Rotational Control of a Dirhodium-Centered Supramolecular Four-Gear System by Ligand Exchange

Article abstract of DOI:10.1021/jacs.5b13515

Self-assembled molecular machines have great potential to enable noncovalent regulation of a coupled motion of the building blocks. Herein we report the synthesis and the rotational control of a lantern-type dirhodium complex with circularly arranged four

Full text of DOI:10.1021/jacs.5b13515

Subscriber access provided by NEW YORK UNIV
Communication
Rotational Control of a Dirhodium-Centered
Supramolecular Four-Gear System by Ligand Exchange
Kazuma Sanada, Hitoshi Ube, and Mitsuhiko Shionoya
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 24 Feb 2016
Downloaded from http://pubs.acs.org on February 24, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 6
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Rotational Control of a DirhodiumꢀCentered Supramolecular Fourꢀ
Gear System by Ligand Exchange
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Kazuma Sanada, Hitoshi Ube, and Mitsuhiko Shionoya*
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7ꢀ3ꢀ1, Hongo, Bunkyoꢀku, Tokyo, 113ꢀ
003, Japan.
Supporting Information Placeholder
gear system can be controlled by fluoride ion17 or by
ABSTRACT: Selfꢀassembled molecular machines have
great potential to enable nonꢀcovalent regulation of a
acid/base,11,16 and, to the best of our knowledge, there is
no example for the rate control of multiple gearing sysꢀ
tems. Moreover, visualization of chemical phenomena
can provide a new insight into molecular dynamics and
chemical reactions.18 Although methods with optical
microscopy enable us to obtain some images of molecuꢀ
lar motions,19 the connection with a subꢀmicrometerꢀ
sized probe that is several hundreds times as large as the
molecular machine possibly varies the original kinetics
and dynamics.20 Alternatively, the correlation between
motions and color changes of molecular machines is
useful as a nonꢀinvasive estimation method as well as
lightꢀsensitive molecules.
coupled motion of the building blocks. Herein we report
the synthesis and the rotational control of a lanternꢀtype
dirhodium complex with circularlyꢀarranged four
2,3,6,7,14,15ꢀhexamethyltriptycene carboxylates as
gears and two axial ligands as the rate control elements.
The rotating rates in solution were markedly affected by
the coordination ability and the bulkiness of axial ligꢀ
ands. Notably, the rate changes were closely correlated
with the changes in the electronic states of the dirhodiꢀ
um center. Such ligand exchangeꢀbased control of rotaꢀ
tional motions with color changes would advance stimuꢀ
lusꢀresponsive metalloꢀmolecular multiꢀrotors.
Biological molecular machines, such as ATP synthase
and motor proteins, are fascinating molecular systems in
which the rate and direction of the motions are highly
controlled in a synergistic manner to integrate functions
of motor, actuator, converter, and transmitter.1 Inspired
by their sophisticated structure and mechanism, a variety
of excellent examples of synthetic molecular machines
have been reported as miniature versions of macroscopic
devices from 1980’s.2,3 So far, several types of functionꢀ
al molecular machines such as unidirectional motion,4
on/off switching by external stimuli,5 transmission of
energy through coupled motions,6 and others are known.
In synthetic molecular machines, selfꢀassembled molecꢀ
ular machines have great potential to enable nonꢀ
covalent regulation of a coupled motion of the building
blocks.7
Since Iwamura8 and Mislow9 independently reported
the first molecular gearing systems, triptycene has been
often used as a threeꢀbladed gear, whereas pentipꢀ
tycene,10 porphyrin,11 and tetraphenylcyclobutadiene10
have been used as fourꢀbladed gears. Most of them are
two gearꢀmeshed molecules, and only a limited number
of examples have been reported for linearlyꢀmeshed
three gear systems.11,13–16 Gear meshing in a two or three
Figure 1. (a) Synthesis and chemical structures of dirhodiꢀ
umꢀcentered circular fourꢀgear complexes 1·L2. (b) Scheꢀ
matic representation of rotational control of the molecular
motions by the axial ligand exchange. (c) Axial ligands
used in this study.
