Porphyrin based molecular turnstiles

Article abstract of DOI:10.1039/b927112k

Molecular turnstiles, based on a hinge composed of a Sn-porphyrin bearing coordinating sites at the meso positions and a handle, equipped with a tridentate coordinating pole or its Pd(ii) complex, connected to the porphyrin through Sn-O bonds, offering open (free rotation of the handle around the hinge) and close states (blockage of rotation through binding of Pd(ii)) were designed, prepared and studied both in solution and in the solid state.

Full text of DOI:10.1039/b927112k

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Porphyrin based molecular turnstilesw
´
Thomas Lang, Aurelie Guenet, Ernest Graf,* Nathalie Kyritsakas and Mir Wais Hosseini*
Received (in Cambridge, UK) 23rd December 2009, Accepted 26th February 2010
First published as an Advance Article on the web 18th March 2010
DOI: 10.1039/b927112k
Molecular turnstiles, based on a hinge composed of a Sn–porphyrin
bearing coordinating sites at the meso positions and a handle,
equipped with a tridentate coordinating pole or its Pd(II) complex,
connected to the porphyrin through Sn–O bonds, offering open
(free rotation of the handle around the hinge) and close states
(blockage of rotation through binding of Pd(II)) were designed,
prepared and studied both in solution and in the solid state.
Controlling movements in molecular systems is a topic of current
interest. Molecular machines for which the amplitude as well as
the direction of motion are imposed by external stimuli have been
attracting considerable interest over the past years.1–7 Among
many systems reported, molecular turnstiles,8–11 by essence
subject to rotational movement, are interesting objects as rather
primitive prototypes for the design of molecular motors and
Fig.
2 Representation of the molecular turnstile in the open
rotors. We have previously reported a molecular gate which may
also be regarded as a molecular turnstile.12
(compound 1) and close (compound 2) states and the assignment of
H atoms based on 2-D NMR studies.
Here, we report on the design, synthesis and structural
characterisation of molecular turnstiles composed of
a
Sn(IV) centre coordinated by the tetraaza core of the porphyrin
offers two apical positions to be used as connecting points
to attach the handle through Sn–O bonds (compound 1).
The handle, a symmetrical fragment, is based on a central
tridentate coordinating pole or its Pd(II) complex connected to
two –CH2CH2OCH2CH2OCH2CH2– units each bearing a
resorcinate type terminus (Fig. 2).
porphyrin based hinge bearing a monodentate coordinating
site and a handle equipped with a tridentate coordinating pole
or its Pd complex.
The design of turnstiles reported here is based on a stator
and a handle (Fig. 1). More precisely, the stator is composed
of a Sn(IV)–porphyrin bearing two anisole and two benzonitrile
groups in trans disposition (Fig. 2). The presence of two
benzonitriles as monodentate sites instead of one is for
synthetic reasons. The introduction of Sn(IV), a strong Lewis
acid and an oxophilic centre adopting octahedral coordination
geometry, within the porphyrin core is essential since it allows
the porphyrin based stator to behave as a hinge. Indeed, the
Compound 1 may be qualified as a turnstile because, in the
absence of metal, the handle, owing to its length and curvature,
freely rotates around the hinge (open state, Fig. 1a). The
turnstile 1 may be considered as a dynamic ligand offering a
monodentate site on the hinge and a tridentate pole on the
handle. In the presence of a metal centre such as Pd(II)
adopting square planar coordination geometry, the two types
of coordinating sites should simultaneously bind the metal
leading to compound 2. Thus, the binding event stops the
rotational movement (close state, Fig. 1b).
The synthetic strategy (see ESIw) adopted for the preparation
of 1 and 2 was based on the coupling reaction between handles
14 or 15 and the dihydroxo Sn–porphyrin 21 respectively
(Scheme 1). The starting materials for the synthesis of the
handle were monochloro triethyleneglycol 3, pyridine dicarboxylic
Fig. 1 Schematic representation of the open (a) and close (b) states of
a molecular turnstile composed of a hinge bearing a monodentate
coordination site (green) and a handle equipped with a tridentate
coordination pole (purple). The yellow sphere represents a metal
adopting the square planar coordination geometry. (c) represents the
opening of 2 upon addition of a competing ligand.
acid
Compound 3 was first converted in 80% yield into the
phthalimido derivative upon treatment by potassium
6 and the monoester derivative of resorcinol 8.
