Synthesis of a nanocar with organometallic wheels

Article abstract of DOI:10.1016/j.tetlet.2009.01.042

A new class of nanovehicles incorporating trans-alkynyl(dppe)₂ruthenium-based wheels is reported. A four-wheeled nanocar and a three-wheeled trimer were synthesized for future studies at the single molecule level.

Full text of DOI:10.1016/j.tetlet.2009.01.042

Tetrahedron Letters 50 (2009) 1427–1430
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Synthesis of a nanocar with organometallic wheels
*
Guillaume Vives, James M. Tour
Departments of Chemistry and Mechanical Engineering and Materials Science and The R. E. Smalley Institute for Nanoscale Science and Technology, Rice
University, MS 222, 6100 Main Street, Houston, TX 77005, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of nanovehicles incorporating trans-alkynyl(dppe)2ruthenium-based wheels is reported. A
four-wheeled nanocar and a three-wheeled trimer were synthesized for future studies at the single mol-
ecule level.
Received 20 November 2008
Revised 7 January 2009
Accepted 9 January 2009
Available online 14 January 2009
Ó 2009 Elsevier Ltd. All rights reserved.
A current trend in physical and biological sciences is the pursuit
of miniaturized machinery.1 To achieve this goal, two complemen-
tary strategies have been developed. The first is the ‘top-down’ ap-
proach which consists of the miniaturization of existing objects
following microscale technologies. However, this approach is
reaching some physical limits due to the incompatibility of such
technology with the nanometer scale. The opposite approach,
known as the ‘bottom-up’ strategy, starts from atoms and mole-
cules and follows a monumentalization2 strategy consisting of
the association of different modular functional units through the
formation of covalent bonds within only one molecule. In this
strategy, the development of molecular machines and nanocarriers
is crucial to control the transport processes at the atomic level.3 In
combination with imaging tools such as scanning tunneling
microscopy (STM),4 our group has developed individually accessi-
ble nanocars, each bearing a chassis, axles and wheels for a re-
stricted rolling motion on a metallic surface.5,6
The first generation of nanocars used C60 fullerene-based
wheels and were studied at the single molecular level.5 Studies
by STM demonstrated a thermally-driven directional motion on a
gold surface.5a,b However, due to the photoactive nature of the ful-
lerenes and the low solubility of the resulting nanocars, a second
generation of nanovehicles using p-carborane wheels has been
synthesized.6 These p-carborane wheels produce more soluble
nanovehicles and are not photoactive, which is crucial for light-
powered nanocars.6a,e
Figure 1. Relative size and space-filling of (a) p-carborane, (b) C60 fullerene, and (c)
trans-[Ru(C„CH)2(dppe)2] wheels.
motion of the wheels,4b,5a,8 should be provided by the diphenyl-
phosphine groups. Furthermore, the overall wheel size of
ꢀ1.2 nm should allow imaging by STM, and the large Z value of
the ruthenium atoms should open new possibilities of imaging
by transmission electron microscopy (TEM). In this Letter we re-
port the synthesis of a new nanocar (1) with organometallic
wheels based on a trans-alkynyl ruthenium complex. In order to
provide evidence for a rolling mechanism, the synthesis of its tri-
mer analogue (2), which should be restricted to a pivoting motion
as we showed for the C60-wheeled trimer,5a is also presented.
The synthesis of nanocar 1 (Fig. 2) was initially considered via a
convergent route consisting of the synthesis of a full axle with the
ruthenium wheels followed by a final coupling with the inner core.
However, the final palladium-catalyzed coupling failed probably
due to the steric hindrance of the bulky ruthenium wheels as seen
in the CPK model (Fig. 2b). A new route based on a modular strat-
egy similar to the one used for the fullerene based nanocars5 was
thus considered. The chassis bearing four alkyne groups was first
synthesized using Sonogashira coupling reactions followed by a
coordination of the four ruthenium wheels in a final step. In order
to increase the solubility of the nanocars, four propoxy groups
were added to the inner core.
In the quest for new molecular wheels, octahedral 18-electron
trans-alkynylbis(1,2-bis-(diphenylphosphino)
ethane)ruthe-
nium(II) complexes7 (Fig. 1) appeared to be good candidates, as
the low rotation barrier around the alkyne bond might allow a free
rotation while the bulky phosphine ligands could act as a tire to
interact with metallic surfaces. Good physisorption on metallic
surfaces, which has been demonstrated to be crucial for a rolling
The synthesis (Scheme 1) started with a double Sonogashira
coupling reaction between the bis-terminal-alkyne 3 and the ary-
liodide 4,6 followed by deprotection to yield 5. The axles were fur-
ther extended by a phenylacetylene unit to allow the coordination
* Corresponding author. Tel.: +1 713 348 6246; fax: +1 713 348 6250.
E-mail address: tour@rice.edu (J.M. Tour).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.01.042