Herein we report the synthesis and rotational control
of a lanternꢀtype dirhodium complex with circularlyꢀ
arranged four 2,3,6,7,14,15ꢀhexamethyltriptycene carꢀ
boxylates as meshed gears and two axial ligands as the
rate control elements (Figure 1). The rotational rates in
solution were markedly affected by the coordination
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 6
ability and the bulkiness of axial ligands, diethyl ether CH3ꢀ, and C2H5ꢀ) at the 2ꢀpositions of the pyridine ligꢀ
1
2
3
4
5
6
7
8
and pyridine derivatives (Figure 1b). Moreover, UVꢀvis
absorption of solutions containing a different axial ligꢀ
and was significantly affected by the type of the axial
ligands. Although a few examples of lanternꢀtype comꢀ
plexes that look like molecular gears have been reportꢀ
ed,21–23 their dynamic behaviors have not been investiꢀ
gated.
ands were significant as shown in the increasing values,
3.50°, 13.6°, and 15.0° for 1·(py)2, 1·(mepy)2, and
1·(etpy)2, respectively (Table 1) (see the Supporting inꢀ
formation).
Table 1. Structural parameters
RhꢀRh (Å)
2.40
RhꢀL (Å)
2.25
RhꢀRhꢀL (°)
3.50
L
py
Tetrakis(2,3,6,7,14,15ꢀhexamethyltriptycene carboxꢀ
ylato)dirhodium·(ether)2 complex, 1·(ether)2, was preꢀ
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2.41
2.28
13.6
mepy
pared by refluxing
a mixture of 2,3,6,7,14,15ꢀ
hexamethyltriptycene carboxylic acid (2), RhCl3·3H2O,
and NaHCO3 in ethanol (Figure 1a). After purification
by column chromatography and recrystallization from
CH2Cl2/diethyl ether, diethyl ether adduct, 1·(ether)2,
was obtained as green crystals in 48% yield. The two
axial diethyl ether ligands could be replaced by several
types of pyridines to form 1·L2 [L = py (pyridine), mepy
(2ꢀmethylpyridine), etpy (2ꢀethylpyridine), dmap (N,Nꢀ
dimethylꢀ4ꢀaminopyridine), min (methyl isonicotinate)]
by adding two equiv. of each pyridine derivative in
CH2Cl2 (see the Supporting Information).
2.41
2.30
15.0
etpy
2.41
2.22
16.2
damp
2.40
2.23
6.30
min
2.37
2.31
2.67
ether
In NMR spectra of 1·L2 in CDCl3 at 300 K, the sigꢀ
nals of methyl groups at the 2,7,14ꢀpositions of tripꢀ
tycenes of each complex were shifted upfield compared
with those of carboxylic acid 2. This result strongly inꢀ
dicates that all the four triptycenes in the circularlyꢀ
arranged structure around the central dirhodium axis
mesh with each other in solution.
The temperature effects on the motional behaviors of
the meshed triptycene parts were then examined by variꢀ
ableꢀtemperature NMR (VTꢀNMR) spectroscopy. For
instance, in the NMR spectrum of 2ꢀmethylpyridine adꢀ
duct 1·(mepy)2, all the signals for the blades of the tripꢀ
tycene parts became broadened and then split at 250 K.
This result suggests that a zigzag conformation of the
triptycene parts is stable in the light of the fact that the
signals of the blade were split in an upfield (orange)ꢀtoꢀ
downfield (blue) ratio of 2:1 (Figure 3). For reference,
another possible tongueꢀin groove conformation would
show an upfieldꢀtoꢀdownfield ratio of 1:2 due to the
shielding effect of two benzene rings on a benzene ring
of the neighboring triptycene. Similar splitting patterns
were also observed in the spectra of a 2ꢀethylpyridine
adduct 1·(etpy)2 and 1·(ether)2. In contrast, the spectra
of a pyridine adduct 1·(py)2, an N,Nꢀdimethylꢀ4ꢀ
aminopyridine adduct 1·(dmap)2 and a methyl isonicoꢀ
tinate adduct 1·(min)2 showed no splitting behaviors
even at 220 K (see the Supporting Information).
Figure 2. ORTEP drawing with 50% probability (side
view) and space filling model (top view) for 1·(mepy)2.
