Ranging correlated motion (1.5 nm) of two coaxially arranged rotors mediated by helix inversion of a supramolecular transmitter

Article abstract of DOI:10.1021/ja8014583

For a long-range transmission of motion between two movable parts apart from each other, transmitters that can precisely correlate these two motions should be properly incorporated into the system. However, such a motional relay is yet to be realized in a

Full text of DOI:10.1021/ja8014583

Published on Web 06/18/2008
Ranging Correlated Motion (1.5 nm) of Two Coaxially
Arranged Rotors Mediated by Helix Inversion of a
Supramolecular Transmitter
Shuichi Hiraoka,*,†,‡ Erika Okuno,† Takaaki Tanaka,† Motoo Shiro,§ and
Mitsuhiko Shionoya*,†
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan, Precursory Research for Embryonic Science and
Technology (PRESTO), Japan Science Technology Agency, 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan, and Rigaku Corporation, 3-9-12 Matsubaracho, Akishima,
Tokyo 196-8666, Japan
Received February 27, 2008; E-mail: shionoya@chem.s.u-tokyo.ac.jp; hiraoka@chem.s.u-tokyo.ac.jp
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This paper contains enhanced objects available on the Internet at http://pubs.acs.org/jacs.
Abstract: For a long-range transmission of motion between two movable parts apart from each other,
transmitters that can precisely correlate these two motions should be properly incorporated into the system.
However, such a motional relay is yet to be realized in artificial systems because of the lack of reliable
methodologies for arranging a discrete number of motional parts. Herein, we report a correlated motion of
two rotor molecules, which are coaxially arranged at a distance of 1.5 nm, through either Ag+- or Hg2+
-
assembled helical transmitters, leading to different frequencies of synchronized motion. A helix inversion
in the transmitter was proven to strongly correlate the motions of both terminals. The X-ray analysis of the
entity determined a quadruple-decker nonanuclear structure of the metal complex comprising two terminal
rotor-like ligands closely attached to a central transmitter moiety. 1H NMR analysis fully demonstrated the
synchronized motion of the two rotors coaxially stacked and connected through the transmitter. Since the
transmitter is composed of simple helical repeating units, the principle of helix inversion would be an efficient
and widely applicable strategy for the long-range transmission of molecular motion.
Introduction
different number of tooth or the direction of rotation axis, have
been realized by simple organic molecules,3–5 metal-coordina-
In a macroscopic machine the rotation of a motor turns a
terminal actuator through intermediate shafts and gears. Es-
sentially, two (rotational) motions, on either side of a simple
mechanical device, are correlated by an in-between part called
a “transmitter” herein. Likewise, elaborate transmission or
transformation of molecular motions is essential for developing
simple molecular motions to more sophisticated mechanical
actions used in artificial molecular machine systems. In early
studies on molecular machines,1 the use of steric interactions
between two covalently linked contiguous gearlike parts is a
classical but rational strategy to join these two motional parts.
The first report on the correlated motion between two artificial
gear molecules was back in the 1970s,2 in which adjacent
benzene rings interact with each other within the molecules
through steric interactions. Since then, a large number of two-
gear systems, that can convert the rate of rotation using a
tion compounds,6–10 and hybridized circular DNA strands.11
Beyond two-gear systems in which two movable parts interact
directly with each other (Figure 1A,B), the multigear systems
that were reported, however, are attained in a circular array of
small aromatic rings attached to a benzene ring12 or linear
oligomers with rotation of aromatic rings along carbon-carbon
single bonds of a main chain.13,14 In our view, it is intrinsically
impossible to transmit motions at a long-range in the former
(3) Iwamura, H.; Mislow, K. Acc. Chem. Res. 1988, 21, 175.
j
(4) Yamamoto, G.; Oki, M. J. Org. Chem. 1983, 48, 1233.
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2002, 233-234, 75.
(7) Stevens, A. M.; Richards, C. J. Tetrahedron Lett. 1997, 38, 7805.
(8) Romeo, R.; Carnabuci, S.; Fenech, L.; Plutino, M. R.; Albinati, A.
Angew. Chem., Int. Ed. 2006, 45, 4494.
(9) Bulo, R. E.; Allaart, F.; Ehlers, A. W.; de Kanter, F. J. J.; Schakel,
M.; Luts, M.; Spek, A. L.; Lammertsma, K. J. Am. Chem. Soc. 2006,
128, 12169.
† The University of Tokyo.
‡ Japan Science Technology Agency.
§ Rigaku Corporation.
(10) Carella, A.; Jaud, J.; Rapenne, G.; Launary, J. P. Chem. Commun.
2003, 2434.
(1) (a) Balzani, V.; Venturi, M.; Credi, A. Molecular DeVices and
Machines-A Journey into the Nano World; Wiley-VCH: Weinheim,
Germany, 2003. (b) Feringa, B. L. J. Org. Chem. 2007, 72, 6635. (c)
Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2006,
46, 72. (d) Sauvage, J.-P. Chem. Commun. 2005, 1507. (e) Saha, S.;
Stoddart, J. F. Chem. Soc. ReV. 2007, 36, 77.
(11) Tian, Y.; Mao, C. J. Am. Chem. Soc. 2004, 126, 11410.
(12) Gust, D.; Patton, A. J. Am. Chem. Soc. 1978, 100, 8175.
(13) Lindner, A. B.; Grynszpan, F.; Biali, S. E. J. Org. Chem. 1993, 58,
6662.
(14) Coluccini, C.; Grilli, S.; Lunazzi, L.; Mazzanti, A. J. Org. Chem. 2003,
68, 7266.
(2) Gust, D.; Mislow, K. J. Am. Chem. Soc. 1973, 95, 1535.
9
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A R T I C L E S
Hiraoka et al.
