Porphyrin-based switchable molecular turnstiles

Article abstract of DOI:10.1002/chem.201100057

The design, synthesis and structural characterisation, in solution, of two new molecular turnstiles based on Sn-porphyrin derivatives are described. The system is composed of a stator (5-(4-pyridyl)-10,15,20-triphenylporphyrin), a hinge (Sn^IV)

Full text of DOI:10.1002/chem.201100057

FULL PAPER
DOI: 10.1002/chem.201100057
Porphyrin-Based Switchable Molecular Turnstiles
Aurꢀlie Guenet, Ernest Graf,* Nathalie Kyritsakas, and Mir Wais Hosseini*[a]
Abstract: The design, synthesis and
structural characterisation, in solution,
of two new molecular turnstiles based
on Sn–porphyrin derivatives are de-
scribed. The system is composed of a
stator (5-(4-pyridyl)-10,15,20-triphenyl-
porphyrin), a hinge (SnIV) and a rotor
(handle equipped with 2,6-pyridinedi-
carboxamide as a tridentate coordinat-
ing site or its PdII complex). The pres-
ence of interaction sites, both on the
stator and the rotor, offers the possibil-
ity of switching between an open state
(free rotation of the handle around the
porphyrin) and a closed state (blockage
of the rotation) by either establishment
of hydrogen bonds between the stator
and the rotor or by the simultaneous
binding of Pd by both coordinating
groups.
Keywords: molecular motion
·
molecular turnstiles · palladium ·
porphyrinoids · synthesis design
Introduction
inga et al.[13] described two other elegant molecular motors
based on thermal or photochemical processes. Examples of
molecular cars,[14] wheelbarrows[15] and turnstiles[16] have also
been reported. The design and synthesis of molecular
motors and machines still remain a challenge.[17–26]
Herein, we report on two new molecular turnstiles, 2 and
3, based on a Sn–porphyrin (stator) combining a peripheral
monodentate coordinating site on the porphyrin core and a
tridentate chelate on the handle (rotor) (Scheme 1).
In living organisms, the control of molecular motion is es-
sential for many biological processes.[1] Translational motors
based on myosine[2,3] or kinesine[4,5] and rotational motors,
such as adenosine triphosphate (ATP) synthase,[6,7] have
been discovered and studied. The complexity of these ma-
chineries is beyond synthetic chemistry. However, molecular
chemists have proposed elegant design principles of molecu-
lar motors and machines that are considerably less sophisti-
cated than the natural ones.[8] In particular, the two pioneer-
ing strategies developed by the groups of Stoddart and Bal-
zani and by Sauvage paved the way to an active research
topic that is still of intense interest.[9,10] The design principles
followed by the two groups are based on rotaxanes and cate-
nanes; two peculiar classes of preorganised architectures.
Whereas Stoddart and co-workers exploited the supramolec-
ular approach by designing translational motors based on
specific interactions between organic moieties in rotaxane-
type assemblies,[9] Sauvage and co-workers, using catenane-
type molecules, exploited the coordination strategy for the
design of both rotational and translational systems based on
geometric and binding-propensity differences for metal cat-
ions in two different oxidation states.[10] Based on the same
type of strategy, the same group has also reported a mova-
ble system described as an artificial molecular muscle.[11]
Based on other molecular platforms, Kelly et al.[12] and Fer-
Results and Discussion
Design of the system: Recently, we have described the
design and synthesis of a series of new molecular turnstiles
based on a Sn–porphyrin backbone.[27,28] These systems are
composed of a stator, a hinge and a handle (Figure 1). The
stator is a porphyrin backbone with a 4-pyridyl unit at one
of the meso positions acting as a monodentate coordinating
site. The connection of the latter to the porphyrin imposes
the outward orientation (Figure 1a). The hinge is a SnIV
atom—a strong oxophilic Lewis acid adopting an octahedral
coordination geometry—complexed by the tetraaza core of
the porphyrin and thus offering two free axial positions (Fig-
ure 1b). The handle is a rather flexible fragment possessing
a pyridyl central coordination site oriented towards the por-
phyrin backbone and two terminal resorcinol moieties con-
nected to the pyridine unit by two oligoethyleneglycol
spacers. The connection of the handle to the stator is en-
[a] Dr. A. Guenet, Dr. E. Graf, N. Kyritsakas, Prof. Dr. M. W. Hosseini
Laboratoire de Chimie de Coordination Organique
UMR UdS-CNRS 7140, Universitꢀ de Strasbourg
Institut Le Bel, 4, rue Blaise Pascal, CS 90032
67081 Strasbourg (France)
À
sured through Sn O bonds between the two terminal resor-
cinol groups and the two free apical positions of the SnIV
centre located at the centre of the porphyrin (Figure 1c).
The behaviour of turnstile 1 (Scheme 1) was investigated
Fax : (+33)368851325
E-mail: egraf@unistra.fr
1
in solution by H NMR spectroscopy in the absence and in
the presence of Ag+ cation acting as an effector. As expect-
ed, whereas in the absence of the cation, the handle freely
rotates around the stator (open state, Figure 1c), in the pres-
hosseini@unistra.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100057.
