A molecular ball bearing mediated by multiligand exchange in concert

Article abstract of DOI:10.1002/anie.200453753

A thrust in the right direction: Two different disk-shaped ligands (a tris-monodentate and a hexa-monodentate ligand) and three Ag⁺ ions have been used as bearings and balls, respectively, in the construction of a molecular thrust ball bearing

Full text of DOI:10.1002/anie.200453753

Communications
double-decker complexes[10,11] and a functionalized ferrocene
derivative.[12] Herein we present a novel rotational device,a
molecular ball bearing mediated by metal ions,which is
quantitatively formed as a thermodynamically favorable
product from three Ag+ ions and disk-shaped tris-monoden-
tate and hexa-monodentate ligands through a heterotopic,
self-assembly process. In the device the three Ag+ ions serve
as the balls for the relative rotational motion of the two disk-
shaped ligands by allowing their efficient rotation through a
combination of intramolecular,reversible multipoint ligand
exchange and flip motions that act in concert.
In a preliminary study we recently constructed a sand-
wich-type trinuclear Ag+ complex [Ag3(2)2] using two disk-
shaped,tris-monodentate thiazolyl ligands 2. In this complex,
each Ag+ ion is bound by the two thiazolyl nitrogen donors of
the two ligands,and all the monodentate ligand rings attached
to 2 tilt toward the same direction to form a helical structure.
This arrangement is further accompanied by a 1208 flipping
rotation of the ligands as a result of the P versus M equilib-
rium.[13] This system was further advanced to a heterogeneous
Ag+-mediated association of a hexa-monodentate and a tris-
monodentate thiazolyl ligands, 1 and 2,respectively,to form a
heterotopic trinuclear complex [Ag3(1)(2)].[14] In this complex
all three nitrogen donors of the tris(thiazolyl) ligand 2 and
every alternate nitrogen donor atoms of the hexa(thiazolyl)
ligand 1 participate in the complexation with the three
Ag+ ions in a linear coordination geometry (Figure 1b). This
heterotopic complex is quantitatively formed by a simple
mixing of 1 and 2 in a 1:1 ratio in the presence of three
equivalents of Ag+ ions in CD3OD. Moreover,the variable-
temperature (VT) 1H NMR spectroscopic studies strongly
suggested that the intramolecular three-point ligand
exchange occurred in concert between the two disk-shaped
ligands (namely, MÐP in Figure 2).[15] In an M!P conver-
sion,for example,alternate coordinatively free ligand rings of
1 (rings B,D,and F) in M (with M configuration) rotates
clockwise and then comes close to the neighboring Ag+ ion
and ligand exchange finally leads to P (with P configuration),
presumably through formation of a three-coordinate transi-
tion state. The thiazolyl rings B,D,and F independently
coordinate to each Ag+ ion in the structure P. The M!P
metal–ligand exchange converts an Ag+-mediated pair of
ligands from a ring 1-Ag-A,3-Ag-E,and 5-Ag-C in M into a
ring 1-Ag-B,3-Ag-F,and 5-Ag-D in P. A 608 rotation from M
to M’ is realized if a subsequent rewinding of complex P by
the flip motion that maintains the linked three pairs take
place (Figure 2). Further motions in the clockwise direction as
well as in the reverse direction (M’!P and P!M) should
occur with equal probability. Therefore,sequential metal–
ligand exchanges and flip motions occurring in concert finally
give rise to the full relative rotation between the two disk-
shaped ligands 1 and 2.