Figure 1. (A-C) Schematic representation of correlated motions between two rotational parts. (A) Two gears with parallel rotation axes (left) and a coaxis
(right). The motions of two gears are correlated by their direct steric interaction. (B) A rotational transmitting system: the motions of two gears (blue) are
correlated through a rotational motion of a central transmitter (red). (C) Outline drawing of an oscillation transmitting system: the motions on both sides are
mediated by oscillating motions of a central transmitter. (Left) A linear oscillating transmitter. (right) A circular oscillating transmitter, in which two parts
(red and blue) are alternately arranged in both the rotor and the transmitter and have a higher affinity for that in the same color. The rotation of the rotor on
one side induces the circular oscillating motion of blades in the transmitter, and thereby the rotor on the other side is allowed to rotate cooperatively. (D)
(Left) A design concept of a molecular rotor-transmitter-rotor device, [Ag912 ·22]9+, composed of a transmitter ([Ag912]9+) and two rotors (2). The rotation
on one side propagates to the other side on a steric demand between the rings both in the rotors and transmitter, for example, rings A to H. (Right) The helix
inversion of transmitter [Ag912]9+ mediates the motions of two rotors, 2 (movie 1). (E) A molecular ball bearing consisting of a hexa-monodentate ligand
2, a tris-monodentate ligand L (4 or 5), and three Ag+ ions. The ligand L arranges the three Ag+ ions in triangle, while six ligand rings in 2 participate in
the intramolecular ligand exchange that causes the relative rotation. (F) A proposed mechanism of a relative rotation of the two rotors in the molecular ball
bearings, [Ag32·L]3+. A 60° relative rotation consists of a ligand exchange and a flip motion (movie 2). The ligand exchange takes place on three Ag+ ions
simultaneously, that is, every second ligand ring of 2 coordinating to Ag+ exchanges with the neighboring coordinatively free ligand rings accompanying
the helix inversion of the complex. The flip motion also accompanies the helix inversion retaining the N-Ag-N bonds.
movie 1, showing the helix inversion of transmitter [Ag912]9+ mediating the motions of two rotors, 2, is available.
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movie 2, showing a 60° relative rotation consisting of a ligand exchange and a flip motion, is available.
case, and even in the latter case it is rather difficult to precisely
control the transmission. Thus, a new synthetic strategy to allow
long-range transmission of motions through a well-designed
transmitter molecule that can precisely synchronize more than
two motions, has been long-awaited.15
Molecular Design of Rotor-Transmitter-Rotors (RTRs).
Recently, we first reported a supramolecular rotational device,
[Ag32·4]3+ complex, which behaves like a ball bearing (Figure
1E).16,17 In this molecule, the random processes of a ligand
exchange (M T P in Figure 1F) and a flip motion (P T M′ in
Figure 1F) allow the two rotor parts, 2 and 4, to rotate relatively
to each other (rotation in either direction, back and forth motion,
and their combination). Both motions take place between the P
and M helical enantiomers of [Ag32·4]3+ (Figure 1F and movie
2),18 and the motions of the rings in the two rotors are precisely
synchronized with each other through their helix inversion. In
this view, a transmitter that can mediate the helix inversion was
expected to correlate the two rotational motions. We have then
newly designed a supramolecular transmitter that can correlate
two rotational motions through an oscillating helix inversion,
as shown in Figure 1C (right). The transmitter has a metal-
assembled helical structure, [Ag6M312](6+3n)+ (Mn+: Ag+ or
Hg2+), and the helix inversion between two helical optical
isomers, P and M, can correlate the motions of two disk-shaped
rotor molecules, 2. The two rotors are ca. 1.5 nm apart and
coaxially connected with the central transmitter through metal
(15) Muraoka, T.; Kinbara, K.; Aida, T. Nature 2006, 440, 512.
(16) Hiraoka, S.; Shiro, M.; Shionoya, M. J. Am. Chem. Soc. 2004, 126,
1214.
(17) Hiraoka, S.; Hirata, K.; Shionoya, M. Angew. Chem., Int. Ed. 2004,
43, 3814.
(18) Hiraoka, S.; Harano, K.; Tanaka, T.; Shiro, M.; Shionoya, M. Angew.
Chem., Int. Ed. 2003, 42, 5182.
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A R T I C L E S
Chart 1. Ligands 1-5 Used in This Studya
a The letters denote the 1H NMR assignments in Figure 2.
coordination (Figure 1D). The ligand 1 has a central benzene
ring carrying alternating oxazolinyl and oxazolinylphenyl groups
on two concentric circles. That is, three oxazoline ligands (G1)
are placed on the inner circle and the other three oxazoline
ligands (G2) on the outer circle. It was thereby expected that
the three ligands G1 of 1 act as a partner of a terminal rotor 2
through Ag+-mediated self-assembly, while the other three
ligands G2 of 1 serve as a joint within the metal-mediated
transmitter. From a molecular modeling study, it was most likely
that in the resulting molecular rotor-transmitter–rotor (RTR),
[Ag6M312 ·22](6+3n)+, four disk-shaped ligands are coaxially
stacked with the aid of nine metal ions, and that both the
transmitter and rotors in the RTR have a helical structure. Thus,
the propagation of a helicity from one side of the rotor 2 to the
other side (ring A to ring H in Figure 1D left) should be
mediated by the synchronous helix inversion that takes place
in the rotors and the transmitter. The helix inversion of on
one side of the rotor 2 (ring A) as a result of the ligand exchange
or the flip motion brings about the conversion of the helicity of
1 (rings B and C) in the transmitter through Ag(I)-N coordina-
tion bonds between the rotor and the transmitter. And then, the
outer oxazoline rings (ring D) in 1 rotate to switch the helicity
of the other side of 1 (ring E) through Ag(I)-N coordination
bonds in the transmitter. As the result, the helicity of the other
side of the rotor (ring H) connected to the transmitter is
simultaneously converted. Since the two disk-shaped ligands 1
are closely placed, it is probable that the propagation of the
helicity should occur face-to-face through a direct contact
between the rings (ring B to G and C to F). Both the propagation
pathways should allow a synchronous motion with the two
terminal rotors, 2, through the helix inversion of the central
transmitter [Ag6M312](6+3n)+ (movie 1).
