A bevel-gear-shaped rotor bearing a double-decker porphyrin complex

Article abstract of DOI:10.1002/chem.201000276

Supramolecular rotor: The interlocking of two porphyrin-based molecular rotors into a bevel-gear-shaped structure is demonstrated (see figure). The mechanical interactions between the teeth of the two porphyrin rotors enabled them to mesh and rotate with almost identical activation energies. Switching the rotational activities of one rotor by using chemical stimulation induced a change in the cooperative rotational motions of both rotors. (Figure Presented)

Full text of DOI:10.1002/chem.201000276

COMMUNICATION
DOI: 10.1002/chem.201000276
A Bevel-Gear-Shaped Rotor Bearing a Double-Decker Porphyrin Complex
Soichiro Ogi,[a, b, c] Tomohiro Ikeda,[b] Rie Wakabayashi,[b] Seiji Shinkai,*[b] and
Masayuki Takeuchi*[a, c]
The combination of molecular interactions and transmit-
ted dynamics generates function in biological systems. In the
natural rotary motor adenosine triphosphatase (ATPase),
the FO and F1 motors exhibit distinct rotational activities
through mutual mechanical and chemical interactions.[1–3]
The meshing of molecule-based rotors is necessary when as-
sembling them into complex machinery systems that resem-
ble those found in natural and mechanical devices. In the
1980s, tripticene-based molecular gears, the blades of which
mesh together, were reported independently by Iwamura
and Mislow.[4] Since their pioneering work, the transmission
of the activities of molecular rotors has attracted much at-
tention.[5–8] One of the synthetic molecular rotors, metal
bis(porphyrinate)s double-decker complexes (DD), can
serve as a rotating module in which two porphyrin rings
rotate around the sandwiched metal ion as their rotational
axis.[9] We have previously reported that the rotational activ-
rotor such that they mesh each other, the rotational activity
of the introduced rotor would be changed as a result of the
switching of the rotational rate of LaDD. This concept that
utilises LaDD as a component for molecular gears would be
conducive to the creation of molecular-based machines,[10]
which are capable of altering the rotational information
among rotors in response to external stimulation.
Herein, we demonstrate the interlocking of two distinct
porphyrin-based molecular rotors into a bevel-gear-shaped
structure[4a–h,5a,e,11] (1 in Figure 1a), which features LaDD
and a porphyrinatorhodiumACTHNURGTNE(NUG III)-based rotor; the former has
the switchable rotational activity and the latter has a higher
rotational activity than that of LaDD. By using 1H NMR
spectroscopy, we found that mechanical interactions be-
tween the teeth of the two molecular rotors enabled them to
mesh and rotate with almost identical activation energies. In
addition, switching the rotational activities of LaDD in-
duced a change in the cooperative rotational motions of the
two rotors.
ity of the porphyrin rotor in a lanthanumACTHNUTRGNEUNG(III) bis(porphyri-
nate)s double-decker (LaDD) is switchable by using chemi-
cal stimulation.[9e] It occurred to us that when a molecular
rotor is introduced at the peripheral position of a LaDD
Scheme 1a illustrates our design for a bevel-gear-shaped
rotor that is bound to two rotors that possess different rota-
tional activities. One is a double-decker metal bis(porphyri-
nate)-based rotor that is driven by heat fluctuation
(Scheme 1a and Figure 1a; top, red).[9] The rotational rate
of LaDDs are almost on the same time scale as that of
NMR spectroscopy. The slow rotational frequency of LaDD
enabled us to evaluate the rotational properties of the top
[a] S. Ogi, Dr. M. Takeuchi
Macromolecules Group, Organic Nanomaterials Center
National Institute for Materials Science (NIMS)
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan)
Fax : (+81)29-859-2101
1
rotor by using variable-temperature (VT) H NMR spectro-
E-mail: Takeuchi.Masayuki@nims.go.jp
scopic techniques. The aryl groups at the meso positions de-
termine the step angle of 908 in LaDD (Scheme 1b). The
[b] S. Ogi, Dr. T. Ikeda, Dr. R. Wakabayashi, Prof. S. Shinkai
Department of Chemistry and Biochemistry
Graduate School of Engineering, Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan)
Fax : (+81)92-802-6990
other rotor (Scheme 1a and Figure 1a; side, blue) possesses
III
a pyridine Rh coordination bond[12] as its rotation axis. In
À
the absence of the top porphyrin unit, the side rotor rotates
too rapidly to evaluate the rotational rate. A porphyrin
ligand presenting a pyridine substituent (Scheme 1a and Fig-
ure 1a; core, black) is shared by the top and side rotors, and
consequently the two rotors are mounted on rotational axes
that are aligned at an angle of almost 908. In the bevel-gear-
shaped molecular rotor 1, four protruding aryl substituents
at the meso positions in each porphyrin-based rotor (top
E-mail: shinkai_center@mail.cstm.kyushu-u.ac.jp
[c] S. Ogi, Dr. M. Takeuchi
Department of Materials Science and Engineering
Graduate School of Pure and Applied Sciences
University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571
(Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000276.
