A [2]catenane displaying pirouetting motion triggered by debenzylation and locked by chloride anion recognition

Article abstract of DOI:10.1002/chem.201101033

Chloride locks the rings: Debenzylation of a chloride-templated N-benzyl pyridinium catenane, allows for a 180° "pirouetting" of the rings in the resulting neutral pyridyl catenane (see scheme). The catenane may be returned to its original co-conformation by chloride recognition as evidenced in solution and in the solid state. Copyright

Full text of DOI:10.1002/chem.201101033

DOI: 10.1002/chem.201101033
A [2]Catenane Displaying Pirouetting Motion Triggered by Debenzylation
and Locked by Chloride Anion Recognition
Nicholas H. Evans, Christopher J. Serpell, and Paul D. Beer*[a]
A primary incentive for the preparation of interlocked
molecules, such as catenanes and rotaxanes, has been the
potential for such species to exhibit relative motion of their
constituent parts.[1] “Pirouetting” of the macrocycles was ob-
served in Sauvageꢀs seminal copper(I)-templated [2]cat-
enane upon removal of the cationic template.[2] Control of
this particular type of molecular motion has since been in-
duced in appropriately designed [2]catenanes by, for exam-
ple, electrochemical[3,4] and chemical means,[4] and in a
highly ingenious [3]catenane by a combination of light, heat
and chemical stimuli.[5] There are few examples of anion-in-
duced molecular motion of interlocked structures,[6–14] and to
the best of our knowledge has only been reported twice in
catenane systems.[8,13]
are able to access an alternative co-conformation, by 1808
rotation, as revealed by solution-phase 1H NMR experi-
ments. The catenane may be returned to the original co-con-
formation by the addition of chloride, which binds within a
convergent hydrogen bond cavity of the catenane, as dem-
onstrated by 1H NMR experiments in solution and the deter-
mination of a solid-state structure.
The preparation of N-benzyl pyridinium catenane 3-Cl is
detailed in Scheme 1. Equimolar amounts of bis-vinyl-ap-
pended precursor 1-Cl[15] and macrocycle 2[16] were dissolved
in dry CH2Cl2, allowing for the self-assembly of pseudo-
In this communication we report the formation of a novel
neutral catenane prepared by debenzylation of a chloride-
templated N-benzyl pyridinium catenane (Figure 1). Upon
formation of the neutral species, the rings of the catenane
Figure 1. Schematic representation of the N-benzyl pyridinium and pyrid-
yl catenanes and their co-conformations.
[a] N. H. Evans, Dr. C. J. Serpell, Prof. P. D. Beer
Inorganic Chemistry Laboratory
Department of Chemistry, University of Oxford
South Parks Road, Oxford. OX1 3QR (UK)
Fax : (+44)1865-272-690
E-mail: paul.beer@chem.ox.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101033.
Scheme 1. Synthesis of N-benzyl pyridinium catenane 3-Cl.
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COMMUNICATION
ACHTUNGTRENNUNGrotaxane 1-Cl·2, templated by a chloride anion and favoura-
ble p–p stacking interactions. Addition of Grubbsꢀ second-
generation RCM catalyst (10% by wt) afforded a crude re-
action product, which upon being subjected to preparative
silica-gel thin-layer chromatography (silica gel prep TLC)
gave catenane 3-Cl in 18% isolated yield. Catenane 3-Cl
1
was characterised by H and 13C NMR spectroscopy and HR
mass spectrometry (see Supporting Information). In the
1H NMR spectrum of the catenane, the protons a and c are
inequivalent and the hydroquinone resonances of protons f
and g are asymmetrically split. These observations are at-
tributed to the presence of the N-benzyl pyridinium group.
Evidence of the chloride anion templating interaction is
found in the upfield shifts of the pyridinium cavity protons b
and c and the concomitant downfield shifts of isophthal-
ACHTUNGTRENNUNGamide cavity protons n and o compared to those of macrocy-
cle 2 and precursor 1-Cl. The p–p stacking interaction be-
tween the electron-rich hydroquinone rings of the isophthal-
ACHTUNGTRENNUNGamide macrocycle and the pyridinium motif is inferred by
Scheme 2. Synthesis (and co-conformations) of neutral pyridyl catenane
4.
an upfield shift and split of protons r and s. There is also a
substantial downfield shift of the benzyl CH2 protons indica-
tive of hydrogen bonding of these protons to the polyether
oxygen atoms of the isophthalamide macrocycle. The co-
conformation of the catenane (as drawn in Scheme 1) is fur-
ther established by the observation of diagnostic through-
space interactions between appropriate protons from the
two different macrocyclic components in the 1H ROESY
NMR spectrum (see Supporting Information).