1428
G. Vives, J. M. Tour / Tetrahedron Letters 50 (2009) 1427–1430
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
H
P
P
P
P
P
P
P
P
Ph
Ru
Ru
Ph
a
Ph
Ph
Ph
Ph
Ph
Ph
TMS
TMS
Ph
Ph
4
I
PrO
OPr
OPr
OPr
1) PdCl2(PPh3)2,CuI
THF, Et3N
PrO
OPr
2) TBAF, THF
48%
PrO
PrO
PrO
OPr
3
PrO
OPr
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
H
P
P
P
P
P
P
P
P
Ph
Ru
Ru
Ph
Ph
Ph
Ph
Ph
Ph
Ph
5
Ph
Ph
H
H
Nanocar 1
b
I
TMS
6
1) PdCl2(PPh3)2, CuI
THF, Et3N
PrO
OPr
2) TBAF, THF
41%
PrO
OPr
H
H
7
Ph
Ph
Cl
Ph
Ph
P
R u
Ph
P
c
Ph
P
•
P
Ph
Ph
H
Ph
P
Ph
P
Ru
P
Ph
Ph
Ph
Ph
Ph
H
•
Ph
Ph
P
Ru
P
Ph
P
Ph
Ph
Ph
HP Ph
P
Ph
Cl
Ph
Ph
PrO
OPr
cis-[RuCl2(dppe)2]
NaPF6
4 PF6
CH2Cl2
PrO
OPr
Ph
Ph
Cl
Ph
Ph
P
Ru
P
Ph
Ph
P
•
P
Ph
H
Ph
Ph
P
Ru
P
Ph
P
Ru
P
Ph
Ph
Ph
Ph
H
Ph
Ph
•
Ph
P
Ph
Ph
P
P
P
P
Ph
Ph
8
P
Ph
Ph
Ru
P
Ph
P
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Cl
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Trimer 2
Ph
Cl
P
P
P
P
P
P
P
d
Cl Ru
Ru
P
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
PrO
OPr
Et3N
THF
1
NaPF6
CH2Cl2,Et3N
43%
34% (from 7)
PrO
OPr
Ph
Ph
Ph
Ph
Ph
Ph
Ph
P
Ph
P
P
P
P
P
P
Cl Ru
Ru
Cl
P
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
9
Figure 2. Nanocar (a, b) and trimer (c, d) with ruthenium wheels and their CPK
model geometry optimized with SPARTAN. The blue arrow represents the expected
motion of the molecule on a surface.
Scheme 1. Synthesis of nanocar with organometallic wheels 1.
amine to give the nanocar precursor 9 in a 34% yield from 7. Since
the remaining chloride ligands are potentially reactive upon depo-
sition onto metallic surfaces, the wheels were capped with phen-
ylacetylene moieties. The ligand exchange reaction required
heating of 9 at 35 °C in the presence of phenylacetylene and tri-
ethylamine to give nanocar 1. The intermediate tetravinylidene
complex leading to 1 was not isolated, since a disubstitution could
not occur. The complexation reactions can be monitored by 31P
NMR with a clear shift of the signals corresponding to the phos-
phine ligands from 49.2 ppm for 9 to 53.6 ppm for 1. The trans
geometry of the complexes was confirmed by the observation of
only one singlet in the 31P NMR spectrum since a cis geometry
would give two signals.
of the bulky ruthenium wheels by a quadruple Sonogashira cou-
pling reaction between 5 and 6. After deprotection with TBAF,
the chassis 7 was obtained in a moderate yield. The activation
of the four terminal alkynes of 6 was realized with the bulky
16-electron species [RuCl(dppe)2]PF6, generated in situ from cis-
[RuCl2(dppe)2]9 in the presence of NaPF6 at room temperature.
This reaction led to the intermediate tetra(vinylidenerutheni-
um(II)) complex 8, as it has been shown for linear monovinylidene
complexes.10 The remaining chloride of the vinylidene complex
being less reactive to further substitution, dissymmetric ruthe-
nium complexes can be selectively obtained. The crude vinylidene
intermediate 8 was subsequently deprotonated with triethyl-