Diagonal triptycenes are displayed in the same color. Hyꢀ
drogen atoms are omitted for clarity in the ORTEP drawꢀ
ing. Methyl groups of the mepy ligands are highlighted in
pink in the spaceꢀfilling model.
Crystal structures of all the six complexes were deꢀ
termined by singleꢀcrystal Xꢀray analyses. As shown in
the structure of 1·(mepy)2 (Figure 2), the dirhodium core
of the lanternꢀtype complex has circularlyꢀarranged four
carboxylate gears meshing with each other. The two axiꢀ
al ligands bind to the dirhodium center along the Rh–Rh
bond axis.
The structural parameters around the axial ligands are
summarized in Table 1. The Rh–Rh distances for pyriꢀ
dine ligands were approximately 2.40 Å, and the Rh–L
distances were roughly in the range from 2.20 to 2.30 Å
regardless of the type of coordination atoms and substitꢀ
uents of the pyridine ligands. On the other hand, the
bending angles of the axial pyridine ligands from the
Rh–Rh bond axis (RhꢀRhꢀL) took on a wide range of
values. Notably, the steric effects of the substituents (Hꢀ,
ACS Paragon Plus Environment

Page 3 of 6
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Figure 4 (a) Visible absorption spectra of molecular gear
complexes (0.6 mM for py, 1 mM for others in CHCl3, 293
K) and the pictures of solutions containing molecular gear
complexes in CHCl3.
Table 2. Absorption maximum and activation energy paꢀ
rameters of 1·L2
1
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3 Dynamic H NMR lineꢀshape simulations (Sim)
to determine the rate constants (k) of a 60° rotation process
for 1·(mepy)2 using the iNMR software (ver. 5.3.3).
k @260 K
L
λ
max (nm)
ꢀH‡ (kcal molꢀ1)
(sꢀ1)
527
554
566
534
520
608
n.a.
14.5 ± 1.2
18.0 ± 0.5
n.a.
n.a.
py
The rotational rates of these complexes at a given
temperature were determined by dynamic NMR experiꢀ
ments. The activation energy parameters were deterꢀ
mined from the temperature dependence of the changes
in the signals for 1·(mepy)2, 1·(etpy)2, and 1·(ether)2
(Table 2). The rotational rate (k) of each complex was
determined for a 60° flipping between orange and blue
blades in a zigzag conformation. The rotational rates and
activation parameters for 1·(py)2, 1·(dmap)2, and
1·(min)2 could not be determined because no splitting
took place at low temperatures due to their faster motion
compared with the NMR timescale. As for these three
complexes, the activation energy barrier (ꢀG‡) can be
roughly estimated lower than 12 kcal molꢀ1 from the
chemical shift gap ꢀν > 0.1 ppm and the coalescence
temperature Tc < 220 K according to the following equaꢀ
tion,24 where Tc is the coalescence temperature:
5.5 × 102
15
mepy
etpy
damp
min
n.a.
n.a.
n.a.
1.0 × 103
11.4 ± 0.2
ether
n.a. = not available
UVꢀvisible spectra of the complexes in CHCl3 are
shown in Figure 4 and Table 2 to compare the effects of
the axial ligands on their electronic states. The effects of
the type of carboxylates bridging RhꢀRh on the electronꢀ
ic states have been well studies.25 It is well known that
there are two peaks around 500~600 nm (band I) and
450 nm (band II) in the visible region, which can be asꢀ
signed to the allowed transitions, π* ꢀ σ* and π* ꢀ
Rh–O σ*, respectively.25 Indeed, band I was markedly
affected by the type of the axial ligands as shown in
Figure 4, while band II around 450 nm remained almost
unchanged (see the Supporting Information). In comparꢀ
ison between the axial pyridine ligands with a substituꢀ
ent at the 2ꢀposition, 1·(py)2, 1·(mepy)2, and 1·(etpy)2,
bulkier axial ligands showed absorption at a longer
wavelength in the order of 1·(py)2 (527 nm) < 1·(mepy)2
(554 nm) < 1·(etpy)2 (566 nm). On the other hand, only
small electronic effects were observed with 1·(dmap)2
(redꢀshift by 7 nm) and 1·(min)2 (blueꢀshift by 7 nm),
which has an electron donating dimethylamino group
and an electron withdrawing methoxycarbonyl group,
respectively, at the 4ꢀposition. It should be noticed that
the band I for 1·(ether)2 appeared in the longest waveꢀ
ꢀG‡ = 4.55 × 10ꢀ3 × Tc{9.97 + logTc ꢀ log(500 ×
ꢀν))
It is apparent that the rotational rates of the pyridine
adducts are influenced by the substituents at the 2ꢀ
positions of the pyridines, as compared between 1·(py)2,
1·(mepy)2, and 1·(etpy)2 (Table 2). This suggests the
possibility of ligand exchangeꢀbased rate control of moꢀ
lecular motions. At 220 K, further splitting was observed
in the spectra of 1·(mepy)2 and 1·(etpy)2 presumably due
to the slowedꢀdown rotation of unsymmetrical axial ligꢀ
ands (see the Supporting Information).