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6443

Figure 1. Schematic representation of a stator, a porphyrin derivative
with a monodentate coordinating site at one of the meso positions (a); a
hinge, an SnIV centre complexed by the tetraaza macrocyclic core of the
porphyrin (b); a turnstile resulting from the interconnection of a handle
with a monodentate (c) or a tridentate coordinating moiety (d) in the
open (c and d) and closed (e and f) states, resulting from the simultane-
ous binding of a metal centre.
ence of the effector, the rotation is blocked by the simulta-
neous binding of a Ag+ cation by both pyridine units locat-
ed on the porphyrin backbone and on the handle (closed
state, Figure 1e). However, the stability constant, Ks, of only
5700 molLÀ1 (DG of ca. À21.4 kJmolÀ1) appeared to be
rather low. To strengthen the interaction between the
handle and the metal centre, it appeared judicious to substi-
tute the monodentate pyridine moiety on the handle by the
2,6-pyridinedicarboxamide group, which is a dianionic tri-
dentate chelating unit (Figure 1d). The choice of the chelat-
ing group was based on previously reported rotaxanes[29–33]
or catenanes[34] and a turnstile[35] leading to rather stable
complexes. As the effector, instead of Ag+ cation, PdII was
chosen because the latter adopts a square-planar coordina-
tion geometry, and furthermore, owing to the dianionic
nature of the chelating site on the handle, should lead to a
neutral turnstile (Figure 1 f).
To explore the above-mentioned design principle, com-
pounds 2 and 3 (Scheme 1) were synthesised. These two
turnstiles are structural analogues and differ only by the
length of the oligoethyleneglycol spacer connecting the che-
lating unit to the resorcinol moieties ((CH2)2O
and (CH2)2O(CH2)2O(CH2)2 for 3).
ACHTUGNERTN(NUNG CH2)2 for 2
N
ACHTUNGTRENNUNG
Scheme 1. Structures of compounds 1–3, 3–Pd and 3–Pd–L complexes
and labelling of hydrogen atoms.
Synthesis of the two turnstiles: The synthetic strategy for
the preparation of compounds 2 and 3 (Scheme 1) was
based on the condensation of the dihydroxy porphyrin 21
with the handles 17 and 18, respectively (Scheme 2).
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Porphyrin-Based Switchable Molecular Turnstiles
FULL PAPER
4 days. In both cases, the completion of the reaction was
1
monitored by H NMR spectroscopy.
Solution behaviour of compound 2: In solution, as expected,
owing to the magnetic anisotropy of the porphyrin core, the
1H NMR spectrum of compound 2 showed significant up-
field shifts of the proton signals belonging to the resorcinol
moieties of the handle.
In CD2Cl2, the signals corresponding to the b-pyrrolic pro-
tons consist of a singlet and two doublets, as observed for
the parent porphyrin 21, indicating that, on the NMR time-
scale, the average local environment of the porphyrin 2 is
similar to that of 21 and implies free rotation of the handle
around the stator. This was further confirmed by a compari-
1
son of the H NMR spectra of free handle 17 and compound
2; this showed the absence of interaction between the
handle and the porphyrin moiety (Table 1). Indeed, in
Table 1. 1H NMR (300 MHz, 298 K) chemical shifts of protons Hr (see
Scheme 1 for proton labelling) for compounds 2, 3, 17 and 18 in CD2Cl2
and [D6]DMSO.
d [ppm]
CD2Cl2
[D6]DMSO
2
17
3
ꢀ8.20[a]
9.48
9.40
9.52
9.42
8.51
9.33
8.84
18
[a] Multiplet.
CD2Cl2, the resonance corresponding to the amide protons
Hr (for signal assignment see Scheme 1) is only slightly up-
field shifted (Dd=0.31 ppm) upon formation of compound
2. The free rotation of the handle was further evidenced by
2D ROESY NMR spectroscopy experiments, which showed
no through-space correlations between protons belonging to
the handle and protons located on the stator (Figure 2). Fi-
nally, a similar behaviour is observed in [D6]DMSO (Dd=
0.08 ppm for the Hr protons when comparing the free
handle 17 and compound 2).
Scheme 2. THP=tetrahydropyranyl, Phth=phthalimidoyl, Py=pyridine.
The synthesis of handle 17 was achieved in about 75%
yield upon condensation of monoprotected resorcinol 4 with
dimesylate 13 in the presence of Cs2CO3 in acetonitrile at
reflux, followed by deprotection at room temperature in
about 80% yield by using a solution of methanol/aqueous
HCl. Dimesylate 13 was prepared in two steps. The reactio-
n[34a] of diacylchloride derivative 6[36] with the commercially
available aminoglycol 7 at room temperature and in the
presence of Et3N afforded in about 72% yield the diol 11,
which was subsequently transformed into the dimesylate 13
in about 72% yield upon treatment with methanesulfonyl
chloride in the presence of triethylamine in dry THF at
room temperature.
Solution behaviour of compound 3: In solution, the struc-
1
ture and dynamics of compound 3 were studied by H NMR
spectroscopy at 298 K in [D6]DMSO and CD2Cl2 (Figure 3)
and by 2D NMR spectroscopic techniques both in
[D6]DMSO at 298 K and in CD2Cl2 at 273 K (Figure 4).
In [D6]DMSO, compound 3 behaves in a similar fashion
to compound 2. In particular, a rather small upfield shift of
0.1 ppm of the amide protons Hr is observed (Table 1).
Again, owing to the magnetic anisotropy of the porphyrin,
upon attachment of the handle to the Sn–porphyrin, protons
belonging to the resorcinol moieties undergo significant
shielding. Furthermore, couplings between the two 117Sn and
119Sn isotopes and the b-pyrrolic protons enabled the assign-
ment of their signals. The signals corresponding to the b-pyr-
rolic protons appear as a sharp multiplet at d=9.12 ppm. Fi-
nally, the ROESY experiment at 298 K shows only the ex-
The syntheses of handle 18[35] and porphyrin 21[28] were re-
ported earlier.