nonanuclear [Ag912 · 22]9+ complex was immediately formed
from 1, 2, and AgOTf (OTf:CF3SO3) in a 2:2:9 ratio in CD3OD
1
at room temperature. The H NMR spectrum of [Ag912 ·22]9+
at 313 K showed well-resolved, simple resonances, suggesting
the quantitative formation of a discrete structure with a high
symmetry (Figure 2). It is notable that the signals for protons e
and f of 1 appeared as four resonances, indicating that two faces
of the disk-shaped ligand 1 are nonequivalent to each other as
a result of the formation of the [Ag912 ·22]9+ complex.16 Its ESI-
TOF mass spectrum showed two prominent signals at m/z )
2304.5 and 1487.1, which are assignable to [Ag912 ·22 ·(OTf)7]2+
and [Ag912 ·22 ·(OTf)6]3+, respectively (Figure S3 in the Sup-
porting Information). We thereby concluded that the
[Ag912 ·22]9+ complex is exclusively formed in solution.
Single-Crystal X-Ray Analysis. A single-crystal suitable for
X-ray diffraction study was obtained from a saturated CH3OH
solution of Ag912 ·22 ·(OTf)9. The crystal structure contained a
nonanuclear [Ag912 ·22]9+ cation together with nine uncoordi-
nated TfO- anions (Figure 3A-C). Its quadruple-decker
structure can be best described as an assembly of two outer (2)
and two inner (1) disk-shaped ligands stacked on top of each
other with the aid of nine Ag+ ions. The total length of the
molecule along the rotation axis is ca. 21 Å and the two terminal
rotors (2) are 1.5 nm apart. Each Ag+ ion is linear two-
coordinate, and therefore every second thiazole nitrogen atom
in 2 is coordinatively free. All the ligand ring planes of the
four disks are not perpendicular to the central benzene ring
planes, resulting in the formation of a helical structure (P or M
form). A set of enantiomers is included in a unit cell to form a
racemate. The oxazoline and phenylene rings attached directly
to the central benzene rings in 1 (G1) and all the thiazolyl rings
in 2 tilt toward the same direction by ca. 30° from the rotation
axis. On the other hand, the outer three oxazoline rings (G2) in
1 tilt in the opposite helical sense by ca. 10°. This observation
indicates the structural interlocking of the four ligands in the
entity. As shown in the molecular packing of [Ag912 ·22]9+ in
the crystal (Figure 3C), the cationic parts, [Ag912 ·22]9+, are
completely separated from each other, and their rotational axes
are arranged in parallel rows.
Results and Discussion
Syntheses of Disk-Shaped Ligands. The disk-shaped multi-
monodentate ligands 1-5 shown in Chart 1 were synthesized
by the cobalt-catalyzed trimerization of alkyne derivatives. The
synthetic procedure for disk-shaped ligands 1,19 2,16 and 418
was previously reported. Disk-shaped hexa-monodentate ligands,
3 and 5, were synthesized according to Scheme S1 (see the
Supporting Information).
Relative Rotation of Rotors in the Molecular RTRs. Rela-
tively free rotation of the two rotors in the RTRs was
demonstrated by examining the symmetry of the complexes,
[Ag6M312 ·32](6+3n)+ (Mn+: Ag+ or Hg2+), including two hexa-
monodentate ligands 3 with C2V symmetry. If the rate of the
rotation is faster than the NMR time scale, the proton signals
for 1 in the transmitter should appear in the same way as their
original ligand 2 with C3 symmetry. That is, the oxazolinyl
proton signals a-d of 1, for example, should be observed as
one set. The proton signals of 3 in the [Ag912 ·32]9+ complex,
on the other hand, are not informative enough to describe the
Formation of Molecular RTRs. Molecular RTRs,
[Ag6M312 · X2](6+3n)+ (X: 2 or 3; Mn+: Ag+ or Hg2+), were
quantitatively formed by simply mixing a set of components in
CD3OD. The resulting entities were fully characterized in
1
solution by H NMR spectroscopy and electrospray ionization
time-of-flight (ESI-TOF) mass spectrometry. For example, a
(19) Hiraoka, S.; Tanaka, T.; Shionoya, M. J. Am. Chem. Soc. 2006, 128,
13038.
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Hiraoka et al.
Figure 2. Partial VT 1H NMR spectra (500 MHz, CD3OD) of a molecular rotor-transmitter-rotor, Ag912 ·22 ·(OTf)9. [Ag912 ·22 ·(OTf)9] ) 3.7 mM.