Molecular Devices
A Molecular Ball Bearing Mediated by
Multiligand Exchange in Concert**
Shuichi Hiraoka, Kaori Hirata, and
Mitsuhiko Shionoya*
There is much interest in the development of devices based on
molecular motion similar to that seen in living organisms.[1–3]
A variety of chemical bonding patterns have been extensively
utilized so far for the realization of such molecular devices at
the molecular level: for example,covalent bonding for a
controlled rotation through single bonds[4,5] and photoisom-
erized double bonds[6] as well as noncovalent bonding for the
formation of interlocked architectures such as rotaxanes[1,7]
and catenanes.[8,9] Among the possible modes of noncovalent
bonding,metal coordinative bonding shows particular prom-
ise for the fabrication of rotational devices,as evidenced by
recent excellent reports on mononuclear bisporphyrinate
[*] Dr. S. Hiraoka, K. Hirata, Prof. M. Shionoya
Department of Chemistry
Graduate School of Science
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax: (+81)3-5841-8061
E-mail: shionoya@chem.s.u-tokyo.ac.jp
The previous VT 1H NMR study of the [Ag3(1)(2)]
complex revealed the 608 relative rotational oscillation of
the two disk-shaped ligands mediated by multipoint metal–
ligand exchanges (MÐP in Figure 2).[15] We thus examined
the dynamic behavior of a similar heterotopic [Ag3(1)(5)]
complex formed from 1 and a tris(thiazolyl) ligand 5 with a
C2v symmetry (Figure 3) to obtain evidence for the free
[**] This study was supported by a Grant-in-Aid for Scientific Research
on Priority Area “Dynamic Complex” (No. 15036216), a Grant-in-Aid
for Encouragement of Young Scientists (B) (No. 1474036), and a
Grant-in-Aid for The 21st Century COE Program for Frontiers in
Fundamental Chemistry from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 1. a) A thrust ball bearing, b) the envisioned scheme of the rotation with molecular ball bearings and the disk-shaped ligands used in this
study.
rotation of the two disks by the mechanism shown in Figure 2.
If the relative rotation between the two disk-shaped ligands 1
and 5 is faster than the NMR timescale,the symmetry of the
signals for 1 would not be affected by the less-symmetrical 5
and,as a result,only one set of thiazolyl proton signals should
be observed in the spectrum. In the case of slower or no
relative rotation,on the other hand,four sets of inequivalent
thiazolyl proton signals for rings I–IV should appear for 1.
The quantitative formation of the [Ag3(1)(5)] complex was
unambiguously evidenced by 1H NMR spectroscopy (Fig-
ure 3c) and ESI-TOF mass spectrometric measurements in
CD3OD (m/z 529.9,842.4,1779.9: dideuterated [Ag 3(1)(5)]3+,
six thiazolyl rings of 1 in the [Ag3(1)(5)] complex are
equivalent. This result reveals that the two disk-shaped
ligands connected by three Ag+ ions rotate freely and faster
than the NMR timescale at 293 K. In contrast,cooling the
sample to 213 K resulted in the Ha resonances dividing into
three distinct signals in a 1:2:3(1:2) ratio (Figure 3g),thus
showing that the thiazolyl rings of 1 are inequivalent as a
consequence of the slower relative rotation of the opposite
ring 5. These results taken together lead us to conclude that
the intramolecular relative rotation between the two disk-
shaped ligands in [Ag3(1)(2)] and [Ag3(1)(5)] takes place in
concert,and that the three Ag + ions positioned between the
tris- and hexa-monodentate ligands can act as both couplers
and mediators for the rotational motion.[18]
To investigate the rotation mechanism in detail,optically
active tris-monodentate nitrogen ligands (S)-3 and (R)-4 were
used in the VT 1H NMR study,in the expectation that the P-
and M-helical isomers could be distinguished as diastereo-
mers at low temperatures. The tris(oxazolyl) and tris(oxa-
zinyl) ligands 3 and 4,respectively,were found to heteroge-
neously associate with 1 in the presence of three equivalents
of Ag+ ions,as observed with 2. The quantitative formation of
[Ag3(1)(3)] and [Ag3(1)(4)] were similarly confirmed by the
[Ag3(1)(5)(CH3SO3)]2+,and [Ag (1)(5)(CH3SO3)2]+,respec-
3
tively).[16] Upon complexation,the aromatic H g, Hh, Hi,and H j
protons of 5 were divided into two sets as a consequence of
the formation of a sandwich complex in which the inner and
outer aromatic protons are inequivalent. This result indicates
that the intermolecular ligand exchanges between Ag+ com-
plexes are slower than the NMR timescale at 293 K,[17] and
that the following dynamic spectral changes should,therefore,
a
arise from intramolecular motions. At 293 K,the H and Hb
protons of 1 were observed as one set of signals at d = 7.90 and
7.67 ppm,respectively (Figure 3c),which indicates that the
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Figure 2. A plausible rotation mechanism of the Ag+ sandwich complex with the aid of the metal–ligand (M-L) exchange and flip motion. In the
M-L exchange from M to P, the binding pairs of the rings between the two ligands through Ag+ ions change from 1-A, 3-E, and 5-C in M to 1-B,
3-F, and 5-D in P with a conversion of the helicity from P into M. The subsequent flip motion maintaining the binding pairs converts the chirality
of the structure from P into M’. A combination of concerted M-L exchange at the three Ag+ centers (MÐP) and a flip motion (PÐM’) finally
allows a 608 rotational motion. Sequential M-L exchanges and flip motions results in the relative free rotation between the two disk-shaped
ligands.