Turnstile 2 was prepared in 70% yield upon condensation
of 21 with a small excess of 17 in dry CHCl3 for 7 days at
room temperature. By using a similar procedure, compound
3 was prepared in 91% yield upon condensation of handle
18 with porphyrin 21 in CHCl3 at room temperature for
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6445

E. Graf, M. W. Hosseini et al.
Figure 4. Portions of the 2D ROESY correlation (1H–1H NMR) of com-
pound
3 in [D6]DMSO (A) (600 MHz, 298 K) and in CD2Cl2 (B)
Figure 2. Portions of the 2D ROESY correlation (1H–1H NMR, 500 MHz,
CD2Cl2, 298 K) of compound 2 and assignment of signals between d=
7.60 and 9.30 ppm (A) and 3.00 and 3.75 ppm (B). For assignment of the
protons, see Scheme 1.
(500 MHz, 273 K). For assignment of the protons, see Scheme 1.
In marked contrast, in CD2Cl2, compound 3 shows rather
different behaviour and a lower symmetry (Figure 3). The
amide protons Hr are shifted upfield in CD2Cl2 (d=
9.33 ppm in CD2Cl2 versus d=9.52 ppm in [D6]DMSO). Pro-
tons s and t (Scheme 1), belonging to the pyridine moiety lo-
cated on the handle, are deshielded by 0.20 ppm and slightly
shielded by 0.07 ppm, respectively. Signals assigned to the b-
pyrrolic protons appear as four doublets (d=9.05, 9.17, 9.27,
9.33 ppm), implying a change in the environment of the por-
phyrin (a sharp multiplet in [D6]DMSO). Furthermore, the
signal corresponding to proton k located on the pyridine
moiety of the stator is shifted downfield by 0.17 ppm. Simi-
lar behaviour was observed previously for the closed state
of turnstile 1 upon binding of Ag+ cations.[26,27] Here, the
only possibility that affords the closed state is the formation
of hydrogen bonds between the two acidic amide protons Hr
of the handle and the pyridine moiety of the stator. This hy-
1
Figure 3. Portions of the 1H NMR spectra (600 MHz, 298 K) in CD2Cl2
(A) and in [D6]DMSO (B) and assignment of signals between d=7.5–
9.6 ppm for compound 3. For assignment of the protons, see Scheme 1.
pothesis is supported by a comparison of the H NMR spec-
tra of the free handle 18 and compound 3 in CD2Cl2.
Indeed, contrary to what observed for compounds 2 and 17,
the establishment of hydrogen bonds between the pyridine
unit of the stator and the amide protons Hr leads to a de-
shielding of the amide protons resonances by about 0.5 ppm.
Further evidence for the existence of such interactions be-
tween the handle and the stator arises from 2D ROESY
NMR spectroscopy experiments performed in CD2Cl2. At
pected correlations between chemically linked protons (Fig-
ure 4A). These observations clearly show that in
[D6]DMSO, as in the case of 2, the handle freely rotates
around the O-Sn-O axis.
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Porphyrin-Based Switchable Molecular Turnstiles
FULL PAPER
Table 2. Selected bond lengths [ꢂ] and angles [8] for 3.
298 K, although a correlation peak between the amide pro-
tons r and protons k belonging to the pyridine of the stator
is observed, one might argue that the observation is ambigu-
ous because of an overlap of signals. To get a clear-cut mea-
surement, the same experiment was carried out at lower
temperature (273 K), which caused a slight downfield shift
of the triplet corresponding to the Hr protons. The above-
mentioned experiment unambiguously confirmed the pres-
ence of the postulated hydrogen bonds (Figure 4B). Finally,
as expected, the ROESY experiment showed a correlation
between proton L located on the porphyrin and those of the
ethyleneglycol chain q1, q3 and q4, indicating their spatial
proximity. The different behaviour observed for compound
3 upon switching from CD2Cl2 to [D6]DMSO, which is a
much more polar solvent, is in agreement with the forma-
tion of intramolecular hydrogen bonds. Indeed, it is expect-
ed that CD2Cl2 would promote the formation of such an in-
teraction, whereas [D6]DMSO would disrupt it. It is also
worth noting that similar hydrogen bonds between amide
protons and pyridine groups have been previously reported
by Leigh et al. for [2]rotaxane-type architectures.[30,31,38]
À
Sn O
À
Sn N
2.040(7), 2.059(6)
2.102(7), 2.103(7), 2.114(6), 2.117(6)
173.9(3)
86.5(3), 88.6(3), 87.0(3), 89.5(3), 91.4(3),
92.3(3), 92.5(3), 92.5(3)
178.6(3), 178.9(3)
O-Sn-O
N-Sn-O
N-Sn-N trans
N-Sn-N cis
89.3(3), 89.5(3), 90.1(3), 91.1(3)
the two apical positions. The three phenyl groups are tilted
with respect to the porphyrin plane (C-C-C-C dihedral
angles: +57.1, +116.35 and +118.168). The pyridine located
on the fourth meso position of the porphyrin is also tilted
(C-C-C-C dihedral angle: +101.598).
Interestingly, hydrogen bonds between the pyridine unit
of the stator and the amide protons are also present in the
solid state (see Table 3 for bond lengths). Indeed, the
À
Namide Nporphyrin distance is about 3.12 ꢂ. Following the clas-
sification proposed by Jeffrey,[39a,b] the hydrogen bonds ob-
served may be considered as “moderate”.
Structural investigations of 3 in the solid state: The presence
of hydrogen bonds between the handle and the hinge was
further confirmed in the crystalline phase by X-ray diffrac-
tion on a single crystal obtained upon vapour diffusion of
pentane into a solution of 3 in CH2Cl2 (Figure 5).
À
Table 3. X···A and H···A distances [ꢂ] and the N H···N angles [8] for 3
in the crystalline phase.