rotation in detail, because these signals should not show any
changes even when the rate of the flip motion becomes slower
than the NMR time scale on the assumption that the energy
barrier of the flip motion, ∆Gqflip, is greater than that of the
ligand exchange, ∆Gqex. The detailed comparative investigation
of the energy profile of these two motions in the molecular ball
bearings with ligand 3 will be reported elsewhere. The quantita-
tive formation of [Ag912 ·32]9+ was demonstrated by 1H NMR
titration studies (Figure S2) and ESI-TOF mass measurements
(Figure S4). The 1H NMR spectrum of [Ag912 ·32]9+ at 303 K
showed two inequivalent oxazoline rings assignable to the inner
and outer rings, indicating the rapid rotation of the rotors with
In this case, the proton signals for oxazoline rings, a-d, are
good indicators to analyze the helix inversion of both the inner
and outer rings, because two geminal protons in the oxazoline
rings are inequivalent, provided that the helix inversion is rather
slow compared with the NMR time scale (Figure 5D). However,
in addition to the helix inversion, the chemical exchange
between these geminal protons would take place due to the ring
flip of oxazoline rings with twisted-boat conformation (Figure
5A). To evaluate the rate of the ring flip, variable-temperature
(VT) 1H NMR measurements of both free ligand 1 and
sandwich-shaped [Ag612]6+ complex19 (Figure 5C), in which
six Ag+ ions are placed between the two ligands, were
performed. The oxazolinyl protons in both 1 and [Ag612]6+
showed almost no changes but became slightly broadened at
temperature as low as 193 K, indicating that the ring flip is so
fast on the NMR time scale or that oxazoline rings are flat (as
shown in the nearly planar conformation of oxazoline rings in
the crystal structure of [Ag912 ·22]9+). Therefore, the ring flip
is negligible in the present VT NMR analysis of the
[Ag912 ·22]9+ complex. As the temperature of solutions of
[Ag912 ·22]9+ in CD3OD was lowered, the oxazolinyl proton
signals, a-d, were divided into two sets arising from slower
helix inversion compared with the NMR time scale (Figure 6A).
The line-shape analysis of the oxazolinyl proton signals clarified
that the helix inversion of the inner and outer oxazoline rings
takes place at almost the same rate at temperatures ranging from
293 to 253 K (Figure 6B). This result demonstrates that all the
rings in the rotors and the transmitter bring about the helix
inversion at the same frequency, that is, a synchronous motion
C2V symmetry on the NMR time scale (Figure 4). Upon cooling
down to 243 K, the signals for 1 and 3 became broadened and
more complicated (Figure S7). This is well explained by the
fact that the rate of the rotation becomes slower than the NMR
time scale around this temperature.
Motional Correlation. As both the ligand exchange and the
flip motion take place with the helix inversion of the structure,
the analysis of the helix inversion is the most useful way to
assess the motional correlation in [Ag912 ·22]9+. In other words,
the rate of helix inversion in the rotors 2 can be determined
from the motion of the inner oxazoline rings (G1), and then
compared with the rate of helix inversion of the outer oxazoline
rings of 1 (G2) in the transmitter. If the motion of the two
terminal rotors 2 are synchronized with each other, the rates of
helix inversion (khelix) of the inner and outer oxazoline rings in
1 should be identical and prove that the helix inversions of both
the rotors and the transmitter take place at the same frequency.
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A R T I C L E S
Figure 3. Crystal structure of the Ag912 ·22 ·(OTf)9 complex. (A) A space filling model. (B) A cylinder model. Hydrogen atoms are omitted for clarity.
Color labels: red, carbon; yellow, silver, blue, nitrogen; purple, oxygen; green, sulfur. (C) A packing diagram of [Ag912 ·22]9+ viewed from a rotation axis
of the complex. Hydrogen atoms and counteranions are omitted for clarity. (D) Schematic depiction of the proposed ground and transition geometries of the
ligand exchange processes taking place in the [Ag912 ·22]9+ complex.
Figure 4. Partial VT 1H NMR spectrum (500 MHz, CD3OD) of Ag912 ·32 ·(OTf)9 ([Ag912 ·32 ·(OTf)9] ) 4.0 mM, 303 K.
of the two terminal rotors 2 mediated by the helix inversion of
temperature of the CD3OD solution of [Ag912 ·22]9+ was
lowered, the proton signals g and h of 2 became broadened
around 263 K and finally divided into two sets at 243 K (Figure
2). The kinetic parameters for the ligand exchange, ∆Hqex and
∆Sqex, were determined to be 77.7 kJ mol-1 and 46.1 J mol-1
K-1, respectively (Table 1). The rate constant for [Ag912 ·22]9+
at 298 K, 1 × 104 s-1, is about twenty times smaller than that
of a monomeric molecular ball bearing, [Ag32·5]3+, and the
the central transmitter. The kinetic parameters for the helix
inversion were determined to be ∆Hq
) 77 kJ mol-1 and
helix
∆Sq
) 38 J mol-1 K-1. It should be noted that a large,
helix
positive entropy change suggests the cooperativity of the motion.
Considering that the helix inversion is a combination of the
ligand exchange and the flip motion, we shall discuss these
parameters along with the ligand exchange and the flip motion
in the following sections.
enthalpy change (∆Hqex) was found greater with [Ag912 ·22]9+
.
Intramolecular Multipoint Ligand Exchanges in an RTR. As
expected from the interlocked structure of [Ag912 ·22]9+, as
evidenced by molecular modeling and the crystal structure, the
rotation of the terminal rotors 2 with a combination of ligand
exchange and the flip motion should be strongly affected by
the supramolecular transmitter. The intramolecular three-point
ligand exchange arising in the rotors of [Ag912 ·22]9+ was then
studied in detail by VT 1H NMR spectroscopy. As the
It is apparent that these significant differences come from the
supramolecular transmitter connecting the terminal rotors. The
larger ∆Hq for [Ag912 ·22]9+ should arise from the steric
ex
interactions of rings between two ligands 1 in the transmitter.
Furthermore, the entropy changes (∆Sqex) showed a significant
difference as shown in a large positive ∆Sqex value, 46.1 J mol-1
K-1, for RTR, [Ag912 ·22]9+, and a negative ∆Sqex value, -9.2
J mol-1 K-1, for molecular ball bearing, [Ag32·5]3+. Three
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A R T I C L E S
Hiraoka et al.