4
phthalimidate in DMF at 120 1C. The ethanolysis in the
presence of hydrazine of 4 afforded the monoamino derivative
5 in 64% yield. On the other hand, the diacid compound 6 was
converted to its acyl chloride derivative 7 in 96% yield upon
treatment with SOCl2. The monoester derivative of resorcinol
8 was first protected in 74% yield (compound 9) upon reaction
Laboratoire de Chimie de Coordination Organique (UMR 7140),
´
Universite de Strasbourg, Institut Le Bel, 4 rue Blaise Pascal,
67000 Strasbourg, France. E-mail: hosseini@unistra.fr
w Electronic supplementary information (ESI) available: Synthetic
procedures. CCDC 759474. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b927112k
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Fig. 3 Portions of the 1H-NMR spectra (500 MHz, CD2Cl2, 25 1C)
and assignment of signals between 7.3–9.5 ppm for 1 (bottom) and 2
(top). For assignment of H atoms see Fig. 2. %, K andEcorrespond,
respectively, to 14 and 20 resulting from the hydrolysis of 1 or 2 and to
the unreacted 21.
Fig. 4 Portions of the 2-D ROESY correlations for 1 (left) and 2
Scheme 1
(right). For attribution of H-atoms see Fig. 2.
with DHP in EtOAc at 25 1C and then saponified upon
treatment with a THF–H2O mixture in the presence of NaOH
affording thus the compound 10 in 97% yield. The condensation
of the amino derivative 5 with 7 in CH2Cl2 afforded the
compound 11 in 75% yield. The latter was activated into its
dimesylate derivative 12 in 74% yield. The condensation of 10
with 12 in CH3CN in the presence of Cs2CO3 afforded the
protected handle 13 in 91% yield which was deprotected under
acidic conditions (37% HCl in MeOH) into the diol
compound 14 in 94% yield. The handle 15 was obtained in
80% yield upon treatment of the diol 14 with Pd(OAc)2 in
CH3CN. For the synthesis of the porphyrin moiety, the two
starting materials were the aldehydes 16 and 17. Treatment of
16 with pyrrole in the presence of TFA afforded the dipyrro-
methane derivative 18 in 63% yield. The condensation of the
latter with the aldehyde 17 in a mixture of CH2Cl2 and EtOH
in the presence of TFA followed by oxidation using DDQ in
THF in the presence of Et3N afforded the desired porphyrin 19
in 12% yield. The formation of the dichloro Sn–porphyrin
derivative 20 was achieved in quantitative yield upon treatment
of 19 with SnCl2ꢁ2H2O in pyridine. The conversion of the
dichloro compound 20 into the dihydroxo complex 21 was
achieved in 83% yield using a solution of K2CO3 in CH2Cl2/
MeOH (4 : 1). Finally, the condensation at room temperature
of the metalloporphyrin 21 with either 14 or 15 in chloroform
afforded compounds 1 and 2 in 91 and 90% yield respectively.
bottom) and b-pyrrolic hydrogen atoms (Hc and Hd, Fig. 3
bottom).
Furthermore, the ROESY sequence at room temperature
only showed the expected intra hinge (Hf and Hg) and intra
handle (Hp, Hq and Ht) through space correlations (Fig. 4
left). This absence of correlation between the H atoms of the
handle and those of the hinge is in favour of the rotation of the
handle around the hinge.
The blockage of the rotational movement of the turnstile 1
through binding of Pd(II) was also investigated in solution by
2-D NMR studies. As in the case of compound 1, for the
palladium complex 2 all H atoms were assigned by COSY and
ROESY sequences (Fig. 2). The 1H-NMR investigation clearly
demonstrated that the symmetry of compound 2 was lower
(Fig. 3 top). Indeed, as expected, the number and multiplicity
of signals corresponding to the anisole moiety (He and Hf)
indicated that the two units were equivalent. In marked
contrast, the number of signals corresponding to the two
0
0
benzonitrile groups was doubled (Ha, Ha , Hb and Hb )
indicating the non-equivalence of the two units. Furthermore,
the number of signals corresponding to the b-pyrrolic H atoms
0
0
has been also multiplied by two (Hc, Hc , Hd and Hd ).
In order to prove that the monodentate benzonitrile group
on the hinge and the tridentate site on the handle bind
simultaneously Pd(II), ROESY experiments were performed
on compound 2 (Fig. 4 right). As stated above, whereas for 1
no correlation could be observed between the hinge and the
handle, for 2, owing to the spatial proximity effects, several
through space correlations were detected (Fig. 5). The most
In solution, the structure and dynamics of
1 were
investigated in CD2Cl2 at 25 1C by 2-D NMR experiments
(COSY and ROESY) which allowed to assign all hydrogen
atoms signals (Fig. 2). The free rotation of the handle around
the O–Sn–O axis at room temperature was evidenced by the
0
relevant ones are between Ha and Hp and Hq and between Hb
0
and Hh and Hl. Finally, the result in solution by NMR was
further confirmed by observation of a peak at 1558.29 by mass
spectrometry (ESI-MS, M + H+).
1
observation of a C2v symmetry by H-NMR. Indeed, a single
set of signals corresponding to the H atoms of the two
benzonitrile units (Ha and Hb, Fig. 3 bottom) was present.
