An oscillating molecular turnstile

Article abstract of DOI:10.1039/c1dt10184f

An oscillating molecular turnstile based on a stator composed of a porphyrin core bearing two trans coordinating pyridyl units at the meso positions and a rotor equipped with a pyridyl group was designed. Whereas, in solution, the rotor freely rotates around the stator, in the presence of a silver cation behaving as an effector, the system oscillates between two stations. The oscillatory movement may be frozen upon cooling at -70 °C generating thus the closed state of the turnstile. The switching of the system between the open and closed states may be achieved using an external stimulus such as addition of Et₄NBr.

Full text of DOI:10.1039/c1dt10184f

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PAPER
An oscillating molecular turnstile†
Thomas Lang, Ernest Graf,* Nathalie Kyritsakas and Mir Wais Hosseini*
Received 2nd February 2011, Accepted 8th March 2011
DOI: 10.1039/c1dt10184f
An oscillating molecular turnstile based on a stator composed of a porphyrin core bearing two trans
coordinating pyridyl units at the meso positions and a rotor equipped with a pyridyl group was
designed. Whereas, in solution, the rotor freely rotates around the stator, in the presence of a silver
cation behaving as an effector, the system oscillates between two stations. The oscillatory movement
may be frozen upon cooling at -70 ◦C generating thus the closed state of the turnstile. The switching of
the system between the open and closed states may be achieved using an external stimulus such as
addition of Et4NBr.
The design of abiotic molecular machines and motors is a topic of
current interest.1–12 Among these dynamic architectures, molecular
turnstiles, which are rotary molecular systems, constitute a partic-
ular class of compounds.13–16 These molecules may be defined as
architectures comprising at least two interconnected mobile parts,
one behaving as a stator and the other as a rotor. The dynamics
of these molecular assemblies may be characterised by their open
and closed states.
Pursuing our effort in this area,17–20 here we report on a
porphyrin based turnstile for which in the open state the rotor
freely rotates around the stator and, in the presence of a Ag+
cation as the effector, the system oscillates, without directional
preference, between two stations at room temperature (Fig. 1).
Fig. 1 Schematic representation of the open (a) and closed (b and
c) states of a molecular turnstile composed of a stator bearing two
identical interaction sites or stations (red) and a handle equipped with
a monodentate coordinating site (black). In the absence of an effector, the
handle rotates freely around the stator and the turnstile is in its open state.
In the presence of a metal cation, its simultaneous binding by one of the
two binding sites belonging to the stator and the monodentate site of the
handle, leads to the closed state of the turnstile. At room temperature, the
turnstile oscillates between the two coordinating stations.
Fig. 2 Representation of the molecular turnstile in its open and closed
states (1 and 1-Ag+), synthetic precursors 2 and 3 as well as the assignment
of H atoms based on 2-D NMR studies.
hexacoordinated Sn(IV) atom located in the centre of the tetraaza
core of the porphyrin. It is worth noting that, as expected from
the difference in coordination propensity between the pyridyl and
benzonitrile groups, one would expect the binding of metals by the
pyridyl unit. The two benzonitrile groups have been introduced
with the aim of switching the coordination process between pyri-
dine and benzonitrile units through external stimuli. This aspect
is out of the scope of the present contribution. The rotor bears
at its centre a pyridyl moiety as a monodentate coordinating site
and two terminal resorcinol groups connected to the pyridyl unit
through two triethyleneglycol chains. The interconnection of the
The molecular turnstile 1 (Fig. 2) is based on a Sn(IV)-porphyrin
behaving as a stator, and a symmetrical handle as a rotor.
The stator is equipped at the meso positions with two pyridyl
and two benzonitrile groups in a trans disposition and a hinge,
UMR UDS-CNRS 7140, Universite de Strasbourg, Institut Le Bel, 4 rue
Blaise Pascal, CS90032 67081 Strasbourg, France
† CCDC reference number 811000 for compound 1. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt10184f
5244 | Dalton Trans., 2011, 40, 5244–5248
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rotor to the stator is ensured by the presence of a Sn hinge through
two apical Sn–O bonds. In the absence of an effector, as a result
of the free rotation of the rotor around the stator (Fig. 1a), the
turnstile 1 may be considered as being in its open position. In
the presence of Ag+ cation as an effector capable of adopting a
linear coordination geometry, owing to the binding of the latter
by the pyridyl unit of the rotor and one of the two pyridyl moieties
located on the stator, the cationic 1-Ag+ complex may be seen as
the closed state of the turnstile (Fig. 1b). Interestingly, resulting
from the design of the turnstile 1 bearing on the stator two pyridyl
monodentate coordinating sites in trans configuration, based on
the rather labile nature of the N–Ag+ bond, one would expect an
oscillating behaviour between the two pyridyl groups which may
be regarded as two distinct stations (Fig. 1c).