Chem. Eur. J. 2010, 16, 8285 – 8290
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8285

tons of the side unit appeared
at 9.08 and 9.25 ppm. The rota-
tional rate of the side rotor in 1
was sufficiently slow to be de-
1
tected on the H NMR spectro-
scopic time scale. Upon increas-
ing the temperature, the signals
of the tert-butyl protons in the
top unit and the b-pyrrole pro-
tons in the side unit gradually
and simultaneously broadened,
eventually
coalescing
into
single peaks at 298 K (see
Figure 2). From the similar ten-
dencies in the coalescence
properties of the top and side
rotors in 1, we deduced their
mutual meshing. To examine
the rotary motion in detail, we
calculated the rotational rates
of the top and side rotors by
Scheme 1. a) Design scheme of a bevel-gear-shaped rotor comprising two rotors (top and side). These rotors
are aligned almost orthogonally and connected through a core (see Figure S9 in the Supporting Information),
b) Each porphyrin ligand in the metal bis(porphyrinate)-based rotor (DD) undergoes a stepping rotation of
908 because of the phenyl groups in the meso positions.
means of
a nonlinear least-
squares method[15] at each tem-
perature. The simulated spectra
of the signals of the tert-butyl
protons of the top unit and the
b-pyrrole protons of the side
and side) produce a stepping rotation of the side rotor
meshing mutually with the top rotor. Thereby, we expect to
transmit the rotational frequency of the top rotor to the side
rotor and turn down the rotational frequency of the side
rotor. The top and side rotors are unsymmetrical because of
the presence of the side and top porphyrin units, respective-
ly; this situation allows the rotational behaviour of each
unit were coincident with those observed experimentally
(see Figure S7 in the Supporting information); the calculat-
ed rotation rates of the 1808 steps are shown in Figure 2.
From plots of logk (rate constants of rotation: ktop and kside
)
versus TÀ1 (Arrhenius plot; correlation coefficient (R):
>0.99), we calculated the activation energies (Ea) for the ro-
tations of the top and side rotors to be 30.1 and
30.7 kJmolÀ1, respectively (see Figure S8 in the Supporting
Information). Moreover, we calculated the following ther-
modynamic parameters for the rotors at 273 K: DGꢀ =
60.5 kJmolÀ1, DHꢀ =27.8 kJmolÀ1, DSꢀ =À120 JKÀ1 molÀ1
for the top rotor; DGꢀ =57.4 kJmolÀ1, DHꢀ =28.4 kJmolÀ1,
DSꢀ =À106 JKÀ1 molÀ1 for the side rotor (a summary of the
thermodynamic and kinetic parameters of the top and side
rotors in 1 is given in Table 1). These similar activation ener-
gies and thermodynamic parameters indicate that the rotary
motions of the side unit were slowed through mutual me-
chanical interactions; in other words, meshing occurred
among the protruding substituents. Although the rates of ro-
tation of the side rotor decreased accordingly, they remained
almost four times larger than those of the top rotor. We
have deduced that the two rotors are meshed in such a
manner that the protruding tooth of the side unit slipped
through two adjoining teeth of the top unit (see Figure S9 in
the Supporting Information).
1
rotor to be evaluated by using H NMR spectroscopy (Fig-
ure 1b).