1H NMR spectroscopic and ESI mass spectrometric measure-
ments.[16] The proton signals for the six thiazolyl rings of 1
were equivalent in both [Ag3(1)(3)] and [Ag3(1)(4)] com-
plexes at 293 K,thus indicating their faster rotation relative to
the NMR timescale (Figure 4a,f). Cooling the sample of
[Ag3(1)(3)] to 213 K resulted in the thiazolyl proton signals
for Ha and Hb being divided into two sets (d = 7.85,7.76
(overlapped),and 7.59 ppm),while the signals of the tris-
monodentate ligand 3 did not show any changes,probably
because the metal–ligand exchange becomes slower than the
NMR timescale while the flip motion is still faster (Fig-
ure 4e). These results suggest that the energy barrier of the
metal–ligand exchange DGꢀM-L is greater than that of the flip
Figure 2). Therefore,the NMR signals do not show any
significant changes. Furthermore,cooling down the solutions
to temperatures at which the metal–ligand exchange becomes
slower than the NMR timescale should result in the signals for
the hexa-monodentate ligand 1 and the tris-monodentate
ligand 4 being divided simultaneously. In fact,the energy
difference between the metal–ligand exchange and the flip
motion was affected by the nature of the monodentate ligand
rings bound to the Ag+ ions. The destabilization of the
transition state in the flip motion was particularly noticeable
for the [Ag3(1)(4)] complex containing the six-membered
ligand rings of 4. Such a marked tendency is consistent with
our previous findings.[13] These results suggest that the rate or
frequency of their rotation is controllable through the effect
of the size of the tris-monodentate ligands on the flip
processes.
motion DGꢀflip [19] In contrast,each signal of both the thiazolyl
.
protons of 1 and the aromatic protons of 4 were divided into
two sets at temperatures around 223 K in the case of the
[Ag3(1)(4)] complex (Figure 4i). Four sets of signals for the
thiazolyl protons of 1 and two sets of signals for 4 were
observed at 203 K (Figure 4k),[20] which indicates that the
coordinating and uncoordinating thiazolyl rings of the hexa-
monodentate ligand 1 are distinguishable in both the P and
M diastereomers of the [Ag3(1)(4)] complex because both the
metal–ligand exchange and the flip processes are slower.
From these spectral changes we concluded that DGꢀflip is equal
to or greater than DGꢀM-L in [Ag3(1)(4)] (DGfꢀlip ꢀ DGMꢀ-L). In
such a case,at temperatures at which the flip motion becomes
slower than the NMR timescale but the metal–ligand
exchange is still faster,the P and M isomers can not be
observed separately because of the rapid intraconversion
through the metal–ligand exchange process (MÐP in
In summary,we have established a quantitative formation
of a novel molecular thrust ball bearing consisting of three
Ag+ ions as balls and two disk-shaped multi-monodentate
ligands as nanosized bearings. This molecular ball bearing
allows the relatively free rotation of two ligands connected by
three Ag+ ions that is mediated by sequential,multiligand
exchanges and flip motions occurring in concert. Finally,a
rotational rate (approximately 8000 rotations per second at
1
298 K) was obtained for [Ag3(1)(2)] from the VT H NMR
study.[20] Furthermore,the energy barriers between the P!M
and the M!P processes in ball bearings using chiral ligands
could be differentiated from each other as a result of the
difference in stability between the P and M diastereomers.