The same also holds for the anisole moiety (He, Hf, Fig. 3
Owing to the high affinity of the dianionic tridentate
coordinating site on the handle and the fact that the SnO
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Pd(II) is tetracoordinated and adopts a distorted square planar
coordination geometry (NPdN angle in the range of
80.51–99.91 (cis) and 162.0–176.71 (trans)). Its coordination
sphere is composed of one N atom of one of the two
benzonitrile groups of the hinge (dCN–Pd = 2.003 A) and three
N atoms belonging to the tridentate coordinating pole of the
handle (dPd–N = 2.040 A and 2.051 A and dPd–N(py) = 1.917 A).
In conclusion, compound 1 bearing two benzonitrile and
two anisole groups on the hinge and a tridentate unit on the
handle behaves as a molecular turnstile, i.e. free rotation of the
handle around the hinge. The rotation may be blocked upon
coordination of Pd(II) by the tridentate site on the handle and
the monodentate sites on the hinge. The system may be
unlocked upon addition of DMAP as competing ligand. The
Pd complex 2 in its turn behaves as a turnstile bearing a stator
and a rotor based on the Pd complex. Both systems may be
seen as models of switch possessing two states (open and
close). It seems interesting to design a turnstile offering one
open and several close states. Work along these lines is
currently under progress.
Fig. 5 Representation of the through space correlations (dotted lines)
in 2 observed by ROESY experiments between the hinge and the
handle.
bond is unstable in acidic media,12b the opening of the system
was achieved using p-dimethylaminopyridine (DMAP) which
is a much stronger ligand13 than the benzonitrile moiety.
Indeed, the addition of 1 eq. of DMAP to a CD2Cl2 solution
of 2 was monitored by 1H-NMR which revealed that: (i) h and
i signals were still shifted upfield implying that the handle was
coordinated to the porphyrin; (ii) r and s signals remained
unchanged indicating that Pd(II) centre was still coordinated to
the handle; (iii) the splitting of protons a, b, c and d was
vanished implying a higher symmetry due to rotation resulting
from the opening of the system; (iv) the two protons at the
ortho positions of the pyridine moiety were downfield shifted
indicating the binding of DMAP to Pd(II). Finally, ROESY
studies at 25 1C revealed the disappearance of through space
correlations between the handle and the porphyrin observed
for 2 and the absence of new correlations. The above
mentioned observations demonstrate that the addition of
DMAP opens the system and restores the rotation (Fig. 1c).
The crystalline structure (Fig. 6) of 2 was determined
by X-ray diffraction on single-crystal obtained upon slow
diffusion of pentane into a CH2Cl2 solution of 2.z
We thank the Universite de 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.
Notes and references
z Crystallographic data for 2: C80H75Cl2N9O14PdSn, M = 1682.48,
ꢀ
triclinic, P1, a = 11.2051(7) A, b = 15.7990(10) A, c = 22.5004(15) A,
a = 99.048(2)1, b = 102.999(2)1, g = 100.826(2)1, U = 3728.3(4) A3,
Z = 2, m = 0.720 mmꢂ1, refls measured : 41 064, independent refls:
16 696 [R(int) = 0.0811], final R indices [I 4 2s(I)]: R1 = 0.0895,
wR2 = 0.2368, R indices (all data): R1 = 0.1502, wR2 = 0.2707, GOF
on F2: 1.059.
ꢀ
The crystal (triclinic, P1) is composed of the compound 2, 2
CH2Cl2 and 4 H2O molecules. The Sn atom is located almost
at the centre of the four pyrrolic units with Sn–N distance in
the range of 2.083–2.109 A and NSnN angle in the range of
89.01–90.81 (cis), 176.81–177.21 (trans). The Sn(IV) cation is
hexacoordinated by four N atoms of the porphyrin and two O
atoms of the two resorcinol groups (dSn–O = 2.039 and
2.050 A). The coordination geometry is a distorted octahedral
with an OSnO angle of 175.31 and OSnN angles in the range
83.9 to 93.31. The two differentiated benzonitrile groups (dCN
of 1.128 A and 1.138 A for the free and bound nitrile groups
respectively) are tilted with respect to the porphyrin plane
(CCCC dihedral angles ꢂ55.21 and 58.81 for PhCN–Pd and
PhCN respectively). For the anisole groups the same trend is
observed (CCCC dihedral angles of ꢂ66.61 and 73.21). The
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Fig. 6 X-Ray structure of the neutral turnstile (compound 2) in the
close position through the binding of Pd(II) by one of the two CN
groups belonging to the hinge and the tridentate site of the handle. For
sake of clarity the C atoms of the porphyrin ring and the handle are
coloured in yellow and blue respectively. H atoms are omitted for
clarity. For bond distances and angles see text.
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3510 | Chem. Commun., 2010, 46, 3508–3510