III
À
Firstly, we confirmed that a pyridine Rh coordination
bond in 1 can be regarded as its rotation axis from the fact
that in the temperature range 193–353 K complexation–un-
complexation dynamics between the pyridine and the por-
phyrinato RhIII in [D2]dichloromethane (193–298 K) and
[D4]tetrachloroethane (353 K) was not observed from VT
1H NMR spectroscopy.[14]
Given the molecular structure of 1, the tert-butyl protons
of the top unit or the b-pyrrole protons of the side unit ex-
change from area (i) to area (ii) upon a 1808 rotation of the
top or side units, respectively (Figure 1b). When the ex-
1
change rate is comparable with the time scale of H NMR
spectroscopy at a certain temperature, the rotary motions of
the top and side units can be monitored readily by using the
coalescence properties of the signals of each pertinent set of
protons, that is, the tert-butyl and b-pyrrole protons, respec-
tively. We evaluated the rotary motions of the top and side
An intrinsic feature of LaDD is that its rotational activity
can be altered. In LaDD, seven of the eight pyrrole nitrogen
atoms in the two porphyrin units are coordinated to the
LaIII atom and one nitrogen atom remains uncoordinated.[16]
Addition of base to LaDD induces dissociation of the unco-
1
rotors in 1 through VT H NMR spectroscopy over the tem-
perature range 253–298 K. At 253 K, in addition to two
sharp singlets at 1.44 and 1.53 ppm, which could be assigned
to the tert-butyl protons of the top unit, the b-pyrrole pro-
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A Bevel-Gear-Shaped Rotor Bound to a Porphyrin Complex
COMMUNICATION
interactions between the por-
phyrin planes of LaDD after
deprotonation. Such treatment
can, therefore, be regarded as a
switch in the rotational activity
of one of the rotors (i.e., the
top rotor), and furthermore
makes 1H NMR spectra of 1
simpler because of the appear-
ance of higher symmetrical por-
phyrin ligand. Indeed, addition
of triethylamine (TEA) to 1 in
dichloromethane induced
a
shift of the absorption maxi-
mum (415.0 nm) of the Soret
band of the LaDD moiety to
shorter wavelength (413.0 nm)
with a hyperchromic effect (see
Figure S10 in the Supporting
Information). When we added
an excess of TEA (25 equiva-
lents per molecule of 1) to dis-
sociate the pyrrole NH proton,
the two signals for the b-pyrrole
protons in the side unit became
a single resonance at 9.17 ppm,
even at 243 K (Figure 3a). We
simultaneously confirmed that
no ligand exchange of the pyri-
III
À
dine Rh coordination bond in
III
À
side rotor to TEA Rh bond
occurs in the presence of
25 equivalents of TEA by the
results from using control com-
pounds (also see Figure S5 in
the Supporting Information).
Meanwhile, the two signals at-
tributable to the tert-butyl pro-
tons of the top unit appeared at
1.44 and 1.51 ppm at 243 K,
even after the addition of TEA,
and remained there without co-
alescence upon increasing the
temperature up to 313 K. These
results suggest that the rota-
tional rate constant ktop was less
than 5 sÀ1 at 313 K and that kside
was greater than 5000 sÀ1, even
at 243 K; therefore, the addi-
tion of TEA altered the cooper-
ative mechanical interactions of
Figure 1. a) Molecular structure of compound 1. In 1, one pyrrole NH proton remains in the porphyrin ligand
at the core, as depicted, because of the presence of the three electron-withdrawing pentafluorophenyl moieties
(see Figure S6 in the Supporting Information);[13] b) From the view of porphyrin planes of the top and side
units, the protons in area (i) experience a different magnetic field from those in (ii), owing to the unsymmetri-
cal circumstances when their rotations are slower than the NMR spectroscopic time scale; the remarkable pro-
tons, the tert-butyl protons of the top unit and the b-pyrrole protons of the side unit in 1, exchange through ro-
tation of 1808 in each rotor. If their rotations are faster than the NMR spectroscopic time scale, the protons in
areas (i) and (ii) will be observed as single peaks.
ordinated pyrrole NH proton (to form an NÀ group); upon
deprotonation, the value of lmax of the Soret band of LaDD
shifts to shorter wavelength with a hyperchromic effect, and
the coalescence temperature (Tc) of the porphyrin rotation
in LaDD increases (i.e., the rotational oscillation frequency
decreases).[9e] This change in Tc is attributed to stronger p–p
the top and side rotors. The side rotor in 1 slipped out of
gear, most likely because of the changes in the conforma-
tions of the LaDD unit.[17] Interestingly, neutralisation of
TEA through the addition of trifluoroacetic acid (TFA) re-
instated the mechanical interaction between the top and
side rotors in 1; again, we observed simultaneous coales-
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M. Takeuchi, S. Shinkai et al.
Figure 3. a) VT 1H NMR (600 MHz) spectra of 1 (1.0ꢁ10À3 moldmÀ3) at
243 K and after addition of TEA at 243–313 K in [D2]dichloromethane.
The descriptors (i) and (ii) denote protons in the areas defined in Fig-
ure 1b. b) Schematic representation of the switch in the meshing of the
two rotors upon the addition of TEA and TFA. The asterisk (*) denotes
the signal of added TEA.
1
temperature range 233–298 K. Indeed, VT H NMR spectro-
scopic studies revealed that the b-pyrrole protons of the top
and the side units in 2 gradually and simultaneously broad-
ened, eventually coalescing into single peaks at 298 K (Fig-
ure S14a in the Supporting Information). The top rotor was
also able to decelerate the rotation of two side rotors and
their cooperative rotational motion was switchable in a simi-
lar manner to 1 (Figure S14b in the Supporting Informa-
tion).[18]
Figure 2. Partial VT 1H NMR spectra of 1 in [D2]dichloromethane ([1]=
1.0ꢁ10À3 moldmÀ3). The observed spectra display the signals of the tert-
butyl protons of the top unit (a) and b-pyrrole protons of the side unit
(b). The calculated rotational rates of the 1808 steps are presented along
with the spectra. The asterisk (*) denotes the signal for water in
[D2]dichloromethane. The descriptors (i) and (ii) denote protons in the
areas defined in Figure 1b.