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 6
K. Y.; Dehez, F.; Zerbetto, F. Nature 2003, 424, 174. (c) Kelly, T. R.;
length side (608 nm). This result indicates that the type
of coordinating atoms is a more effective factor affecting
the electronic states of the complexes.
De Silva, H.; Silva, R. A. Nature 1999, 401, 150.
(5) (a) Kelly, T. R.; Bower, M. C.; Bhaskar, K. V.; Bebbington, D.;
Garcia, A.; Lang, F. Kim, M. H.; Jette, M. P. J. Am. Chem. Soc. 1994,
116, 3657. (b) Takeuchi, M.; Imada, T.; Shinkai, S. Angew. Chem.,
Int. Ed. 1998, 37, 2096. (c) Tashiro, K.; Konishi, K.; Aida, T. J. Am.
Chem. Soc. 2000, 122, 7921.
(6) (a) Muraoka, T.; Kinbara, K.; Kobayashi, Y.; Aida, T. J. Am.
Chem. Soc. 2013, 125, 5612. (b) Muraoka, T.; Kinbara, K.; Aida, T.
Nature 2006, 440, 512.
1
2
3
4
5
6
7
8
In conclusion, we have constructed a dirhodiumꢀ
centered circular fourꢀgear system, in which the gears
have a circularly meshed structure both in solution and
in the crystal state. Dynamic NMR analysis revealed that
the exchangeable axial ligands can remarkably affect the
rotational behaviors of the molecular gear system mainly
due to the steric effects of the substituents and the type
of the coordination atoms of the axial ligands as shown
by NMR spectroscopy and visible absorption study.
These findings would provide a useful design guide for
stimulusꢀresponsive metalloꢀmolecular rotors that realize
ligand exchangeꢀbased rotational motions in a selfꢀ
assembled system.
(7) Samanta, S. K.; Bats, J. W.; Schmittel, M. Chem. Commun.
2014, 50, 2364.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(8) Kawada, Y.; Iwamura, H. J. Org. Chem. 1980, 45, 2547.
(9) Hounshell, W. D.; Johnson, C. A.; Guenzi, A.; Cozzi, F.; Misꢀ
low, K. Proc. Natl. Acad. Sci. USA 1980, 77, 696.
(10) Kao, C.ꢀY.; Hsu, Y.ꢀT.; Lu, H.ꢀF.; Chao, I.; Huang, S.ꢀL.; Lin,
Y.ꢀC.; Sun, W.ꢀT.; Yang, J.ꢀS. J. Org. Chem. 2011, 76, 5782.
(11) Ogi, S.; Ikeda, T.; Wakabayashi, R.; Shinkai, S.; Takeuchi, M.
Chem. Eur. J. 2010, 16, 8285.
(12) Stevens, A. M.; Richards, C. J. Tetrahedron Lett. 1997, 38,
7805.
(13) Koga, N.; Kawada, Y.; Iwamura, H. Tetrahedron 1986, 42,
1679.
ASSOCIATED CONTENT
Experimental procedures and characterization data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(14) Chance, J. M.; Geiger, J. H.; Okamoto, Y.; Aburatani, R.;
Mislow, K. J. Am. Chem. Soc. 1990, 112, 3540.