À
Namide Nporphyrin
3.053, 3.197
2.685, 2.690
2.264, 2.377
2.291, 2.312
149.29, 155.29
À
Namide Nhandle
À
H Nporphyrin
H Nhandle
Namide-H-Nporphyrin
À
In the light of structural features observed for 3, the ab-
sence of hydrogen bonding for compound 2 mentioned
above, even in a rather apolar solvent such as CD2Cl2, is
probably due to the length of the spacers connecting the tri-
dentate unit to the resorcinol moieties ((CH2)2OACHTNURTGENUNG(CH2)2).
In summary, the molecular turnstile 3 displays solvent-de-
pendent behaviour. Whereas in [D6]DMSO, which is a hy-
drogen-bond-disrupting solvent, the handle rotates freely
around the stator (open state), in a relatively apolar solvent,
such as CD2Cl2, owing to the formation of hydrogen bonds
between the pyridyl moiety located on the stator and the
amide protons belonging to the handle, the rotation of the
latter is blocked (closed state).
Figure 5. Solid-state structure of 3. A fragment of the triethyleneglycol
units was found to be disordered. Hydrogen atoms and solvent molecules
are omitted for the sake of clarity. The C atoms of the stator (grey) and
the handle (black) are shown in different colours for clarity. For bond
lengths and angles, see Table 2.
Coordination of palladium(II) by precursors: Both com-
pounds 2 and 3, with a tridentate moiety on the handle,
were designed to strongly bind square-planar metal centres
such as PdII. As mentioned above, since for compound 2,
the handle was found to be too short to allow the formation
of intramolecular hydrogen bonds, we thought that the same
would apply to the binding of PdII, and thus, only the behav-
iour of compound 3 was explored. To investigate the struc-
Compound 3 crystallises in the triclinic system (space
group P1) with four dichloromethane molecules. One of the
¯
ethyleneglycol units was found to be disordered. The struc-
tural features (Table 2) are similar to those already reported
for similar complexes[28] and other tin phenolate porphyr-
ins.[38] The porphyrin plane is slightly distorted and the Sn
atom is located almost at the centre of the four pyrrolic
units. The SnIV cation is hexacoordinated and adopts an
almost octahedral coordination geometry. Its coordination
sphere is composed of four N atoms belonging to the por-
phyrin and two O atoms of the resorcinol groups occupying
1
tural features of 3–Pd by H NMR spectroscopy, reference
complexes 11–Pd, 12–Pd, 17–Pd and 18–Pd (Scheme 2) were
prepared. Their synthesis was based on reported procedures
by Hirao et al.[40] and Leigh et al.[34a] for similar complexes.
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E. Graf, M. W. Hosseini et al.
At room temperature, the reaction of precursors 11, 12, 17
and 18 with a small excess of palladium acetate in acetoni-
trile afforded the desired complexes in 31 to 52% yield.
Complexes with pyridine as the fourth ligand appeared to
be more stable than those with acetonitrile.
While for the free ligands 11, 12, 17 and 18, the triplet
corresponding to the proton t located on the pyridine is sys-
tematically more shielded than protons s (doublet), the re-
verse situation, that is, an upfield shift of the doublet, is ob-
served for the Pd complexes (Table 4 and Figure 6 for 18
and 18–Pd). As expected, the three signals of the pyridine
acting as the fourth ligand in complexes 12–Pd, 17–Pd and
18–Pd are shifted downfield when compared with free pyri-
dine.
sation with tin porphyrin 21. A further advantage of this
strategy could be a template effect of the palladium(II)
cation[29–31,41] previously employed for the synthesis of cate-
nanes and rotaxanes.[42,43] Thus, at room temperature, the re-
action of 21 with 18–Pd for 4 days afforded the desired het-
erobimetallic porphyrin 3–Pd in 90% yield. The second
method (method B) was based on the direct condensation of
compound 3 with [PdACTHNUTRGENN(UG CH3CN)4]AHCTUNGTRENN(UGN PF6)2. At room tempera-
ture, the condensation in the presence of triethylamine in a
mixture of CH2Cl2 and CH3CN for 3 h afforded 3–Pd in
85% yield. Compound 3–Pd was characterised both by
1H NMR spectroscopy and mass spectrometry. Independent
of the method used, the MALDI-TOF spectra showed, in
addition to the molecular ion signal at 1449.3 for
[3ÀPd+H]+, two other signals at m/z 958.2 and 768.1 corre-
sponding to an adduct formed with the matrix used (dithra-
nol) and to the hydrolysed porphyrin 21, respectively. A
1
Table 4. Selected H NMR (300 MHz, 298 K) chemical shifts (d) for com-
pounds 11, 12, 17, 18, 11–Pd, 12–Pd, 17–Pd and 18–Pd.
1
comparison of the H NMR spectra of 3 and 3–Pd clearly in-
Ht [ppm]
Hs [ppm]
dicates the simultaneous coordination of palladium to the
tridentate site on the handle and the monodentate site locat-
ed at the meso position of the porphyrin. As mentioned
above for 18–Pd, protons s are shifted upfield by about
0.63 ppm (Figure 7), whereas proton t is barely affected (Dd
11[a]
8.07
8.11
8.01
8.01
8.15
8.20
7.99
8.04
8.24
7.59
8.28
7.68
8.20
7.65
8.27
7.75
11–Pd[a]
12[b]
12–Pd[b]
17[c]
17–Pd[c]
18[b]
18–Pd[b]
[a] Recorded in CD3CN. [b] Recorded in CD2Cl2. [c] Recorded in
[D6]DMSO.
Figure 7. Portions of the 1H NMR spectra (600 MHz, CD2Cl2, 298 K) for
compound 3 (A) and its palladium complex 3–Pd (B). For labelling of
the protons, see Scheme 1. * indicates the triethylammonium salt.