Figure 5. (A) Ring flip of an oxazoline ring between two twisted-boat conformations. (B) Chemical structure of asymmetric tris-monodentate ligand 6. VT
NMR spectra of [Ag32·6]3+ were reported in ref 17. (C) Schematic representation of [Ag612]6+ complex. The synthesis and structural characterization of
the complex were reported in ref 19. (D) Schematic representation of the helix inversion in [Ag6M312 ·22](3n+6)+ complexes (Mn+ ) Ag+ or Hg2+). PP and
MM denote screw senses of the two molecular ball bearing moieties ([Ag31·2]3+) in their structures.
1
Figure 6. (A) Partial VT H NMR spectra of [Ag912 ·22]9+ complex (500 MHz, CD3OD, [1] ) [2] ) 3.7 mM). (B) Simulated spectra of Ha, Hb, Hc, and
Hd protons for various exchange rates. (C) Eyring plots.
additional Ag-N bonds in the plausible three-coordinate Ag+
observed for [Ag32·5]3+ 16 Although a negative ∆Sq was
.
ex
centers in the transition state (TSex) of the intramolecular ligand
similarly expected owing to the six additional Ag-N bonds
exchange should be the primary reason for the negative ∆Sq
formed in the transition state of [Ag912 ·22]9+; in fact, a large
ex
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Ranging Correlated Motion of Two Rotors
A R T I C L E S
barrier of the flip motion in molecular RTR, [Ag912 ·22]9+, is
considerably heightened and becomes larger than that of the
ligand exchange. This is probably because the stretched structure
in the transition state of the flip motion, in which all the rings
attached to the central benzene rings are parallel to the rotation
axis, gets rather unstable compared with that of the intramo-
lecular ligand exchange.
Table 1. Kinetic Parameters of Intramolecular Ligand Exchange
for Molecular Ball Bearing and Rotor-Transmitter-Rotor Devicesa
∆Hq
∆Sq
∆Gq
k at
298 K (s-1
)
complex
(kJ mol-1
)
(J mol-1 K-1
)
(kJ mol-1
)
[Ag32·5]3+
51.7
77.7
67.9
-9.2
46.1
5.3
54.4
63.9
66.3
2 × 105
1 × 104
4 × 103
[Ag912 ·22]9+
[Ag6Hg312 ·22]12+
[Ag6Hg312]12+ Transmitter. As described above, the connec-
tion of the two terminal rotors by a supramolecular transmitter
[Ag912]9+ resulted in an intramolecular motional correlation
between them at a distance of 1.5 nm. The changes in the size
and the charge number of metal ions in the [Ag6M312](6+3n)+
transmitter would affect the kinetic characteristic of the motional
correlation. Recently, we have found that monovalent Ag+ and
divalent Hg2+ ions, both of which prefer a linear two-coordinate
geometry, are hierarchically arranged on the G1 and G2 metal
binding sites of 1, respectively, in an electrostatically controlled
manner.19 On the basis of this finding, we have developed a
sandwich-shaped [Ag6Hg312 · 22]12+ complex quantitatively
formed in solution. In this case, the central three Ag+ ions on
the G2 ligand sites of the transmitter were expected to be
a Determined by the line shape analysis of the chemical exchange of
thiazolyl protons of 2.
positive ∆Sq value was obtained. This is probably because
ex
two ligands 1 in the transmitter are bent in the ground-state so
that the motion of the rings in [Ag912 · 22]9+ may be highly
restrained (Figure 3D), as was observed in the crystal structure
and judged from the molecular modeling. On the other hand,
the structure of [Ag912 ·22]9+ in the transition state of the ligand
exchange process is considered stretched along the rotation axis
so that the rings can move without hindrance. Thus, it appears
that the large positive ∆Sq value for [Ag912 ·22]9+ arises from
the partially compacted structure around the metal centers of
rotors in the ground state. These results indicate that the helix
inversion of the transmitter would strongly affect the rotational
motions of the two terminal rotors.
1
specifically substituted by three Hg2+ ions. In fact, H NMR
and ESI-TOF mass measurements of a mixture of 1, 2, AgOTf,
and Hg(OTf)2 in a 2:2:6:3 ratio demonstrated the exclusive
formation of an alternative RTR, [Ag6Hg312 ·22]12+ complex.
Flip Motion. In the flip motion, which is the other basic
motion in the rotation mechanism (Figure 1F), the rate of flip
motion (kflip) in [Ag912 ·22]9+ should become considerably
slower than a molecular ball bearing, [Ag32·5]3+, as expected
from its slower ligand exchange. The flip motion is accompanied
by a helix inversion between two enantiomers with retention
of all the Ag-N bonds during the process. In general, it is hard
to analyze the flip motion of an achiral molecular ball bearing.