(Fig. 4a) and thus the absence of specific interactions between the
rotor and the stator. Indeed, as expected, two doublets for the b-
pyrrolic hydrogen atoms Hc and Hd, as well as only two doublets
for Ha and Hb protons of the pyridyl moiety and two doublets for
He and Hf protons of the benzonitrile units were observed.
The synthesis of turnstile 1 was achieved in 60% yield at room
temperature upon condensation of the dihydroxy Sn-porphyrin
derivative 220 with the handle 3 in CH2Cl2.17,18 The closed turnstile
1-Ag+ was obtained in quantitative yield at room temperature
upon reaction of silver hexafluoroantimonate with the turnstile 1
in a 1 : 1 CH3CN–CH2Cl2 mixture.
The solid state structure (Fig. 3) of turnstile 1 was determined
by X-ray diffraction on single crystal obtained upon slow diffusion
of diethyl ether into an acetonitrile solution of 1. The latter
crystallizes in the monoclinic Pc space group and the crystal
is composed of two molecules of 1 and an acetonitrile solvent
molecule. No distortion of the planarity of the porphyrin core
is observed and the tin atom is located almost at the centre of
the four pyrrolic units with the Sn–N bond distance in the range
Fig. 4 Portion of the 1H-NMR spectra between 9.5–7.0 ppm and
assignment of signals for turnstiles 1 in CD3CN at 25 ◦C (a) and for
1-Ag+ in CD2Cl2–CD3CN at 25 ◦C (b) and at -70 ◦C (c).
The closed state of the turnstile 1-Ag+ is obtained upon addition
of AgSbF6 to a 1 : 1 CH3CN–CH2Cl2 solution of the turnstile 1.
At room temperature, the 1D 1H-NMR study of the turnstile in its
closed state 1-Ag+ (Fig. 4b) showed significant downfield shifts of
signals corresponding to Hs and Hr hydrogen atoms of the pyridyl
group as well as the methylene bridge protons Hu by +0.75, +0.38
and +0.29 ppm, respectively. Interestingly, signals corresponding
to Ha and Hb hydrogen atoms of the two pyridyl groups attached
to the stator are also downfield shifted by +0.10 and +0.18 ppm,
respectively, however with no splitting of the two doublets. These
observations indicate that the Ag+ cation is complexed by the
pyridyl unit of the rotor and those of the stator. However, the
absence of splitting of the Ha and Hb signals into (Ha, Ha¢) and
(Hb, Hb¢), as well as for Hc and Hd implies a fast exchange process
with respect to the NMR timescale between these two sites.
In order to investigate the dynamics of the exchange process,
variable temperature 1H-NMR studies were performed in the -70
to 25 ◦C range. As shown on Fig. 5, the decrease in temperature
˚
of 2.087–2.102 A. The two types of substituents, i.e. two pyridyl
and two benzonitrile groups, located at the meso substituents of
the porphyrin are not coplanar with the porphyrin main plane
but tilted with CCCC dihedral angles of -67.4◦ and 85.9◦ for the
pyridyl groups and -69.4◦ and -89.6◦ for the benzonitrile units.
The coordination geometry around the metallic centre is almost
octahedral with N–Sn–N, N–Sn–O and O–Sn–O angles of 179.4–
179.7◦ (N–Sn–Ntrans), 89.5–90.7◦ (N–Sn–Ncis), 87.9–92.2◦ and
177.7◦, respectively. The two apical positions of the tin atoms
are occupied by the oxygen of the two terminal resorcinol groups
˚
with Sn–O distances of 2.042 and 2.053 A.
Fig. 3 X-Ray structure of the turnstile 1. For the sake of clarity the carbon
atoms of the porphyrin moiety and the handle are differentiated by colour.
Hydrogen atoms and solvent molecules are omitted for clarity. For bond
distances and angles see text.
In solution, the turnstile in its open (compound 1) and in its
closed (complex 1-Ag+) states as well as the switching process
between them (Fig. 1) were studied by variable temperature 1- and
2-D NMR experiments.