In this paper, we prepared synthetic molecular bevel-gear-
shaped rotors (1 and 2) in which the intrinsic rotational rate
of one rotor differs from the
other, with the two distinct top
and side rotor(s) being mount-
Table 1. Thermodynamic and kinetic parameters of the top and side rotors in 1.
Rotor k [sÀ1
]
Ea [kJmolÀ1
]
DGꢀ [kJmolÀ1
]
DHꢀ [kJmolÀ1
]
DSꢀ [JKÀ1 molÀ1
]
k (with TEA) [sÀ1
]
ed on rotational axes aligned at
an angle of almost 908. The ro-
tational activities of these top
and side units are partially
transmitted through mechanical
top
side
17.0
66.2
30.1
30.7
60.5
57.4
27.8
28.4
À120
À106
<5
5000<
cence properties at 298 K (see Figure S13 in the Supporting
Information).
interactions between the two rotors. In the case of systems 1
and 2, it was possible to switch the frequency of the rotary
motion from a meshing state (Etop ꢀEside) to an independent
state (ktop !kside), and back again to the original state,
through chemical stimulation.
To demonstrate further extension of the DD-based rotor
system, we synthesised a supramolecular rotor 2 in which
LaDD is bound by two side rotors at the distal meso posi-
tions through coordination bonds (Figure 4). We evaluated
the transmission of the rotational activity from one top to
1
two side rotors in 2 by using H NMR spectroscopy over the
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A Bevel-Gear-Shaped Rotor Bound to a Porphyrin Complex
COMMUNICATION
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Experimental Section
General methods and materials: All starting materials and solvents were
purchased from Tokyo Kasei Chemicals or Wako Chemicals and used as
received. 1H NMR spectra were recorded on
a Bruker DRX 600
(600 MHz) spectrometer. Chemical shifts are reported in ppm downfield
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Acknowledgements
M.T. and S.O thank Dr. K. Sugiyasu, Dr. O. Hirata and Dr. M. Ikeda for
critical discussions; N. Yazawa and S. Kanae for technical help of
1H NMR spectroscopy; and Dr. P. Glink for reading the manuscript. S.O.
thanks the JSPS Research Fellowship for Young Scientists for financial
support. This study was supported partially by a Grant-in-Aid for Scien-
tifc Research for Priority Area “Coodination Programming” (area 2107)
and “Innovative Nanobio” (area 521) to M.T. from MEXT, Japan.
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Keywords: molecular gears
porphyrinoids · supramolecular chemistry
·
molecular devices
·
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process takes place in this system, the side rotor can rotate within
the lifetime of the pyridine RhIII coordination bond. Under the con-
À
ditions we applied in this study, we deduce that this dynamic behav-
III
À
iour in the complexation–uncomplexation of the pyridine Rh
III
À
bond is negligible and the pyridine Rh coordination bond can be
regarded as the robust rotational axis of the side rotor (also see Fig-
ure S4 in the Supporting Information).
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[14] Compound 1 was purified through size exclusion column chromatog-
raphy, and during the purification process no decomposition into the
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[17] To obtain direct structural information on 1, we have been trying to
obtain single crystals that are suitable for X-ray analyses with and
without TEA, but unfortunately we have been unsuccessful at the
present stage. As far as we know, there is no report of the single-
crystal analysis of lanthanumACTHNUGRTENUNG(III) bis(porphyrinate). Instead, we uti-
lised a computer-generated model to discuss the mechanical interac-
tion between top and side rotors (see Figure S9 in the Supporting
Information).
corresponding LaDD and porphyrinatorhodiumACHTNUTRGNEUNG(III) took place. In
the temperature range 193–353 K, complexation–uncomplexation
dynamics between the pyridine and the porphyrinato RhIII in
[D2]dichloromethane (193–298 K) and [D4]tetrachloroethane
(353 K) was not observed from VT 1H NMR spectroscopy. These
observations suggests that these complexation–uncomplexation dy-
namics should be much slower than the NMR time scale if they
exist, which indicates that even if a complexation–uncomplexation
[18] We assume that the slippage would still occur although the activa-
tion energies for the rotations of the top and side rotors are almost
same.
Received: February 1, 2010
Published online: June 22, 2010
8290
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