Crystals suitable for single-crystal X-ray diffraction struc-
tural analysis of 3-Cl were grown by the diffusion of diiso-
propylether into a solution of the catenane in chloroform.
As well as confirming the co-conformation of the catenane,
the solved structure reveals the interactions inferred from
the 1H NMR spectrum: multiple hydrogen bonds to the
chloride anion template, as well as intercomponent p–p
stacking and hydrogen bonding (Figure 2).
1
which was characterised by H and 13C NMR spectroscopy
1
and HR mass spectrometry. Inspection of the H NMR spec-
trum of the catenane in CDCl3 (see Supporting Information)
reveals loss of the benzyl protons is accompanied by proto-
ns a and c becoming equivalent and the hydroquinone reso-
nances of protons f and g becoming a symmetric multiplet.
Hydroquinone protons r and s are no longer split and have
moved downfield, reflecting loss of the positive charge of
the intercalating pyridyl ring.
1
Assignment of the full H NMR spectrum of pyridyl cate-
nane 4 in CDCl3 was achieved by use of 2D H COSY and
1
ROESY NMR experiments. As seen in Figure 3a, the ex-
pected correlations between pyridyl protons a, b and c and
hydroquinone protons r and s of the isophthalamide-con-
taining macrocycle are present in the ROESY spectrum.
However, there are also strong through-space correlations
between pyridyl proton a and isophthalamide protons n and
o, and an apparent lack of through-space interactions be-
tween the isophthalamide protons l, m, n and o and hydro-
quinone protons f and g. This implies that the macrocycles
of the catenane have rotated through 1808, that is, molecular
motion induced by chemical transformation has occurred.
This is further supported by the analysis of the section of
the 1H ROESY NMR spectrum, depicted in Figure 3b, in
which interactions 9, 11 and 13 would not be observed in
the “unrotated” co-conformation. It is proposed that the
driving force for such a rearrangement is the inter-ring hy-
drogen-bonding of the nitrogen lone pair of the pyridyl mac-
rocycle to the isophthalamide cleft of the other macrocy-
cle.[18]
Removal of the benzyl functionality was achieved by stir-
ring a mixture of 3-Cl with excess triphenylphosphine in
acetonitrile
subjected
to
microwave
irradiation
(Scheme 2).[17] Silica gel prep TLC purification led to the
isolation of the neutral pyridyl catenane 4 in a yield of 45%,
The intriguing possibility of switching the co-conforma-
tion of the catenane by chloride-anion recognition was in-
1
vestigated by use of H NMR titration experiments. Initial
investigations carried out in CDCl3, revealed catenane 4
only bound chloride weakly, apparently with little effect on
the relative populations of the co-conformations (see Sup-
porting Information). This is attributed to the strength of
Figure 2. Crystal structure of N-benzyl pyridinium catenane 3-Cl. Only
hydrogen atoms involved in hydrogen-bonding interactions (represented
as dashed lines) are depicted.
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P. D. Beer et al.
Figure 3. Sections of 1H ROESY NMR spectra of neutral pyridyl catenane 4 (solvent: CDCl3, T=293 K) with intercomponent through-space couplings
highlighted. For atom labels see Scheme 1.
the inter-ring pyridyl–isophthalamide hydrogen-bonding in-
teraction and significant ion pairing of TBA+ and ClÀ in this
uncompetitive
solvent
(TBA=tetrabutylammonium).
Hence, investigations were repeated in [D6]acetone.[19] A H
ROESY NMR spectrum of 4 recorded in [D6]acetone con-
tained cross-coupling peaks consistent with both possible co-
conformations being present. However, upon the addition of
TBACl, the cavity protons b, c, n and o all moved signifi-
cantly downfield, with their signals being notably broad and
relatively ill-defined, until one equivalent of halide anion
had been added, whereupon the resonances associated with
these protons sharpened and underwent negligible further
shift (Figures 4 and 5). These observations imply very strong
1:1 binding of chloride (i.e., K>104 mÀ1), with the anion
being bound within a convergent hydrogen-bond array of
the isophthalamide and pyridyl amide motifs of the cate-
nane.[20] Importantly, the 1H ROESY NMR spectrum re-
corded in the presence of excess chloride also shows the
vast majority of unambiguously assignable intercomponent
cross-coupling peaks are consistent with catenane 4 being in
the “unrotated” co-conformation as depicted in Scheme 2
(see Supporting Information).[21]
1
Figure 4. Partial 1H NMR spectra of neutral pyridyl catenane 4 upon the
addition of a) 0, b) 0.2, c) 0.6, d) 1 and e) 2 equivalents of TBACl (sol-
vent: [D6]acetone, T=293 K).
tous determination of a crystal structure of the chloride-
bound complex of catenane 4 as shown in Figure 6. The
crystals suitable for X-ray diffraction structural analysis
were obtained by the diffusion of diisopropyl ether into a
Corroboration for chloride-anion recognition favouring
the “unrotated” co-conformer is provided by the serendipi-
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Supramolecular Chemistry
COMMUNICATION
junction with Johnson-Matthey) and for post-doctoral funding (Ph.D.