1
Figure 6. Portions of the H NMR spectra (300 MHz, CD2Cl2, 298 K) and
assignment of signals for compounds 18 (A) and 18–Pd (B). For labelling
of the protons, see Scheme 2. * indicates residual free pyridine.
ꢀ0.01 ppm). Possibly, owing to the rigidification of the
system induced by the coordination of PdII, signals corre-
sponding to the oligoethyleneglycol chain (q1, q2, q3, q4, q5
and q6) are broadened. No significant change is observed
for signals corresponding to the b-pyrrolic protons (four
doublets, the same as for 3 in CD2Cl2), indicating that com-
pounds 3 and 3–Pd display the same symmetry. As expected,
protons k and L, belonging to the pyridine attached to the
stator, experience deshielding upon coordination to PdII.
Synthesis and solution behaviour of 3–Pd: It should be
noted that the synthetic method followed for the prepara-
tion of reference Pd complexes mentioned above could not
be used for the synthesis of 3–Pd. Indeed, this procedure
leads to the formation of acetic acid, which dissociates the
handle through the replacement of the two resorcinate moi-
eties by acetate anions.[27] To overcome this difficulty, two
different strategies were developed for the synthesis of 3–
Pd. The first one (method A) was based on the complexa-
tion of the metal centre by the handle 18 prior to its conden-
Decoordination of palladium: As stated above, compound
3–Pd corresponds to the closed state of the turnstile (Fig-
ure 1 f). To switch to the open state, two demetallation strat-
egies have been explored. The first one aimed at generating
the partially decoordinated state 3–Pd–L (Scheme 1), corre-
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Porphyrin-Based Switchable Molecular Turnstiles
FULL PAPER
sponding to the displacement of the pyridyl unit belonging
to the stator. The second strategy was based on the com-
plete removal of PdII and the generation of 3. For the partial
decoordination, p-dimethylaminopyridine (DMAP), which
is a more strongly coordinating ligand than pyridine, was
used.[32a,e,35,44] Unfortunately, even in large excess (10 equiv)
and after six days, no decoordination of the palladium was
observed, as evidenced by the absence of shift of the signal
related to protons k. A possible reason might be steric hin-
drance preventing DMAP from accessing the Pd centre. To
overcome this, CNÀ anions, which are less sterically demand-
ing and stronger ligands, were used. The decoordination pro-
cess was monitored by 1H NMR spectroscopy. Different
amounts of Bu4NCN were added to aliquots of a solution of
[M+H]+; 1690.6 [M+TBA]+ TBA=tetrabutylammonium
and 3–Pd–CN (m/z 1474.26 [M]À). However, compound 3–
Pd is the major component of the mixture. Upon the addi-
tion of two equivalents of cyanide anions, the pyridyl unit is
displaced, thus leading to 3–Pd–CN as the major species.
Indeed, signals assigned to compound 3–Pd disappear and
the intensity of those corresponding to 3–Pd–CN increase
(Figure 8C). It is interesting to note that signals correspond-
ing to the b-pyrrolic protons appear as a singlet and two
doublets. This pattern is similar to that observed for com-
pound 3 in [D6]DMSO, indicating free rotation of the
handle. The above-mentioned observations are further sub-
stantiated by mass spectrometry, which shows the presence
of 3–Pd–CN (m/z 1474.41 [M]À) and the absence of a signal
at m/z 1449.3 (in positive mode) corresponding to 3–Pd.
However, while 3–Pd–CN is the dominant complex in solu-
tion, traces of the fully decoordinated species 3 are seen
(d=9.29 (d), 9.00 (J), 8.10 ppm (t)). Upon the addition of
10 equivalents of Bu4NCN, complete demetallation of 3–Pd
to afford 3 occurs. Indeed, as expected, a triplet correspond-
ing to proton r (d=9.35 ppm) and a downfield shift of the
signals corresponding to proton s are observed (Figure 8D).
Moreover, b-pyrrolic protons appear as four doublets (d=
9.28 (d), 9.22 (c), 9.12 (i), 9.00 ppm (J)). The small differen-
ces in chemical shifts observed between signals observed for
pure compound 3 and 3–Pd in the presence of 10 equiva-
lents of Bu4NCN are probably due to differences in ionic
strength. Again, assumptions made on the basis of the
1H NMR spectroscopic studies are confirmed by mass spec-
trometry, which reveals the presence of signals at m/z 1345.3
[M+H]+ and 1586.6 [M+TBA]+ corresponding to 3.
1
3–Pd in CD2Cl2 at room temperature and H NMR spectra
were recorded at different times until an equilibrium state
was reached (Figure 8).
In summary, compound 3–Pd may be partially or com-
pletely demetallated upon addition of tetrabutylammonium
cyanide. Although substitution of the monodentate pyridyl
unit, leading to 3–Pd–CN, by a cyanide ligand is feasible, the
reaction is not quantitative because traces of 3–Pd or 3 are
also present.
1
Figure 8. Portions of the H NMR spectra (300 MHz, CD2Cl2, 298 K) and
assignment of signals of 3–Pd in the presence of 0 (A), 1 (B), 2 (C) and
10 equivalents (D) of Bu4NCN and of complex 3 (E). For labelling of the
protons, see Scheme 1. * indicates an impurity.