Indeed, to assess the flip motion, diastereomeric complexes
having chiral tris-monodentate ligands17 or counteranions18 are
needed. Since both the ligand exchange and the flip motion
accompany a helix inversion, the khelix should be the summation
of the rate constants of both motions (kex and kflip), that is, khelix
) kex + kflip. Therefore, kflip can be estimated from kex and khelix
which were determined experimentally by analyzing the chemi-
cal exchanges of the thiazolyl protons of 2 and oxazolinyl
protons of 1, respectively. Thus, khelix at every temperature
should be greater than kex. However, the line shape analysis of
these proton signals at various temperatures indicates slower
rate constants of the helix inversion. This result contradicts the
above-mentioned considerations. The slower rate constants of
the chemical exchange of the oxazolinyl protons could be
attributed to their relatively broadened signals. This is probably
due to the temperature-dependent conformational changes of
the oxazoline rings. These factors should affect evenly all the
oxazolinyl protons so that one can compare the chemical
exchange of these protons. The VT NMR measurement of
[Ag922 ·32]9+ containing 3 with C2V symmetry clearly showed
that the rotation of the terminal rotors is fast on the NMR time
scale at 303 K, indicating that the flip motion is also fast on
the NMR time scale at the temperature. In light of these results,
we conclude that the flip motion is slower than the ligand
exchange but still fast on the NMR time scale at 303 K. Thus,
kflip would reach at least 1000 s-1 at 298 K. The relatively slower
flip motion in [Ag912 ·22]9+ is contradistinctive. In the case of
a monomeric ball bearing, the flip motion is much faster than
the ligand exchange. For example, any diastereotopic proton
signals were not observed for [Ag32·6]3+ with asymmetric tris-
monodentate ligand 6 (Figure 5B) even at 193 K, whereas two
sets of thiazolyl signals of 2 were observed, indicating that the
flip motion is faster than the ligand exchange.17 Thus, the energy
1
The H NMR spectrum of the complex at 323 K (Figure 7)
displayed resonances for both 1 and 2 with C3 symmetry as
observed with [Ag912 ·22]9+. This result suggests the formation
of a quadruple-decker [Ag6Hg312 ·22]12+ complex with an
identical structural symmetry to that of [Ag912 ·22]9+. Moreover,
the resonances for the protons of the oxazoline rings G2, a and
b, for [Ag6Hg312 ·22]12+ shifted to 5.1 and 4.6 ppm, respectively,
whose magnitude is greater than [Ag912 ·22]9+. On the other
hand, the proton signals of ring G1,
c and d, for
[Ag6Hg312 ·22]12+ stayed almost constant as seen with those for
[Ag912 ·22]9+. This suggests that the divalent Hg2+ ions were
site-selectively placed on the G2 ligand rings as expected
from our previous study.19 In addition, the ESI-TOF mass
spectrum showed a signal at m/z ) 1728.7 assignable to
[Ag6Hg312 ·22 ·(OTf)9]3+ (Figure S5). These results lead to a
conclusion that the three Hg2+ ions are bound by the outer rings
G2 of 1 in the center of the [Ag6Hg312 ·22]12+ complex.20 This
heteronuclear complex was also formed by the selective
replacement of the three Ag+ ions in the middle of [Ag912 ·22]9+
.
Upon the addition of 3 equiv of Hg(OTf)2 to a solution of
[Ag912 · 22]9+, the complex was quickly converted into
[Ag6Hg312 ·22]12+ (Figure 8). Then, [Ag912 ·22]9+ regenerated
as a result of the trapping of the three Hg2+ ions from
[Ag6Hg312 ·22]12+ by [2,2,2]-cryptand which has a higher affinity
to Hg2+ than to Ag+ (Figure 8).21 Thus, these complexes are
interconvertible with each other by addition of Hg2+ and [2,2,2]-
cryptand one after the other.
The intramolecular ligand exchange in the [Ag6Hg312 ·22]12+
complex was examined by VT 1H NMR study. Proton signals,
g and h, became broadened upon cooling down to 273 K, and
then both signals were finally divided into two sets at 243 K
(Figure 7). The kinetic parameters for the ligand exchange are
(20) The 1H NMR spectrum of [Ag6Hg312 ·32]12+ complex at 323 K
displayed three resonances for protons a, c, and d of 1, exhibiting the
relatively free rotation of the two rotors. VT 1H NMR spectra are
shown in Figure S10. The formation of [Ag6Hg312 ·32]12+ was also
demonstrated by ESI-TOF mass spectrometry (Figure S6).
(21) Bessiere, J.; Lejaille, M. F. Anal. Lett. 1979, 12, 753.
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A R T I C L E S
Hiraoka et al.
1
Figure 7. Partial VT H NMR spectra (500 MHz, CD3OD) of a molecular rotor-transmitter-rotor, Ag6Hg312 ·22 ·(OTf)12. [Ag6Hg312 ·22 ·(OTf)12] ) 3.7
mM.
Figure 8. Interconversion between the Ag912 ·22 ·(OTf)9 and Ag6Hg312 ·22 ·(OTf)12 complexes. (A) Schematic representation of an interconversion between
the Ag912 ·22 ·(OTf)9 and Ag6Hg312 ·22 ·(OTf)12 complexes. (B-D) 1H NMR spectra for the replacement of the central three metal ions, Ag+fHg2+fAg+
(500 MHz, CD3OD, 313 K, [1] ) [2] ) 7.5 mM). (B) Ag912 ·22 ·(OTf)9. (C) Upon addition of Hg(OTf)2 (3 equiv) to the sample for panel B,
Ag6Hg312 ·22 ·(OTf)12 complex was formed. (D) Upon addition of [2,2,2]-cryptand (3 equiv) to the sample for panel C, Ag912 ·22 ·(OTf)9 complex was
regenerated as a result of the formation of Hg2+⊂[2,2,2]-cryptand.
summarized in Table 1. The comparison of smaller ∆Hqex and
larger than that of Ag+. The distance between two ligands 1 in
the transmitter [Ag6Hg312]12+ should be longer when larger
Hg2+ ions are placed between them. The structural change
would reduce the steric hindrance between the two ligands 1,
which makes the ∆Hqex value smaller, and would further relax
∆Sq values found for [Ag6Hg312 · 22]12+ with [Ag912 · 22]9+
ex
indicates the lesser extent of interlocking between the central
two ligands 1 in the [Ag6Hg312]12+ transmitter. These results
are well explained by the fact that an ionic radius of Hg2+ is
9
9096 J. AM. CHEM. SOC. VOL. 130, NO. 28, 2008

Ranging Correlated Motion of Two Rotors
A R T I C L E S
Preparation of Molecular RTRs. Ag912 ·22 ·(OTf)9 complex:
To a solution of 1 (2.2 mg, 3.0 µmol) in CD3OD, 2 (1.7 mg, 3.0
µmol) was suspended, and then a CD3OD solution of AgOTf (51
µL of 0.26 M solution, 13.4 µmol) was added. The mixture was
allowed to stand at room temperature for a few minutes. 1H NMR
spectra for the titration studies demonstrated the formation of the
the strained structure in the ground state, which results in
lowering the ∆Sq value.