Plus). We are grateful to Diamond Light Source for the award of beam-
time on I19 (MT1858) and to the beamline scientists for help and sup-
port. Useful discussions with Dr. Alexandre Leontiev regarding the deal-
kylation of pyridinium salts are acknowledged.
Keywords: anions
·
catenanes
·
molecular devices
·
supramolecular chemistry · template synthesis
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Figure 5. Plot of chemical shift of cavity protons of neutral pyridyl cate-
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Figure 6. Crystal structure of chloride complex of catenane 4 (larger
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solution of the catenane in chloroform. A chloride anion is
bound within a convergent hydrogen-bond array formed by
the isophthalamide and 3,5-bis-amide-substituted pyridyl
motifs, while a sodium counter-cation is to be found coordi-
nating to the polyether oxygen atoms of the isophthalamide
macrocycle and to the pyridyl nitrogen.[22]
In summary, we have prepared a neutral pyridyl catenane
by debenzylation of an N-benzyl pyridinium chloride cate-
nane. Upon formation of the neutral species, the macrocy-
clic rings of the catenane may undergo a 1808 rotation rela-
tive to one another, and the original co-conformation may
be locked by chloride-anion recognition as demonstrated in
[D6]acetone. Further investigations into exploiting this novel
anion modulated pirouetting motion are being pursued in
our laboratories.
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3718–3721; Angew. Chem. Int. Ed. 2001, 40, 3606–3609.
[17] The conditions used for the debenzylation were based on a litera-
ture procedure to dealkylate pyridinium salts using a sealed tube:
J. P. Kutney, R. Greenhouse, Synth. Commun. 1975, 5, 119–124.
1
[18] Repeating the H ROESY NMR experiments on more sensitive ma-
chines (e.g., 500 and 700 MHz fitted with Cryoprobes) revealed
weak correlations between isophthalamide protons l, m, n and o and
hydroquinone protons f and g (see Supporting Information) which
suggests the “unrotated” co-conformer is also present to a very lim-
ited extent in CDCl3 solution.
[19] Despite being a more competitive solvent than CDCl3, stronger
anion binding has been observed in [D6]acetone when using TBA
salts compared to deuterated chlorinated solvents, for example see:
P. D. Beer, M. Shade, Chem. Commun. 1997, 2377–2378.
[20] Of the other resonances which moved during this titration, that of
alkene proton k is of most interest (see Supporting Information).
The observed downfield shift of ꢀ0.2 ppm cannot be due to partici-
pation in an anion-binding event. The neighbouring alkyl proton j
also undergoes a (smaller) downfield shift. These shifts suggest upon
rotation these protons move from being in the vicinity of the poly-
ether portion of the isophthalamide macrocycle to be in proximity
Acknowledgements
N.H.E. wishes to thank the EPSRC for a DTA studentship. C.J.S. also
wishes to thank the EPSRC for a CASE sponsored studentship (in con-
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P. D. Beer et al.
to the isophthalamide group itself, and hence experience the effect
of aromatic ring currents. Further evidence for this may be found in
the 1H ROESY NMR spectrum recorded in the presence of excess
chloride, where new correlations between protons k and isophthala-
mide protons l and m may be observed. Re-inspection of the 1H
ROESY NMR spectra recorded for the titration carried out in
CDCl3 also reveal the appearance of these new correlations in the
presence of excess chloride.
[22] The presence of NaCl is presumed to arise from trace contamination
of the chloroform solution used to grow the crystals. The chloride
anion of 3-Cl would have been removed during the synthesis of cat-
enane 4 as the counter-anion of the triphenylphosphonium benzyl
cation generated by dealkylation of 3-Cl. The impact the sodium
cation (along with anions other than chloride) may have on the co-
conformation of such catenane species is part of ongoing investiga-
tions in our laboratories.
[21] Only three weak interactions indicating the presence of the “rotat-
ed” co-conformation are observed in the 1H ROESY NMR spec-
trum.
Received: April 5, 2011
Published online: May 25, 2011
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Chem. Eur. J. 2011, 17, 7734 – 7738