G. Vives, J. M. Tour / Tetrahedron Letters 50 (2009) 1427–1430
1429
H
1)
TMSA
PdCl2(PPh3)2, CuI
THF, Et3N
Br
Br
Br 2)
K2CO3
MeOH, THF
H
H
69%
10
H
6
1)
PdCl2(PPh3)2, CuI
THF, Et3N
K2CO3
2)
MeOH,T HF
56%
H
H
11
Cl
Ph
Ph
P
Ph
Ph
P
Ru
P
Ph
P
Ph
Ph
Ph
1)
cis-[RuCl2(dppe)2]
NaPF6,C H2Cl2
NaPF6
2
2)
CH2Cl2,E t3N
Et3N
74%
61%
Figure 3. (a) UV–vis spectra of nanocar 1 (blue, top line) and trimer 2 (red, middle
line) and 7 (green, bottom line) in CH2Cl2. (b) Normalized fluorescence spectra
(excitation at 330 nm) of Nanocar 1 (blue) and trimer 2 (red) and 7 (green) in
CH2Cl2.
Ph
P
Ru
P
Ph
P
Ru
P
Ph
Ph
Ph
Ph
Ph
Ph
Cl
Ph
Ph
Ph
P
P
P
P
Ph
12
Ph
Cl
Ph
Ph
around 460 nm (457 nm for 1 and 462 nm for 2) similar to the
one observed for the chassis 7. This makes the molecules suitable
for surface studies using fluorescence.
Ph
Scheme 2. Synthesis of trimer 2.
In summary we have synthesized a new nanocar with ruthe-
nium-based wheels and its threefold symmetric analogue trimer.
Studies at the single molecule level of 1 and 2 by STM on metallic
surfaces are now in progress.
In order to demonstrate the rolling versus sliding mechanism, a
threefold symmetric trimer model 2 (Fig. 2) was also synthesized.
Similar to the fullerene wheeled nanovehicles,5a the three wheel
trimer should exhibit a pivoting motion if a rolling mechanism is
effective. The synthesis of 211 (Scheme 2) is based on a stepwise
Acknowledgments
We thank the Welch Foundation, C-1489, the NSF NIRT (ECCS-
0708765), and the NSF Penn State MRSEC and Center for Nanoscale
Science, for financial support. The NSF, CHEM 0075728, provided
partial funding for the 400 MHz NMR. We thank Drs. I. Chester of
FAR Research, Inc., and R. Awartari of Petra Research, Inc., for pro-
viding trimethylsilyacetylene.
construction of the inner core followed by
attachment.
a final wheel
The triangular core was synthesized starting from 1,3,5-tri-
bromobenzene using a Sonogashira coupling reaction with TMSA
followed by a deprotection reaction to provide 10.12 Compound
10 was further coupled to 6 using a Sonogashira coupling reaction
to afford, after deprotection, the inner core 11. Similarly to the
nanocar synthesis, the ruthenium wheels were added by reaction
with cis-[RuCl2(dppe)2] to yield the trichlororuthenium trimer 12.
The chloride ligands were finally exchanged with phenylacetylene
to obtain trimer 2.
Supplementary data
Synthetic procedures, 1H NMR, 13C NMR, IR, and mass spectra or
elemental analysis of compounds 1–9 and 12 are available. Supple-
mentary data associated with this article can be found, in the on-
line version, at doi:10.1016/j.tetlet.2009.01.042.
As an alternative to STM, it is possible that the optical proper-
ties could be exploited to image nanocars on a nonmetallic surface
by using confocal fluorescence microscopy,13 hence optical studies
of the products were done. The UV–vis spectra of nanocar 1 and tri-
mer 2 (Fig. 3a) exhibit metal-to-ligand charge transfer (MLCT)
transitions of the ruthenium complexes at 414 and 408 nm, respec-
tively.14 Both complexes show strong high-energy bands around
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