ex
Conclusion
In conclusion, we have first developed a molecular rotor–
transmitter–rotor (RTR) device, in which the rotational motions
of two terminal rotors 1.5 nm apart are strongly correlated
through a metal-mediated transmitter. Furthermore, the rate of
rotational motions were successfully regulated by changing the
kind of metal ions in the transmitter. The single-crystal X-ray
analysis of Ag912 ·22 ·(OTf)9 revealed that the two terminal
rotors 2 are coaxially stacked through the transmitter, and that
they have an identical helicity. Large entropy changes of both
the intramolecular ligand exchange and the helix inversion were
found in the RTR devices, [Ag6M312 ·22](6+3n)+ (Mn+ ) Ag+
or Hg2+), whereas [Ag32·5]3+, which is a part of the RTR,
showed a negative entropy change. In addition, the helix
inversions of all the rings in the device take place with
comparable rate constants. Taken all together, we conclude that
the rotational motions of the remotely stacked rotors are highly
correlated with each other through the oscillating transmitter.
The mechanism of motion has two elements, the ligand
exchange and the flip motion, both of which accompany a
simultaneous, oscillating motion between P and M helical
isomers of the two rotors and the transmitter put between them.22
The helix inversion of a transmitter would be a highly
controllable mode for a long-range transmission of motions,
because the change in helicity on one side of the transmitter
could propagate efficiently to the other side along the rotational
axis of the transmitter consisting of simple repeating units.23
Therefore, the propagation distance of the motions between two
rotors would be easily altered by the number of repeating units
in the transmitter. In our [Ag6M312](6+3n)+ transmitters, for
example, the insertion of a predetermined number of disk-shaped
ligands in the transmitters with the aid of metal-coordination
would form a multilayered transmitter to realize a longer-range
transmission of motions. Toward more complex molecular
machine systems, one can connect a variety of motor and
actuator molecules to the transmitter mediated by a terminal
rotor containing an anchoring functional group such as molecule
3. Such a strategy would be widely applicable to transmission
of motions within a molecular machine system fitted with
artificial molecular devices or natural macromolecules, for
example, motor proteins.
1
Ag912 ·22 ·(OTf)9 complex as shown in Figure S1. H NMR (500
MHz, CD3OD, 313 K) δ 8.17 (dd, J ) 8.1, 1.2 Hz, 6H), 8.00 (dd,
J ) 8.1, 1.2 Hz, 6H), 7.91 (d, J ) 3.4 Hz, 12 H), 7.79 (d, J ) 3.2
Hz, 12H), 7.68 (d, J ) 7.8 Hz, 6H), 7.26 (d, J ) 7.8 Hz, 6H), 4.77
(t, J ) 9.7 Hz, 12H), 4.32 (t, J ) 9.5 Hz, 12H), 4.19 (t, J ) 9.2
Hz, 12H), 3.78 (t, J ) 9.7 Hz, 12H). ESI-TOF mass (positive) m/z
2304.5 [Ag912 ·22 ·(OTf)7]2+, 1487.1 [Ag912 ·22 ·(OTf)6]3+. HRMS
(ESI) m/z: exact mass [Ag912 · 22(OTf)6]3+ 1486.7638,
C138H96Ag9N24O30S18, requires 1486.7618.
Ag912 ·32 ·(OTf)9 complex: To a solution of 1 (2.3 mg, 3.2 µmol)
and 3 (1.9 mg, 3.2 µmol) in CD3OD, a CD3OD solution of AgOTf
(36 µL of 0.39 M solution, 14.2 µmol) was added. The mixture
1
was allowed to stand at room temperature for a few minutes. H
NMR titration studies demonstrated the formation of the
1
Ag912 ·32 ·(OTf)9 complex as shown in Figure S2. H NMR (500
MHz, CD3OD, 303 K) δ 8.16 (dd, J ) 8.1, 1.2 Hz, 6H), 7.99 (dd,
J ) 7.8, 1.5 Hz, 3H), 7.93 (d, J ) 3.2 Hz, 4H), 7.92 (d, J ) 3.4
Hz, 2H), 7.90 (d, J ) 3.4 Hz, 4H), 7.79 (d, J ) 3.7 Hz, 4H), 7.78
(d, J ) 3.7 Hz, 4H), 7.77 (d, J ) 3.2 Hz, 2H), 7.67 (dd, J ) 7.8,
1.5 Hz, 3H), 7.64 (s, 2H), 7.25 (dd, J ) 8.1, 1.5 Hz, 3H), 4.32 (t,
J ) 9.7 Hz, 12H), 4.18 (br, 12 H), 3.77 (t, J ) 9.3 Hz, 12H).
ESI-TOF mass (positive) m/z 1507.5 [Ag912 ·3′2 ·(OTf)6]3+, in
which 3′ denotes the deuterated form of hydroxy groups of 3.
Ag6Hg312 ·22(OTf)12 complex: To a solution of 1 (2.1 mg, 2.9
µmol) in CD3OD (0.4 mL), 2 (1.7 mg, 2.9 µmol) in CD3OD (0.4
mL) was suspended, and then a CD3OD solution of Hg(OTf)2 (22
µL of 0.2 M solution, 4.4 µmol) and a CD3OD solution of AgOTf
(19 µL of 0.46 M solution, 8.8 µmol) were added. The mixture
was allowed to stand at room temperature for a few minutes. Its
1H NMR spectrum showed the quantitative formation of
1
Ag6Hg312 ·22(OTf)12 complex. H NMR (500 MHz, CD3OD, 313
K) δ 8.10 (d, J ) 8.1 Hz, 12 H), 7.92 (d, J ) 3.2 Hz, 12H), 7.89
(d, J ) 7.8 Hz, 6H), 7.83 (d, J ) 2.9 Hz, 12H), 7.33 (d, J ) 8.1
Hz, 6H), 5.1 (t, J ) 9.7 Hz, 12H), 4.56 (br, 12H), 3.84 (br, 12H).
