adsorption might permit the formation of a
better ordered SAM.
Conclusion
In summary, molecular tripod 1 and its 4-halophenyl deriva-
tives, 2 and 3, were synthesized, and the formation of their
SAMs on Au(111) by three-point adsorption was confirmed.
STM analyses indicated a hexagonal packing with a nearest
intermolecular distance of 8.7 Å for the SAM of 1. Unlike the
SAM of monothiol 15, where the intermolecular distance is
1
Analytical Industry JAIGEL 1H+2H, CHCl3); H NMR (CDCl3)
δ 1.47 (d, J ) 12.3 Hz, 3H), 1.54 (d, J ) 12.3 Hz, 3H), 2.16 (s,
6H), 4.22 (s, 6H); 13C NMR (CDCl3) δ 37.1, 38.1, 47.6, 55.9, 81.9,
118.5 (q, 1JCF ) 319 Hz). Anal. (after purification by GPC) Calcd
for C16H18BrF9O9S3: C, 27.40; H, 2.59. Found: C, 27.51; H,2.53.
1-Bromo-3,5,7-tris(acetylthiomethyl)adamantane (7). A solu-
tion of tritriflate 6 (0.212 g, 0.302 mmol), 18-crown-6 (1.12 g),
and potassium thioacetate (0.384 g, 3.36 mmol) in acetonitrile (25
mL) was stirred at room temperature for 2 h. The resulting
precipitate was removed by filtration, and the filtrate washed with
(37) The roughness factor of the surface of our Au substrate was
estimated to be at most 1.05 from the underpotential deposition of Pb2+
.
For the method, see: Wong, S.-S.; Porter, M. D. J. Electroanal. Chem.
2000, 485, 135.
(38) Fujii, S., Akiba, U.; Fujihira, M. J. Am. Chem. Soc. 2002, 124,
13629.
(39) Yamada, R.; Sakai, H.; Uosaki, K. Chem. Lett. 1999, 667.
(40) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966.
(41) Demmark, S. E.; Henke, B. R. J. Am. Chem. Soc. 1991, 113, 2177.
J. Org. Chem, Vol. 71, No. 4, 2006 1367

Kitagawa et al.
5% NaCl and dried (MgSO4). Evaporation of the solvent followed
by separation by GPC (JAIGEL 1H+2H, CHCl3) afforded 7 (0.116
g, 80%) as colorless crystals: mp 110-111 °C; 1H NMR (CDCl3)
δ 1.18 (d, J ) 12.2 Hz, 3H), 1.28 (d, J ) 12.2 Hz, 3H), 1.95 (s,
6H), 2.37 (s, 9H), 2.80 (s, 6H); 13C NMR (CDCl3) δ 30.7, 38.6,
40.0, 42.7, 50.7, 62.3, 194.9. Anal. Calcd for C19H27BrO3S3: C,
47.59; H, 5.68. Found: C, 47.87; H, 5.71.
1-(4-Iodophenyl)-3,5,7-tris[(trifluoromethylsulfonyloxy)methyl]-
adamantane (11). A solution of 10 (0.273 g, 0.391 mmol), C6H5I-
(OCOCF3)2 (0.194 g, 0.451 mmol), I2 (98.9 mg, 0.390 mmol) in
CCl4 (1.3 mL) was stirred for 4 h at room temperature. The mixture
was diluted with chloroform (30 mL), washed with 5% NaHSO3
and 10% NaCl, and dried (MgSO4). Evaporation of the solvent
followed by GPC separation (JAIGEL 1H+2H, CHCl3) afforded a
colorless solid (0.272 g) consisting of 11 and byproduct 12 in a
4:1 ratio. The solid was recrystallized from chloroform to yield
0.179 g (56%) of pure 11. The mother liquor was concentrated
and chromatographed (SiO2, hexane-ether 2:1) to give 27.4 mg
(7.4%) of 12.