At room temperature, 1D H-NMR investigation in CD3CN
of 1 revealed the free rotation of the handle around the stator
Fig. 5 Portions of 1H-NMR spectra (500 MHz, CD2Cl2–CD3CN) of
1-Ag+ at 25 (a), -8 (b), -20 (c), -25 (d), -30 (e), -40 (f), -45 (g) and -70 ◦C
(h). * corresponds to traces of grease.
1
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leads to a frozen system as evidenced by the differentiation of the
two pyridyl groups of the stator. The H-NMR investigation at
and an activation energy of 48.5 kJ, (iii) complete immobilization
of the system at -70 ◦C. However, for the oscillation of the handle
between the two coordinating sites located on the stator, two
possible scenarios may be envisaged. Starting from 1-Ag+ complex
for which the metal cation is simultaneously complexed by the
pyridyl unit of the rotor and one of the two pyridyl groups of
the stator, the first possibility could be a complete dissociative
mechanism consisting in decomplexation of the metal cation thus
leading overall to three free pyridyl units followed by rotation of
the handle and subsequent binding of Ag+ by the coordinating site
of the handle and by the other pyridyl unit. The second possibility
could be a partially dissociative mechanism for which the metal
cation would remain bound to the handle and the rotation of
the complexed handle would lead to the formation of the locked
system through coordination by the other pyridyl unit of the
stator. The solution studies carried out do not allow one to choose
between the two possible mechanisms.
Interestingly, starting from the closed state of the turnstile 1-
Ag+, one may regenerate the open state upon addition of a com-
peting ligand. Indeed, addition of tetraethylammonium bromide
leads to the formation of insoluble AgBr and soluble Et4NSbF6
salts and the open state of the turnstile 1. The opening process was
followed by 1H-NMR at room temperature (Fig. 7) which revealed
that the spectra of 1 (Fig. 7a) and of the medium after treatment
with Et4NBr (Fig. 7c) were perfectly superimposable.
1
-70 ◦C (Fig. 4c and Fig. 5h) clearly showed that the symmetry of
the molecule is lowered and as expected for a blocked system, the
number of signals corresponding to the b-pyrrolic hydrogen atoms
(Hc, Hc¢, Hd and Hd¢) is doubled with respect to the parent turnstile
1. Furthermore, the two pyridyl groups at the meso positions of
the porphyrin are differentiated, i.e. observation of four doublets
corresponding to protons Ha, Ha¢, Hb and Hb¢ while He and Hf
protons of the two benzonitrile units remained unchanged (two
doublets).
1
In order to further confirm the observations made by 1D H-
NMR, the system has been studied by 2D 1H-NMR experiments
at 25 and -70 ◦C. The ROESY experiments at room temperature
on 1-Ag+ (Fig. 6, top) showed through space correlations between
the handle and both pyridyl groups of the stator. Indeed, protons
Ha are correlated with protons Ho, Hp, Hq and Hl of the handle.
Furthermore, a correlation between Hb and Hl is observed. This
behaviour confirms the oscillating movement between those two
pyridyl units mentioned above -25 ◦C.
Fig. 7 Portions of the 1H-NMR spectra (25 ◦C, 300 MHz, CD3CN) of 1
(a), 1-Ag+ (b) and of the mixture resulting from the addition of Et4NBr (c)
showing the opening of the turnstile upon removal of Ag+ cation as AgBr
insoluble salt. * corresponds to CH2N protons of the Et4N+ cation.
Fig.
6
Portion of the 2-D ROESY investigations (500 MHz,
CD2Cl2–CD3CN) for 1-Ag+ at 25 ◦C (top) and at -70 ◦C (bottom). For
attribution of hydrogen atoms see Fig. 2.
In conclusion, compound 1 is a molecular turnstile composed
of three parts, a porphyrin based stator equipped with two
sets of substituents in trans configuration (two pyridyl and two
benzonitrile units) at the meso positions, a hinge Sn(IV) atom
located at the centre of the porphyrin backbone and a symmetrical
handle bearing a central pyridyl group connected to the stator
through two Sn–O bonds. Resulting from the design of the system,
in the presence of a silver cation, the turnstile oscillates in solution
at room temperature between two closed states resulting from
the simultaneous binding of the cation by the coordinating site
located on the handle and the pyridyl units belonging to the stator.
By lowering the temperature to -70 ◦C, the oscillating movement
may be frozen thus generating the blocked state of the system.
The ROESY experiment carried out at -70 ◦C (Fig. 6, bottom)
confirms the blockage of the turnstile. Owing to the overlap
between Ha¢ and Ha signals, the correlation observed with Ho and
Hp do not confirm unambiguously the closed state of the turnstile.