Conclusion
As expected, depending on the number of equivalents of
cyanide anions added, the presence of different species in
solution was observed. Upon the addition of one equivalent
of CNÀ, compounds 3–Pd and 3–Pd–CN coexist. Indeed, two
sets of resonances for protons k, L, s and t corresponding to
3–Pd (d=9.66 (k), 8.44 (L), 8.10 (t), 7.75 ppm (s)) and to 3–
Pd–CN (d=9.07 (k), 8.15 (L), 7.96 (t), 7.66 ppm (s)) were
observed in a ratio of 2:1 (Figure 8B). With respect to the
last set of resonances, the signals corresponding to the pro-
tons located on the tridentate site are shifted upfield. Fur-
thermore, protons s—more shielded than proton t, which re-
mains almost unaffected—are shifted upfield relative to 3–
Pd, indicating that Pd is still coordinated to the tridentate
site and the fourth coordination site on the metal is occu-
pied by the cyanide anion. Electrospray mass spectrometry
in positive and negative modes performed on the NMR
sample confirmed the presence of 3–Pd (m/z 1449.3
Two new molecular turnstiles have been designed and pre-
pared. The two systems, based on a porphyrin backbone
with a pyridyl unit at one meso position (stator), connected
through a SnIV atom complexed by the tetraaza core of the
porphyrin (hinge) to a handle composed of two terminal re-
sorcinol units connected to a 2,6-pyridinedicarboxamide tri-
dentate coordinating site or its PdII complex by oligoethyle-
neglycol spacers (rotor), differ only by the length of the
spacer. In an apolar solvent, such as CD2Cl2, owing to the
presence of the tridentate unit on the handle and the pyridyl
moiety on the stator acting as hydrogen-bond donor and ac-
ceptor sites, respectively, the formation of hydrogen bonds
between the two units leads to the closed state of turnstile 3.
This may be opened (free rotation of the handle around the
porphyrin) upon addition of a hydrogen-bond disrupting sol-
vent, such as DMSO. The same compound 3, upon treat-
Chem. Eur. J. 2011, 17, 6443 – 6452
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6449

E. Graf, M. W. Hosseini et al.
1257.29 (44) [MÀH]+, 958.13 (46) [19ÀSnÀC14H10O3]+, 733.14 (10)
[19ÀSn]+.
ment with a PdII salt, leads to the formation of the neutral
palladium complex 3–Pd. The formation of the latter, result-
ing from the simultaneous binding of the cation by both the
deprotonated tridentate unit and the pyridyl group, gener-
ates the closed state of the turnstile. Again, the system may
be opened upon addition of CNÀ anions as competitive li-
gands.
Work on analogous systems with several coordinating
sites located at the meso positions of the porphyrin back-
bone is currently under progress.
Synthesis of 3: Under argon, a solution of 12 (35.3 mg, 0.057 mmol,
1.1 equiv) in dry CHCl3 (40 mL) was added dropwise, by using a cannula,
to a solution of 21 (40.1 mg, 0.052 mmol, 1 equiv) in dry CHCl3 (30 mL).
The resulting mixture was stirred at room temperature for 4 days. Evolu-
tion of the reaction was monitored by 1H NMR spectroscopy. After re-
moval of the solvent, the residue was recrystallised from toluene/pentane
(12 mL/60 mL) to afford a deep red–purple powder (63.9 mg, 91%).
Single crystals suitable for X-ray analysis were obtained by slow diffusion
of pentane into a solution of 3 in dichloromethane. 1H NMR (400 MHz,
CD2Cl2) (assignments according to COSY and ROESY 2D 1H–1H NMR
experiments): d=1.26 (t, 4J
7.9, 4J(H,H)=2.1 Hz, 2H; Hn), 3.20 (t, 3J
(t, 3J
(H,H)=4.6 Hz, 4H; Hq2), 3.56 (m, 4H; Hq6), 3.67–3.71 (m, 12H;
A
ACHTUNGTRENNUNG(H,H)=
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Experimental Section
3
4
H
ACHTUNGTRENNUNG
q3, Hq4, Hq5), 5.35 (dd, J
(H,H)=8.0 Hz, 2H; Ho), 7.83–7.92 (m, 9H; Ha, Hb, Hg, Hh), 8.09 (t,
(H,H)=7.8 Hz, 1H; Ht), 8.22–8.25 (m, 6H; Hf, HL), 8.37–8.41 (m, 4H;
(H,H)=4.7, 4J
(Sn,H)=16.7 Hz, 2H; HJ), 9.13 (d,
(Sn,H)=16.7 Hz, 2H; Hi), 9.16 (dd, 3J
(H,H)=4.3,
(H,H)=1.3 Hz, 2H; Hk), 9.23 (m, 4H; He, Hw), 9.30 ppm (d, J
(Sn-H)=17.3 Hz, 2H; Hd); 13C NMR (100 MHz, CD2Cl2): d=(as-
ACHUTGTNREN(NGU H,H)=7.9, JHCATUTGNREN(NUGN H,H)=2.4 Hz, 2H; Hp), 5.54 (t,
3J
3J
General: Compounds 5, 7 and 9 were commercially available and used
without further purification. Compounds 4,[28] 6, 8, 10, 12, 14, 16, 18,[35]
and 19–21[28] were synthesised according to published procedures. The
syntheses of compounds 11, 11–Pd, 12–Pd, 13, 15, 17, 17–Pd and 18–Pd
are described in the Supporting Information. CH3CN and CHCl3 were
dried over molecular sieves; THF and CH2Cl2 were dried and distilled
over sodium and CaH2. Triethylamine was dried and distilled over KOH.
Analytical EtOH and MeOH were used without further purification. 1H
and 13C NMR spectra were recorded at 258C, unless stated otherwise, on
either Bruker AV300 (300 MHz), Bruker AV400 (400 MHz), Bruker
AV500 (500 MHz) or Bruker AV600 (600 MHz) spectrometers, with the
deuterated solvent as the lock and residual solvent as the internal refer-
ence. Absorption spectra were recorded by using a Uvikon XL spectro-
photometer. Melting points were measured by using a Stuart Scientific
Melting Point SMP-1 apparatus without further correction. MALDI-TOF
spectra were recorded on a Bruker autoflex II TOF TOF (equipped with
a nitrogen laser and used in reflectron mode) instrument with dithranol
(1,8,9-trihydroxyanthracene) as the matrix. A microTOF LC (Bruker
Daltonics, Bremen) spectrometer equipped with an electrospray source
was used for the electrospray mass spectrometry measurements.