1-Bromo-3,5,7-tris(mercaptomethyl)adamantane (1). A solu-
tion of potassium hydroxide (0.26 g) in methanol (10 mL) was
added to 7 (42.8 mg, 0.089 mmol) in methanol (30 mL). The
mixture was stirred at 0 °C for 1 h and acidified with 10% HCl (3
mL). The solvent was evaporated and the residue extracted with
20 mL of ethyl acetate. Evaporation of the solvent and subsequent
recrystallization from chloroform gave 17.3 mg (55%) of 1 as
1
11: colorless crystals; mp 147-148 °C; H NMR (CDCl3) δ
1.51 (s, 6H), 1.73 (s, 6H), 4.26 (s, 6H), 7.05 (d, J ) 8.0 Hz, 2H),
7.69 (d, J ) 8.0 Hz, 2H); 13C NMR (CDCl3) δ 35.8, 37.0, 37.9,
1
colorless crystals: mp 107-108 °C; H NMR (CDCl3) δ 1.21 (t,
1
42.3, 83.1, 92.5, 118.6 (q, JCF ) 320 Hz), 126.6, 137.8, 145.8;
J ) 9.0 Hz, 3H), 1.27 (d, J ) 12.0 Hz, 3H), 1.36 (d, J ) 12.0 Hz,
3H), 2.03 (s, 6H), 2.44 (d, J ) 9.0 Hz, 6H); 13C NMR (CDCl3) δ
36.6, 39.0, 42.3, 51.0, 63.5. Anal. Calcd for C13H21BrS3: C, 44.18;
H, 5.99. Found: C, 44.44; H, 5.94.
HRMS (FAB+) m/z calcd for C22H22F9IO9S3 (M+) 823.9327, found
823.9327.
12: colorless oil; 1H NMR (CDCl3) δ 1.51 (s, 6H), 1.71 (s, 6H),
4.27 (s, 6H), 7.00 (dd, J ) 8.1, 2.1 Hz, 1H), 7.75 (d, J ) 2.1 Hz,
1H), 7.84 (d, J ) 8.1 Hz, 1H); 13C NMR (CDCl3) δ 35.8, 36.9,
1-(4-Bromophenyl)-3,5,7-tris[(trifluoromethylsulfonyloxy)-
methyl]adamantane (8). To a solution of tritriflate 6 (0.374 g,
0.533 mmol) in CH2Cl2 (60 mL) and bromobenzene (6 mL) was
added AgSbF6 (0.699 g, 2.03 mmol). The mixture was refluxed
for 7 h and washed with 5% NaHCO3 and water. The solvent was
evaporated under reduced pressure, and the residue was purified
by GPC (JAIGEL 1H+2H, CHCl3) to give 0.293 g (71%) of 8 as
1
37.7, 42.1, 83.0, 106.1, 108.8, 118.6 (q, JCF ) 320 Hz), 125.8,
135.9, 139.5, 147.8; HRMS (FAB+) m/z calcd for C22H21F9I2O9S3
(M+) 949.8293, found 949.8302.
1-(4-Iodophenyl)-3,5,7-tris(acetylthiomethyl)adamantane (13).
A solution of tritriflate 11 (70.0 mg, 0.085 mmol), 18-crown-6
(0.340 g), and potassium thioacetate (0.110 g, 0.96 mmol) in
acetonitrile (17 mL) was stirred at room temperature for 4 h. The
solvent was evaporated under reduced pressure and the residue
dissolved in ethyl ether. The solution was washed with 5% NaCl,
dried (MgSO4), and evaporated. Purification of the residue by GPC
(JAIGEL 1H+2H, CHCl3) afforded 13 (45.9 mg, 90%) as a
colorless oil: 1H NMR (CDCl3) δ 1.24 (s, 6H), 1.48 (s, 6H), 2.36
(s, 9H), 2.84 (s, 6H), 7.04 (d, J ) 8.6 Hz, 2H), 7.62 (d, J ) 8.6
Hz, 2H); 13C NMR (CDCl3) δ 30.7, 35.8, 38.2, 40.9, 43.6, 45.2,
91.4, 127.1, 137.3, 148.2, 195.2; HRMS (FAB+) m/z calcd for
C25H31IO3S3 (M + H+) 603.0558, found 603.0554.
1-(4-Iodophenyl)-3,5,7-tris(mercaptomethyl)adamantane (3).
A solution of potassium hydroxide (0.26 g) in methanol (15 mL)
was added to 13 (24.8 mg, 0.041 mmol) in methanol-ethyl ether
(3:1, 13 mL), and the mixture was stirred at room temperature for
1 h. The mixture was acidified with 10% HCl (2 mL) and
evaporated under reduced pressure. The residue was extracted with
ethyl ether (20 mL), and the ethyl ether was evaporated to give 3
(16.8 mg, 86%) as a yellow oil: 1H NMR (CDCl3) δ 1.20 (t, J )
9.0 Hz, 3H), 1.30 (s, 6H), 1.55 (s, 6H), 2.47 (d, J ) 9.0 Hz, 6H),
7.11 (d, J ) 8.1 Hz, 2H), 7.64 (d, J ) 8.1 Hz, 2H); 13C NMR
(CDCl3) δ 35.9, 37.4, 38.4, 43.1, 45.3, 91.4, 127.2, 137.3, 148.6;
HRMS (EI+) m/z calcd for C19H25IS3 (M+) 476.0163, found
476.0157.