However, as expected, the observation of a single correlation
between Hb¢ and Hl of the handle clearly demonstrate that the
turnstile is frozen at that temperature.
The va◦riable temperature study revealed the following features:
(i) at 25 C, the turnstile is oscillating between two degenerated
states corresponding to the binding of the cation by the two
pyridyl units attached to the stator, (ii) a coalescence temperature
of -25 ◦C for the proton Hb, leading to a kinetic constant of 317 Hz
5246 | Dalton Trans., 2011, 40, 5244–5248
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Interestingly, one may switch between the open and closed states of
the system using an external stimulus such as addition of Et4NBr.
Currently, we are pursuing the same design principle using other
types of stations and metal centres.
We thank the University of Strasbourg, the International Centre
for Frontier Research in Chemistry (FRC), Strasbourg, the Institut
Universitaire de France, the Ministry of Education and Research
and the CNRS for financial support.
temperature and in the exclusion of light during 1.5 h. After
removal of the solvent, the desired complex 1-Ag+ was obtained
in quantitative yield (12.5 mg) as a purple solid.
UV-Vis (CH3CN, lmax (nm) (loge)): 418 (5.5), 555 (4.2), 594
1
(3.7). H-NMR (CD3CN, 500 MHz) d (ppm): 1.14 (t, 2H, Arh,
3
4
4J = 2.0 Hz), 1.46 (dd, 2H, Ari, J = 8.0 Hz, J = 2.0 Hz), 3.18
(m, 4H, OCH2l), 3.45 (m, 4H, OCH2m), 3.63 (m, 4H, OCH2n), 3.72
(m, 4H, OCH2o), 3.87 (m, 8H, OCH2p,q), 4.84 (s, 4H, OCH2u), 5.31
(dd, 2H, Ark, 3J = 8.0 Hz, 4J = 2.0 Hz), 5.52 (t, 2H, Arj, 3J = 8.0
Hz), 7.52 (d, 2H, Pyr, 3J = 8.0 Hz), 7.99 (t, 1H, Pys, 3J = 8.0 Hz),
8.25 (d, 4H, Arf, 3J = 8.0 Hz), 8.33 (dd, 4H, Pyb, 3J = 4.5 Hz, 4J =
Experimental section
3
3
CH3CN was dried over molecular sieves; CH2Cl2 was dried and
distilled over CaH2.
1.5 Hz), 8.39 (d, 4H, Are, J = 8.0 Hz), 9.16 (dd, 4H, Pya, J =
4
3
4.5 Hz, J = 1.5 Hz), 9.17 (d, 4H, b-pyr.d, J = 5.0 Hz), 9.20 (d,
4H, b-pyr.c, 3J = 5.0 Hz). 13C-NMR (CD3CN, 100 MHz) d (ppm):
66.8, 70.0, 70.8, 70.9, 71.1, 73.2, 103.6, 103.7, 110.7, 113.5, 119.5,
119.8, 121.7, 124.2, 127.4, 130.8, 132.0, 133.7, 134.0, 135.9, 140.6,
145.6, 147.5, 150.4, 150.7, 156.2, 158.3, 158.4.
1H- and 13C-NMR spectra were acquired at 25 ◦C unless
stated otherwise on either a Bruker AV 300 or a Bruker AV 500
spectrometer, with the deuterated solvent as the lock and residual
solvent as the internal reference. Absorption spectra were recorded
using an Uvikon XL spectrophotometer. Mass spectrometry
analyses were performed by the Service de Spectrometrie de
Masse, UdS, Strasbourg. X-ray analysis were performed on a
Bruker APEX8 CCD Diffractometer equipped with an Oxford
Cryosystem liquid N2 device at 173(2) K using a molybdenum
microfocus sealed tube generator with mirror-monochromated
1H-NMR (CD2Cl2–CD3CN (1 : 4), 500 MHz, 203 K) d (ppm):
3
1.04 (br, 2H, Arh), 1.32 (d, 2H, Ari, J = 7.5 Hz), 3.11 (br, 4H,
OCH2l), 3.43 (br, 4H, OCH2m), 3.63 (br, 4H, OCH2n), 3.74 (m, 8H,
OCH2o,q), 3.82 (br, 4H, OCH2p), 4.74 (br, 4H, OCH2u), 5.17 (d, 2H,
Ark, 3J = 7.5 Hz), 5.39 (t, 2H, Arj, 3J = 8.0 Hz), 7.43 (d, 2H, Pyr,
3J = 7.5 Hz), 7.92 (t, 1H, Pys, 3J = 7.5 Hz), 8.12 (br, 2H, Pyb¢), 8.24
(d, 4H, Arf, 3J = 7.5 Hz), 8.40 (br, 2H, Pyb), 8.45 (d, 4H, Are, 3J =
7.5 Hz), 8.92 (d, 2H, b-pyr.c¢, 3J = 4.5 Hz), 9.03 (br, 4H, Pya,a¢), 9.15
˚
Mo-Ka radiation (l = 0.71073 A), operated at 50 kV/600 mA. The
structure were solved using SHELXS-97 and refined by full matrix
least-squares on F2 using SHELXL-97 with anisotropic thermal
parameters for all non-hydrogen atoms.21 The hydrogen atoms
were introduced at calculated positions and not refined (riding
model).