Such ball bearings could work as parts of chiral molecular
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Chemie
motors which make unidirectional rotation feasible in a
controlled manner with the aid of external energy sources.
Received: January 14,2004
Revised: April 28,2004 [Z53753]
Keywords: ligand exchange · molecular recognition ·
.
self-assembly · silver · supramolecular chemistry
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Figure 3. 1H NMR spectra (500 MHz, CD3OD): a) 1 + 3 equiv of
AgCH3SO3,b) [Ag3(5)2] at 293 K, c) [Ag3(1)(5)] at 293 K, d) [Ag3(1)(5)] at
243 K, e) [Ag3(1)(5)] at 233 K, f) [Ag3(1)(5)] at 223 K, g) [Ag3(1)(5)] at
213 K.
[15] The 1H NMR spectrum of [Ag3(1)(2)] showed only one set of the
thiazolyl protons of 1 (d = 7.90 (d, J = 3.3 Hz,6H),7.64 ppm (d,
J = 3.3 Hz,6H)) above 263 K. Upon cooling the sample down to
213 K the thiazolyl signals were divided into two sets (d = 7.96
(d, J = 3.3 Hz,3H),7.83 (d, J = 3.3 Hz,3H),7.64 (d, J = 3.3 Hz,
3H),7.62 ppm (d, J = 3.3 Hz,3H)),whereas the signals of 2 did
not show any changes. The VT 1H NMR results of the
[Ag3(1)(2)] and the discussion on the energy barrier of the
metal–ligand exchange are described in detail in ref. [14].
[16] See Supporting Information.
[17] In the 1H NMR titration studies using 1 and 5,the signals of
[Ag3(1)(5)] and [Ag3(5)2] ([5] > [1]) or those of [Ag3(1)(5)] and
unidentified Ag+ complexes with 1 ([5] < [1]) were independ-
ently observed. These results also support the relatively slow
intermolecular ligand exchanges in the Ag+ complexes.
[18] The relatively free rotation of the two disks in the ball bearings
contains every rotational motion produced by the combination
of the metal–ligand exchange and the flip motion between the P
and M isomers,such as a 360 8 full rotation and a back and forth
movement.
Figure 4. Schematic representation of a reaction coordinate for 608
rotation, and partial 1H NMR spectra (500 MHz, CD3OD) of [Ag3(1)(3)]
(a)–(e) and [Ag3(1)(4)] (f)–(k). a) 293 K, b) 253 K, c) 233 K, d) 223 K,
e) 213 K, f) 293 K, g) 243 K, h) 233 K, i) 223 K, j) 213 K, k) 203 K.
[19] In the chiral [Ag3(1)(3)] and [Ag3(1)(4)] complexes,the stabil-
ities of the P and M isomers would be different from each other
for each case,and therefore the energy barriers of both the
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metal–ligand exchange and the flip motion from P to M
(DGꢀM-LðP!MÞ and DGfꢀlipðP!MÞ) should be different from those of
M to P (DGꢀM-LðM!PÞ and DGꢀflipðM!PÞ,respectively; namely,
DGMꢀ-LðP!MÞ ¼ DGꢀM-LðM!PÞ and DGfꢀlipðP!MÞ ¼ DGꢀflipðM!PÞ). Here
we discuss the relative energy differences between the energy
barriers of the ligand exchanges and the flip motions using the
greater one in each motion.
[20] Four sets of thiazolyl signals for the [Ag3(1)(4)] complex at
203 K were confirmed by its H-H COSY spectrum,as shown in
the Supporting Information.
1
[21] From the VT H NMR studies for the molecular motion of the
[Ag3(1)(2)] complex,in which the metal–ligand exchange is the
rate-determining step (DGꢀM-L @ DGfꢀlip),the rate constant of the
intramolecular ligand exchange at 298 K can be calculated to be
approximately 49000 sꢁ1 [14]
The rate of a single 3608 rotation is
.
at least six times greater than the rate of metal–ligand exchange,
and hence the maximum rotation rate can be estimated to be
about 8000 rps.
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