1
colorless crystals: mp 129-130 °C; H NMR (CDCl3) δ 1.51 (s,
6H), 1.74 (s, 6H), 4.27 (s, 6H), 7.18 (d, J ) 8.7 Hz, 2H), 7.50 (d,
J ) 8.7 Hz, 2H); 13C NMR (CDCl3) δ 35.8, 37.0, 37.9, 42.3, 83.1,
118.6 (q, 1JCF ) 320 Hz), 121.0, 126.4, 131.9, 145.1. Anal. Calcd
for C22H22BrF9O9S3: C, 33.99; H, 2.85. Found: C, 33.90; H, 2.89.
1-(4-Bromophenyl)-3,5,7-tris(acetylthiomethyl)adamantane (9).
A solution of tritriflate 8 (0.177 g, 0.228 mmol), 18-crown-6 (0.716
g), and potassium thioacetate (0.260 g, 2.28 mmol) in acetonitrile
(25 mL) was stirred at room temperature for 3 h. The solvent was
evaporated under reduced pressure and the residue dissolved in ethyl
ether. The solution was washed with 5% NaCl and water, dried
(MgSO4), and evaporated. Purification of the residue by MPLC
(SiO2, hexane-ether 3:1) afforded 9 (99.6 mg, 79%) as a colorless
oil: 1H NMR (CDCl3) δ 1.25 (s, 6H), 1.49 (s, 6H), 2.36 (s, 9H),
2.84 (s, 6H), 7.16 (d, J ) 8.7 Hz, 2H), 7.42 (d, J ) 8.7 Hz, 2H);
13C NMR (CDCl3) δ 30.7, 35.8, 38.1, 40.9, 43.6, 45.3, 119.9, 126.8,
131.3, 147.5, 195.3. Anal. Calcd for C25H31BrO3S3: C, 54.04; H,
5.62. Found: C, 54.34; H, 5.58.
1-(4-Bromophenyl)-3,5,7-tris(mercaptomethyl)adamantane (2).
A solution of potassium hydroxide (0.16 g) in methanol (10 mL)
was added to a solution of 9 (28.1 mg, 0.051 mmol) in methanol-
ethyl ether (4:1, 12 mL), and the mixture was stirred at room
temperature for 1 h. The mixture was acidified with 10% HCl (1.5
mL) and evaporated under reduced pressure. The residue was
dissolved in CS2 and quickly passed through a short column of
SiO2 to give 2 (17.1 mg, 79%) as a colorless oil: 1H NMR (CDCl3)
δ 1.20 (t, J ) 9.0 Hz, 3H), 1.31 (s, 6H), 1.56 (s, 6H), 2.48 (d, J )
9.0 Hz, 6H), 7.24 (d, J ) 9.0 Hz, 2H), 7.44 (d, J ) 9.0 Hz, 2H);
13C NMR (CDCl3) δ 35.9, 37.4, 38.3, 43.1, 45.4, 119.9, 126.9,
131.3, 147.9; HRMS (EI+) m/z calcd for C19H25BrS3 (M+)
428.0302, found 428.0307.
1-Phenyl-3,5,7-tris[(trifluoromethylsulfonyloxy)methyl]-
adamantane (10). To a solution of tritriflate 6 (0.213 g, 0.304
mmol) in CH2Cl2 (35 mL) and benzene (4 mL) was added AgSbF6
(0.522 g 1.52 mmol). The mixture was refluxed for 90 h and then
washed with 5% NaHCO3 and water, dried (MgSO4), and evapo-
rated. Purification of the residue by GPC (JAIGEL 1H+2H, CHCl3)
afforded 10 (0.177 g, 83%) as a colorless oil: 1H NMR (CDCl3) δ
1.50 (s, 6H), 1.76 (s, 6H), 4.27 (s, 6H), 7.24-7.41 (m, 5H); 13C
NMR (CDCl3) δ 35.8, 37.0, 37.8, 42.4, 83.4, 118.6 (q, 1JCF ) 320
Hz), 124.5, 127.0, 128.7, 146.1; HRMS (EI+) m/z calcd for
C22H23F9O9S3 (M+) 698.0360, found 698.0355.