(15) Yamamoto, G.; Ohta, S.; Kaneko, M.; Mouri, K.; Ohkuma,
M.; Mikami, R.; Uchiyama, Y.; Minoura, M. Bull. Chem. Soc. Jpn.
2005, 78, 487.
(16) Ogi, S.; Ikeda, T.; Wakabayashi, R.; Shinkai, S.; Takeuchi, M.
Eur. J. Org. Chem. 2011, 1831.
(17) Setaka, W.; Nirengi, T.; Kabuto, C.; Kira M. J. Am. Chem.
Soc. 2008, 130, 15762.
(18) (a) Koshino, M.; Tanaka, T.; Solin, N.; Suenaga, K.; Isobe, H.;
Nakamura, E. Science 2007, 316, 853. (b) Koshino, M.; Niimi, Y.;
Nakamura, E.; Kataura, H.; Okazaki, T.; Suenaga, K.; Iijima, S. Nat.
Chem. 2010, 2, 117.
(19) Noji, H.; Yasuda, R.; Yoshida, M.; Kinosita Jr, K. Nature
1997, 386, 299.
(20) Ikeda, T.; Tsukahara, T.; Iino, R.; Takeuchi, M.; Noji, H. An-
gew. Chem., Int. Ed. 2014, 53, 10082.
(21) Demonceau, A.; Noels, A. F.; Hubert, A. J.; Teyssié, P. Bull.
Soc. Chim. Belg. 1984, 93, 945.
(22) Friedle, S.; Kodanko, J. J.; Fornace, K. L.; Lippard, S. J. J.
Mol. Struct. 2008, 890, 317.
(23) Vagin, S.; Ott, A.; Weiss, H.ꢀC.; Karbach, A.; Volkmer, D.;
Rieger, B. Eur. J. Inorg. Chem. 2008, 2601.
(24) Hesse, M.; Maier, H.; Zeeh, B. Spectroscopic Methods in Or-
ganic Chemistry; Thieme Publishing Group: Stuttgart, 2007.
(25) Boyar, E. B.; Robinson, S. D. Coord. Chem. Rev. 1983, 50,
109.
AUTHOR INFORMATION
Corresponding Author
shionoya@chem.s.uꢀtokyo.ac.jp
Funding Sources
No competing financial interests have been declared.
ACKNOWLEDGMENT
This study was supported by JSPS KAKENHI Grant Numꢀ
bers 26248016 (for M.S.).
REFERENCES
(1) Frank, J. Molecular machines in biology: Workshop of the cell;
Cambridge University Press: Cambridge, 2011.
(2) (a) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew.
Chem., Int. Ed. 2000, 39, 3348. (b) Browne, W. R.; Feringa, B. L.
Nat. Nanotechnol. 2006, 1, 25. (c) Kay, E. R.; Leigh, D. A.; Zerbetto,
F. Angew. Chem., Int. Ed. 2007, 46, 72.
(3) Molecular machines in the crystalline state have been also
widely investigated. See, (a) Vogelsberg, C. S.; GarciaꢀGaribay, M.
A. Chem. Soc. Rev. 2012, 41, 1892. (b) Jiang, X.; RodriguezꢀMolina,
B.; Nazarian, N.; GarciaꢀGaribay, M. A. J. Am. Chem. Soc. 2014, 136,
8871.
(4) (a) Koumura, N.; Zijlstra, R. W. J.; van Delden, R. A.; Harada,
N.; Feringa, B. L. Nature 1999, 401, 152. (b) Leigh, D. A.; Wong, J.
ACS Paragon Plus Environment

Page 5 of 6
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 6
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Synthesis of a lantern-type dirhodium complex with circularly-arranged four 2,3,6,7,14,15-
hexamethyltriptycene carbox- ylates as gears and two axial ligands as velocity control elements are
reported. The rotating velocities in solution were mark- edly-affected by the donating properties of
coordinating atoms and the bulkiness of axial ligands. Notably, the velocity changes were accompanied by
the color changes of each solution.
85x47mm (300 x 300 DPI)
ACS Paragon Plus Environment