3
3
(d, 2H, b-pyr.d¢, J = 4.5 Hz), 9.34 (d, 2H, b-pyr.d, J = 4.0 Hz),
9.39 (d, 2H, b-pyr.c, 3J = 4.0 Hz).
MS (ESI) for C75H63AgN9O10Sn+: 1476.94 g mol-1. m/z (M+):
calculated = 1476.278; measured = 1476.283.
Compound 1. In
a dry 100 mL round-bottom flask,
Crystallographic data for 1. C152H129N19O20Sn2, M = 2779.12,
trans-dihydroxy[5,15-bis(4-pyridyl)-10,20-bis (4-cyanophenyl)-
porphyrinato] tin(IV) 2 (100 mg, 120 mmol, 1 eq.) and the handle 3
(76 mg, 120 mmol, 1 eq.) were dissolved in 55 mL of CH2Cl2. The
reaction mixture was stirred at room temperature under argon,
for 7 days. 50 mL of pentane was added to the reaction mixture in
order to precipitate unreacted products. After filtration, complex
1 (100 mg, 60%) was precipitated as a purple solid upon addition
of 100 mL of n-pentane.
˚
˚
monoclin◦ic, Pc, a = 13.9419(3) A, b =◦10.2272(2) A, c = 24.4742(5)
◦
3
˚
˚
A, a = 90 , b = 103.4480(10) , g = 90 , U = 3394.01(12) A , Z = 1,
m = 0.444 mm-1, refls measured: 60 652, independent refls: 15 078
[R(int) = 0.0482], final R indices [I > 2s(I)]: R1 = 0.0526, wR2 =
0.1248, R indices (all data): R1 = 0.0695, wR2 = 0.1379, GOF on
F2: 1.010.
IR (ATR, cm-1): 2229 (CN). H-NMR (CD3CN, 300 MHz) d
1
Notes and references
4
3
(ppm): 1.18 (t, 2H, Arh, J = 2.5 Hz), 1.39 (ddd, 2H, Ari, J =
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4
4
8.0 Hz, J = 2.0 Hz, J = 0.5 Hz), 3.13 (m, 4H, OCH2), 3.43 (m,
4H, OCH2), 3.50 (m, 4H, OCH2), 3.56 (m, 4H, OCH2), 3.65 (m,
8H, OCH2), 4.55 (s, 4H, OCH2u), 5.34 (ddd, 2H, Ark, 3J = 8.0 Hz,
4
3
4J = 2.5 Hz, J = 0.5 Hz) 5.51 (t, 2H, Arj, J = 8.0 Hz), 7.14 (d,
2H, Pyr, 3J = 8.0 Hz), 7.24 (dd, 1H, Pys, 3J = 8.5 Hz, 3J = 7.0 Hz),
8.15 (dd, 4H, Pyb, 3J = 4.5 Hz, 4J = 1.5 Hz), 8.21 (d, 4H, Arf, 3J =
3
3
8.0 Hz), 8.31 (d, 4H, Are, J = 8.0 Hz), 9.06 (dd, 4H, Pya, J =
4.5 Hz, 4J = 1.5 Hz), 9.10 (d, 4H, b-pyr., 3J = 5.0 Hz, JSn-H = 17.0
Hz), 9.18 (d, 4H, b-pyr., 3J = 5.0 Hz, JSn-H = 17.0 Hz). 13C-NMR
(CD2Cl2, 100 MHz) d (ppm): 66.3, 69.5, 70.5, 70.8, 74.0, 103.3,
103.4, 110.1, 113.1, 118.9, 120.0, 120.8, 126.9, 129.7, 131.3, 133.2,
135.6, 137.2, 145.3, 146.9, 148.9, 155.3, 158.2, 158.3. MS (ESI)
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5248 | Dalton Trans., 2011, 40, 5244–5248
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