AHCTUNGTRENNUNG
Hs, Hc), 9.00 (d, 3J
N
ACHTUNGTRENNUNG
3J
4J
(H,H)=4.7, 4J
E
ACHTUNGTRENNUNG
3
ACHTUNGTRENNUNG(H,H)=
4.8, 4J
U
signments according to HSQC and HMBC 2D 1H–13C NMR experi-
ments): 40.0 (Cq6), 66.9 (Cq1), 69.9 (Cq2), 70.4 (Cq5), 70.9 (Cq4, Cq3), 103.4
(Cp), 103.8 (Cm), 110.8 (Cn), 118.6 (Cj3), 122.4 (Ce2), 122.8 (Cc2, Cj2), 125.1
(Cs), 127.1 (Co), 127.4 (Ca, Ch), 128.9 (Cb, Cg), 130.1 (CL), 132.0 (CJ),
133.3 (Ce), 133.4 (Cd, Ci), 135.1 (Cc1), 135.4 (Cf), 135.5 (Cc), 139.3 (Ct),
140.8 (Ce3), 146.5 (Cj1), 147.2 (Cc3), 147.5 (Ci1), 147.9 (Ce1), 149.4 (Cs1),
149.6 (Ck), 164.2 ppm (Cs2) (labelling of the hydrogen and carbon atoms
is given in the Supporting Information); UV/VIS (CH2Cl2): lmax (e)=427
(390000), 561 (15000), 600 nm (6000 molÀ1 m3 cmÀ1); MALDI-TOF: m/z
(%): 1345.61 (33) [MÀH]+, 958.32 (53) [19ÀSnÀC14H10O3]+, 733.24 (14)
[19ÀSn]+.
Crystal data for 3: C78H72Cl8N8O10Sn; Mr =1683.73; purple crystal; 0.05ꢃ
¯
0.05ꢃ0.02 mm; triclinic; space group P1; a=9.8866(5), b=14.8757(8), c=
27.7531(14) ꢂ; a=90.905(3), b=95.079(3), g=108.221(3)8; V=
3857.7(3) ꢂ3; T=173(2) K; Z=2; 1calcd =1.450 gcmÀ3
; ;
m=0.672 mmÀ1
X-ray crystal-structure analyses: Data were collected on
a Bruker
28114 collected reflections, 15911 independent (RACHTNUTRGNEUNG(int)=0.0453), GooF=
SMART CCD diffractometer with MoKa radiation. The structures were
solved by using the SHELXS-97 program and refined by full-matrix least
squares on F2 using SHELXL-97 with anisotropic thermal parameters for
all non-hydrogen atoms. The hydrogen atoms were introduced at calcu-
lated positions and not refined (riding model). CCDC-803619 contains
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
1.167, R1=0.1073, wR2=0.2685 for I>2s(I) and R1=0.1458, wR2=
0.2918 for all data.
Synthesis of 3–Pd
Method A: Under argon, a solution of 18–Pd (10.5 mg, 0.013 mmol,
1.1 equiv) in distilled CH2Cl2 (25 mL) was added dropwise, by using a
cannula, to a solution of 21 (9.3 mg, 0.012 mmol, 1 equiv) in distilled
CH2Cl2 (10 mL). The resulting solution was stirred at room temperature
for 4 days. Evolution of the reaction was monitored by 1H NMR spectros-
copy. After removal of the solvent, a purple powder was obtained
(16.0 mg, 90%).
Synthesis of 2: Compound 21 (70 mg, 0.091 mmol, 1 equiv) was added as
a solid to a solution of 17 (51 mg, 0.097 mmol, 1.1 equiv) in CH2Cl2
(160 mL). The resulting solution was stirred at room temperature for
7 days. After removal of the solvent, the residue was recrystallised from
CH2Cl2/pentane (7 mL/50 mL) to afford a deep red powder (80 mg,
70%). 1H NMR (500 MHz, CD2Cl2) (assignments according to COSY
Method B: Et3N (25 mL, 0.182 mmol, 25 equiv) was added to a solution
of 3 (10 mg, 7.44ꢃ10À3 mmol, 1 equiv) in distilled CH2Cl2 (10 mL). A so-
(PF6)2 (4.2 mg, 7.44ꢃ10À3 mmol, 1 equiv) in dry
lution of [PdACHTNUTRGENNUG(CH3CN)4]ACHUTNGTRENNGUN
1
3
and ROESY 2D H–1H NMR experiments): d=1.17 (t, J
2H; Hm), 1.52 (ddd, 3J(H,H)=8.0, 4J(H,H)=2.1, 4J
(H,H)=0.9 Hz, 2H;
Hn), 3.09–3.15 (m, 4H; Hq1), 3.46–3.51 (m, 4H; Hq2), 3.62–3.66 (m, 4H;
q3), 3.67–3.73 (m, 4H; Hq4), 5.36 (ddd, 3J(H,H)=8.0, 4J
(H,H)=2.5,
(H,H)=0.9 Hz, 2H; Hp), 5.53 (t, 3J
(H,H)=8.0 Hz, 2H; Ho), 7.77–7.85
(m, 9H; Ha, Hb, Hg, Hh), 8.13–8.21 (m, 3H; HL, Hr, Ht), 8.21–8.27 (m,
3H; Hc and Hf), 8.45 (d, 3J(H,H)=7.9 Hz, 2H; Hs), 9.07 (dd, 3J
(H,H)=
4.2, 4J(H,H)=1.6 Hz, 2H; Hk), 9.10 (d, 3J
(H,H)=4.7 Hz, 2H; HJ), 9.17
(s, 4H; He, Hd), 9.18 ppm (d, 3J(H,H)=4.8 Hz, 2H; Hi); 13C NMR
ACHTUNGTREN(NUNG H,H)=2.3 Hz,
CH3CN (10 mL) was added dropwise to the mixture. The resulting mix-
ture was stirred at room temperature for 3 h. No colour change was ob-
served. The solvent was evaporated and the crude product was dissolved
in CH2Cl2 (2 mL) and precipitated upon addition of pentane (10 mL).