ESI-TOF mass (positive) m/z 1728.7 [Ag6Hg312 ·22(OTf)9]3+
.
HRMS (ESI) m/z: exact mass [Ag6Hg312 ·22(OTf)9]3+ 1728.4446,
C141H96Ag6Hg3N24O39S21, requires 1728.4449.
Crystal data for Ag912 ·22 ·(OTf)9 ·(CH3OH)7: C148H100Ag9F27-
j
N24O46S21, M ) 5107.58, T ) 93.1 K, triclinic, P1, Z ) 2, a )
21.2194(9), b ) 21.3055(8), c ) 24.4632(19) Å, R ) 89.325(3), ꢀ
) 79.310(3), γ ) 63.263(3)°, V ) 9673.3(7) Å3, 56643 measured
reflections, 32928 unique reflections, R ) 0.1345, wR ) 0.3904,
GOF ) 1.054. Material details for the crystal structure are available
free of charge from the Cambridge Crystallographic Data Centre
under deposition number CCDC 627131.
Experimental Section
All ambient and variable-temperature 1H NMR spectra were
recorded on a Bruker DRX 500 (500 MHz) spectrometer using TMS
as the internal reference. Electrospray ionization-time-of-flight (ESI-
TOF) mass spectra were recorded on a Micromass LCT mass
spectrometer KB 201. High-resolution mass spectra of [Ag912 ·22]9+
and [Ag6Hg312 ·22]12+ complexes were recorded on a QFT-7
(IonSpec FTMS Systems, Varian Inc.) mass spectrometer. Intensity
data for X-ray crystallographic analysis were obtained on a Rigaku
RAXIS-RAPID Imaging Plate diffractometer with graphite mono-
chromated Cu KR radiation.
Ag6Hg312 ·32(OTf)12 complex: To a solution of 1 (2.1 mg, 2.9
µmol) and 3 (1.7 mg, 2.9 µmol) in CD3OD (0.4 mL), a CD3OD
solution of Hg(OTf)2 (21 µL of 0.2 M solution, 4.3 µmol) and a
CD3OD solution of AgOTf (18.5 µL of 0.46 M solution, 8.6 µmol)
were added. The mixture was allowed to stand at room temperature
for a few minutes. 1H NMR (500 MHz, CD3OD, 313 K) δ 8.11 (d,
J ) 6.9 Hz, 6H), 8.09 (d, J ) 7.1 Hz, 6H), 7.94-7.77 (m, 26H),
7.33 (d, J ) 7.6 Hz, 6H), 5.11 (t, J ) 8.3 Hz, 12H), 4.57 (br,
12H), 3.83 (br, 12H). ESI-TOF mass (positive) m/z 1748.8
[Ag6Hg312 ·3′2(OTf)9]3+, in which 3′ denotes the deuterated form
of hydroxy groups of 3.
(22) In this study the synchronous helix inversions were demonstrated for
the RTRs. Since the helix inversion takes place both by the ligand
exchange (EX) and the flip motion (flip), if both motions take place
at the same frequency, every co-occurring combination of the ligand
exchange and the flip motion on both sides, that is, EX(rotor A)-
EX(rotor B), EX(rotor A)-flip(rotor B), flip(rotor A)-EX(rotor B),
or flip(rotor A)-flip(rotor B) is possible. In the present case, the
detailed information is missing on this point.
Preparation of [Ag32·5]3+ Complex. To a solution of 2 (2.3
mg, 3.9 µmol) and 5 (2.0 mg, 3.9 µmol) in CD3OD (0.55 mL) was
added a solution of AgOTf (3.0 mg, 11.7 µmol), and then the
mixture was allowed to stand at room temperature for a few
minutes. Variable-temperature 1H NMR spectra are shown in Figure
S11. 1H NMR (500 MHz, CD3OD, 293 K) δ 7.89 (d, J ) 8.5 Hz,
3H), 7.88 (d, J ) 3.5 Hz, 6H), 7.75 (d, J ) 3.5 Hz, 6H), 7.57-7.55
(23) Pijper, D.; Feringa, B. L. Angew. Chem., Int. Ed. 2007, 46, 3693.
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A R T I C L E S
Hiraoka et al.
(m, 6H), 7.33-7.31 (m, 3H), 7.26-7.22 (m, 3H), 4.03 (t, J ) 9.8
Hz, 6H), 3.68 (t, J ) 9.8 Hz, 6H); ESI-TOF mass (positive) m/z )
the metal complexes. This work was supported by Grants-in-Aid
for Scientific Research (S) to M.S. (No. 16105001) and Young
Scientists (A) to S.H. (No. 17685005) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan,
CASIO science promotion foundation, and Inamori foundation.
781.4 [Ag32·5·(OTf)]2+, 471.3 [Ag32·5]3+
.
Analysis of the Ligand Exchange in Molecular RTR
Devices. The kinetic parameters for the ligand exchange of the RTR
devices and molecular ball bearing, [Ag32·5]3+, are summarized
in Table 1. The rate constant at each temperature was estimated
from the line shape analysis for thiazolyl proton signals of 2, and
∆Hq and ∆Sq values were determined by Eyring plot. Simulated
spectra and Eyring plots are shown in Figures S11-S13.
Supporting Information Available: Synthetic procedures, VT
1H NMR spectra, ESI-TOF mass spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank A. Sato of JASCO International
Co., LTD. for measuring the high-resolution FT mass spectra of
JA8014583
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9098 J. AM. CHEM. SOC. VOL. 130, NO. 28, 2008