1-[(Trifluoromethylsulfonyloxy)methyl]adamantane (1-AdCH2-
OTf). A solution of trifluoromethanesulfonic anhydride (0.84 g,
3.0 mmol) in CH2Cl2 (8 mL) was added dropwise to a solution of
1-(hydroxymethyl)adamantane (0.262 g, 1.58 mmol) and pyridine
(1.0 mL) in CH2Cl2 (9 mL) with stirring at 0 °C over a period of
15 min, and the reaction mixture was stirred at 0 °C for 2 h. The
mixture was diluted with CH2Cl2 (30 mL), washed with 10% HCl
and 5% NaHCO3, and dried (MgSO4). The solvent was removed
under reduced pressure to give essentially pure 1-AdCH2OTf (0.454
g, 96%) as a pale yellow oil: 1H NMR (CDCl3) δ 1.58 (d, J ) 2.4
Hz, 6H), 1.65 (d, J ) 12.5 Hz, 3H), 1.76 (d, J ) 12.5 Hz, 3H),
2.04 (br. s, 3H), 4.08 (s, 2H); 13C NMR (CDCl3) δ 27.6, 33.8,
1
36.5, 38.2, 86.3, 118.6 (q, JCF ) 319 Hz). Anal. Calcd for
C12H17F3O3S: C, 48.31; H, 5.74. Found: C, 47.97; H, 5.87.
1-(Acetylthiomethyl)adamantane (1-AdCH2SAc). A solution
of 1-AdCH2OTf (0.252 g, 0.845 mmol), 18-crown-6 (0.766 g), and
potassium thioacetate (0.302 g, 2.64 mmol) in acetonitrile (30 mL)
was stirred at room temperature for 2 h. The acetonitrile was
1368 J. Org. Chem., Vol. 71, No. 4, 2006

Rigid Molecular Tripod with Adamantane Framework
evaporated, and the residue was dissolved in 30 mL of ethyl ether.
The ether solution was washed with 5% NaCl and dried (MgSO4).
Evaporation of the solvent gave a brown oil, which was passed
through a short column of SiO2 and separated by GPC (JAIGEL
1H+2H, CHCl3) to afford 1-AdCH2SAc (0.101 g, 53%) as colorless
crystals: mp 30-31 °C; 1H NMR (CDCl3) δ 1.50 (d, J ) 2.7 Hz,
6H), 1.60 (d, J ) 12.2 Hz, 3H), 1.69 (d, J ) 12.2 Hz, 3H), 1.96 (s,
3H), 2.35 (s, 3H), 2.73 (s, 2H); 13C NMR (CDCl3) δ 28.4, 30.7,
33.2, 36.7, 41.3, 42.4, 196.0. Anal. Calcd for C13H20OS: C, 69.59;
H, 8.98. Found: C, 69.59; H, 9.07.
Water System). STM analysis of the gold film prepared in this way
confirmed the formation of large Au(111) terraces.
For the preparation of the SAMs of 1 and 2, the substrate was
washed with EtOH and immersed in a 0.1 mmol/L solution of the
thiol. For the preparation of the SAMs of 3, which is not well
soluble in EtOH, the substrate was washed with CH2Cl2 after
washing with EtOH, and then immersed in a 0.1 mmol/L solution
of 3 in CH2Cl2. In both cases, the substrates were kept immersed
at room temperature in the dark for 24 h, rinsed with pure EtOH
or CH2Cl2 and dried in air.
Polarization Modulation-Infrared Reflection Absorption Spec-
troscopy. PM-IRRAS spectra were recorded with a Thermo-
Mattson Infinity spectrometer equipped with a HgCdTe detector
cooled with liquid N2 and a photoelastic modulator (Hinds, PEM-
90) with the optical layout similar to that reported in the literature.43
The signal was demodulated with a synchronous sampling de-
modulator (GWC Instruments). The spectra were collected over
10000 scans at a resolution of 4 cm-1. More precisely, one cycle
of recording, which consists of 10 consecutive scans for baseline,
100 scans for sample, 100 scans for sample, and 10 scans for
baseline, was repeated 50 times. It took about 4 h for each sample.
The differential reflectance was numerically converted to absor-
bance. All measurements were made in the air at room temperature.