The violet powder was further recrystallised from CH2Cl2/pentane (3 mL/
15 mL) to afford the desired compound 3–Pd as a purple powder (9.2 mg,
85%). 1H NMR (500 MHz, CD2Cl2) (assignments according to COSY
G
E
ACHTUNGTRENNUNG
H
G
R
ACHTUNGTRENNUNG
4J
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
and ROESY 2D 1H-1H NMR experiments): d=1.15 (t, 4J
2H; Hm), 1.48 (dd, 3J(H,H)=7.7, 4J
(H,H)=1.7 Hz, 2H; Hn), 2.66 (m,
4H; Hq6), 3.28 (m, 4H; Hq1), 3.61 (m, 4H; Hq2), 3.68 (m, 4H; Hq3), 3.76
(m, 4H; Hq4), 3.97 (m, 4H; Hq5), 5.31 (m, 2H; Hp), 5.53 (t, 3J
(H,H)=
8.0 Hz, 2H; Ho), 7.68 (d, 3J
(H,H)=7.5 Hz, 2H; Hs), 7.80–7.89 (m, 9H;
Ha, Hb, Hg, Hh), 8.02 (t, 3J
(H,H)=7.7 Hz, 1H; Ht), 8.24 (m, 4H; Hf),
8.40–8.43 (m, 4H; HL, Hc), 8.89 (d, J ACTHNUTRGNEUNG(Sn,H)=17.0 Hz, 2H;
ACHTUNGTREN(NUNG H,H)=2.5 Hz,
G
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(125 MHz, CD2Cl2) (assignments according to HSQC and HMBC 2D
1H–13C NMR experiments): d=39.8 (Cq4), 65.9 (Cq1), 69.6 (Cq2), 69.8
(Cq3), 102.8 (Cm), 103.7 (Cp), 110.2 (Cn), 125.2 (Cs), 126.6 (Co), 127.0 (Ca,
Ch), 128.6 (Cb, Cg), 129.8 (CL), 131.7 (CJ), 132.8 (Cd, Ce), 139.1 (Ct), 140.3
(Cc1, Ce3), 146.2 (CJ1), 148.3 (CJ3), 148.7 (Ck), 155.6 (Cm1), 156.5 (Cm2),
163.4 ppm (Cs2) (labelling of the hydrogen and carbon atoms is given in
the Supporting Information); UV/VIS (CH2Cl2): lmax (e)=425 (330000),
561 (25000), 600 nm (14000 molÀ1 m3 cmÀ1); MALDI-TOF: m/z (%):
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
3
4
U
HJ), 9.12 (d, 3J
(H,H)=4.5, 4J
E
3J
4J
A
U
ACHTUNGTRENNUNG
A
A
ACHTUNGTRENNUNG
6450
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 6443 – 6452

Porphyrin-Based Switchable Molecular Turnstiles
FULL PAPER
1.5 Hz, 2H; Hk); 13C NMR (125 MHz, CD2Cl2) (assignments according to
HSQC and HMBC 2D 1H-13C NMR experiments): d=47.3 (Cq6), 66.5
(Cq1), 69.6 (Cq4), 70.0 (Cq2), 70.8 (Cq3), 70.9 (Cq5), 101.9 (Cp), 103.9 (Cm),
110.7 (Cn), 117.0 (Cc2), 123.9 (Cs), 126.4 (Co), 127.1 (Ca, Ch), 127.5 (Ce2),
128.5 (Cb, Cg), 128.9 (Cc1), 130.9 (CJ), 131.2 (CL), 132.9 (Ce), 133.3 (Cd),
133.4 (Ci), 133.6 (CJ2), 134.9 (Cf), 135.2 (Cc), 140.6 (Ct), 140.7 (Ce3), 146.4
(CJ1), 147.2 (Cc3), 147.3 (Ci1), 148.0 (Ce1), 151.9 (CJ3), 152.2 (Ck), 153.1
(Cs1), 156.0 (Cm1), 157.2 ppm (Cm2) (labelling of the hydrogen and carbon
atoms is given in the Supporting Information); UV/VIS (CH2Cl2): lmax
(e)=426 (290000), 561 (19000), 601 nm (12000 molÀ1 m3 cmÀ1); MALDI-
TOF: m/z (%): 1449.26 (38) [MÀH]+, 958.17 (40) [19ÀSnÀC14H10O3]+,
768.08 (22) [21ÀH]+.
Addition of cyanide salts: Different amounts of solid Bu4NCN (1, 2 or
10 equiv) were added to a solution of 3–Pd (5 mg, 3.45ꢃ10À3 mmol,
1 equiv) in CD2Cl2 (0.6 mL). The resulting mixture was stirred for a few
minutes at room temperature and transferred into an NMR tube.
1H NMR spectra were recorded at different reaction times to ensure that
the reaction was complete.
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The Universitꢀ de Strasbourg, the International Centre for Frontier Re-
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