Scanning Tunneling Microscopy. STM images were recorded
under 0.1 M aq. HClO4 using a Pico-SPM (Molecular Imaging Co.)
and NanoScope III controller (Digital Instruments Inc.). An
electrochemical system composed of a Pt wire counter electrode
and a Ag wire reference electrode was used. Measurements were
performed in the constant current mode using Pt0.8Ir0.2 tips coated
with Apiezon wax.
Cyclic Voltammetry. Cyclic voltammograms of the reductive
desorption were recorded in 0.5 mol/L KOH using a Ag/AgCl/sat.
KCl electrode as the reference electrode and a platinum wire as
the counter electrode. The gold substrate was mounted at the bottom
of a cone-shaped cell using an O-ring and a clamp. The exposed
area of the working electrode was 0.152 cm2 (2.2 mm diameter
circle). The solution in the cell was deaerated by bubbling H2O-
saturated argon for 30 min. Voltammograms were recorded on a
BAS 100B electrochemical analyzer.
1-(Mercaptomethyl)adamantane (15). A solution of potassium
hydroxide (0.58 g) in methanol (15 mL) was added to 1-AdCH2-
SAc (0.101 g, 0.450 mmol) in methanol (15 mL), and the mixture
was stirred at room temperature for 1 h. The methanol was
evaporated, and the residue was dissolved in 20 mL of ethyl ether.
The ether solution was washed with 10% HCl and 5% NaHCO3
and dried (MgSO4). The solvent was removed under reduced
pressure to give 15 (70.9 mg, 86%) as a colorless oil: 1H NMR
(CDCl3) δ 1.10 (t, J ) 8.6 Hz, 1H), 1.52 (d, J ) 2.4 Hz, 6H), 1.62
(d, J ) 12.0 Hz, 3H), 1.71 (d, J ) 12.0 Hz, 3H), 1.99 (br. s, 3H),
2.30 (d, J ) 8.6 Hz, 2H); 13C NMR (CDCl3) δ 28.5, 33.1, 36.9,
38.7, 41.0; HRMS (EI+) m/z calcd for C11H18S (M+) 182.1129,
found 182.1132.
X-ray Structural Analysis. Single crystals of 1 were obtained
by recrystallization from chloroform. Intensity data were collected
at 100 K on a Bruker SMART APEX diffractomater with Mo KR
radiation (λ ) 0.71073 Å) and a graphite monochromator. The
structure was solved by direct methods (SHELXTL) and refined
by the full-matrix least-squares on F2 (SHELXL-97). All non-
hydrogen atoms were refined anisotropically, and all hydrogen
atoms were placed using AFIX instructions: C13H21BrS3; FW )
353.39, crystal size 0.40 × 0.40 × 0.30 mm3, monoclinic, P2(1)/
n, a ) 9.5204(6) Å, b ) 10.3261(7) Å, c ) 15.3043(10) Å, â )
101.7830(10)°, V ) 1472.84(17) Å3, Z ) 4, Dc ) 1.594 g cm-3
.
The refinement converged to R1 ) 0.0270, wR2 ) 0.0738 (I >
2σ(I)), GOF ) 1.001.
Preparation of Self-Assembled Monolayers on Au(111). Gold
substrates were prepared by the method reported previously.33,34,42
Gold (99.99%) was vapor-deposited on freshly cleaved mica sheets
at a reduced pressure, <10-3 Pa. The mica was baked at 580 °C
for 6 h prior to the deposition of the gold and this temperature was
maintained during the deposition. The typical evaporation rate and
thickness of the gold films were 1.0-1.5 nm/s and 200 ( 5 nm,
respectively. The Au-deposited mica sheets were cut into 2 cm ×
2 cm (for cyclic voltammetry) or 1 cm × 2 cm (for STM analysis)
pieces, annealed at 530 °C for 8 h, and then quenched in Millipore
purified water (resistivity >18 MΩ‚cm, Millipore Simplicity 185
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Supporting Information Available: 1H and 13C NMR spectra
of new compounds and crystallographic data (table and CIF) for
1. This material is available free of charge via the Internet at
http://pubs.acs.org.
JO051863J
(42) (a) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.;
Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33. (b) Kakiuchi, T.; Iida,
M.; Imabayashi, S.; Niki, K. Langmuir 2000, 16, 5397. (c) Ooi, Y.; Hobara,
D.; Yamamoto, M.; Kakiuchi, T. Langmuir 2005, 21, 11185.
(43) Barner, B. J.; Green, M. J.; Saez, E. I.; Corn, R. M. Anal. Chem.
1991, 63, 55.
J. Org. Chem, Vol. 71, No. 4, 2006 1369