Threading the calix[5]arene annulus

Article abstract of DOI:10.1002/chem.200902945

Slowly does it! Di-n-alkylammonium ions (such as 2 H⁺) thread the annulus of calix[5]arene 1 to yield stable [2] pseudorotaxanes. The ease of formation of this hitherto unknown family of interpenetrated supermolecules is predominantly determine

Full text of DOI:10.1002/chem.200902945

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DOI: 10.1002/chem.200902945
Threading the Calix[5]arene Annulus
Giuseppe Gattuso,[a] Anna Notti,[a] Melchiorre F. Parisi,*[a] Ilenia Pisagatti,[a]
Maria E. Amato,[b] Andrea Pappalardo,[b] and Sebastiano Pappalardo*[b]
Among the fast-expanding collection of molecular systems
available for nanoscale applications, mechanically inter-
locked molecules,[1] such as rotaxanes and catenanes, have
already moved towards becoming technological realities.[2]
Rotaxanes (and their pseudorotaxane precursors), in partic-
ular, have been constructed by using the most diverse mac-
rocyclic receptors as wheel components. Crown ethers,[1,3]
cyclodextrins,[4] cyclic amides,[5] and more recently cucurbi-
turils[6] have all been threaded onto suitable complementary
linear axle components. Calix[n]arenes,[7] on the other hand,
have received much less attention as building blocks for the
construction of interlocked supermolecules,[8] despite their
tunable size, their versatility of derivatization (both at the
wide and narrow rims), and their ready availability. The
only notable exceptions are the findings of Arduini, Pochini,
and co-workers, who have extensively investigated pseudor-
otaxanes and rotaxanes based on heterotopic calix[6]arene
receptors adorned with ureido groups and viologen-derived
linear components.[9]
been carried out to evaluate whether or not a linear thread
can interpenetrate the calix[5]arene annulus. Herein we de-
scribe the first examples of [2]pseudorotaxanes derived from
a calix[5]arene and linear secondary alkylammonium ions
and we show that the ease of formation of these species is
predominantly determined by salt ion-pairing effects, where-
as the time course of the threading/dethreading process de-
pends on the length of the cation alkylammonium chains.
Very recently we have reported the solid-state structure
of a calix[5]arene/n-butylammonium endo-cavity complex.[14]
Inspection of this structure revealed that the spatial arrange-
ment of the oxygen atoms around the nitrogen atom of the
included guest is reminiscent of the oxygen array present in
crown ether/secondary ammonium ion complexes.[15] Even
though the cavity size of a calix[5]arene (at its narrow rim)
appears to be slightly smaller than that found in the solid-
state structure of the dibenzo[24]crown-8 encircling the di-
butylammonium ion (ca. 5 vs. 6 ꢀ,[16] respectively), calix[5]-
arenes were judged to be sufficiently flexible to allow for
the inclusion of secondary alkylammonium cations. After a
preliminary screening, penta-tert-butylpentakis(tert-butoxy-
carbonylmethoxy)calix[5]arene[17] (1) was selected as the
prototype wheel component, whereas di-n-butylammonium
(2·H+) and di-n-hexylammonium (3·H+) were chosen as
axle components. The axle components were all tested as
chloride, picrate (PicÀ), and hexafluorophosphate salts to
evaluate the influence of ion pairing[18] on the pseudorotax-
ane assembly process.
Leaving aside calix[4]arenes,[10] the cavity of which is too
small to be threaded by a linear guest, the slightly larger cal-
ix[5]arenes are the next potential candidates for pseudoro-
taxane formation. Although calix[5]arenes have previously
been shown to efficiently perform a number of tasks, which
range from the complexation of alkyl(di)ammonium ions[11]
and ion pairs[12] to the self-assembly of supramolecular poly-
mers,[13] to the best of our knowledge, no studies have so far
[a] Dr. G. Gattuso, Dr. A. Notti, Prof. M. F. Parisi, Dr. I. Pisagatti
Dipartimento di Chimica Organica e Biologica, Universitꢁ di Messina
Salita Sperone 31, 98166 Messina (Italy)
Fax : (+39) 090-393895
E-mail: mparisi@unime.it
[b] Prof. M. E. Amato, Dr. A. Pappalardo, Prof. S. Pappalardo
Dipartimento di Scienze Chimiche, Universitꢁ di Catania
Viale A. Doria 6, 95125 Catania (Italy)
Fax : (+39) 095-580138
E-mail: spappalardo@unict.it
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902945.
Chem. Eur. J. 2010, 16, 2381 – 2385
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Formation of the pseudorotaxanes was investigated by
1H NMR spectroscopy. In a typical experiment, the appro-
priate amount of dialkylammonium salt was added to a
5 mm solution of 1 in (CDCl2)2, so as to reach a 1:1 host/
guest ratio. Data on the threading experiments (Table 1)
degree of conformational freedom of 3·H+, with respect to
the shorter 2·H+ guest, is responsible for the time-depen-
dent process observed, which sees the former moving slowly
into the cavity of calix[5]arene 1.
Formation of the [2]pseudorotaxane 3·H+ꢀ1 is further
corroborated by the ESIMS spectra (Figure S1 in the Sup-
porting Information) and ROESY experiments. Under iden-
tical ROESY conditions, complexes 2·H+ꢀ1 and 3·H+ꢀ1
show different behavior. The 3·H+ꢀ1 pseudorotaxane gives
rise to ROE correlations among the b–e-methylenes (and
the methyl end group) and the aromatic hydrogen atoms of
the tert-butylphenoxy rings (Figure 2), as well as between
the a-methylene and the axial hydrogen atoms of the bridg-
ing methylene units (Figure S5 in the Supporting Informa-
tion). Conversely, the inclusion complex with the shorter di-
n-butylammonium guest showed only negative cross-peaks
that are indicative of chemical exchange between the includ-
ed and the free guest species (Figure S4 in the Supporting
Information).[21] Assignment of the resonances belonging to
the a- and a’-CH2 groups of pseudorotaxane 3·H+ꢀ1 (d=
2.45 and 3.80 ppm, respectively) followed from a COSY
spectrum. Interestingly, a comparison of the upfield com-
plexation-induced shifts (CISs) experienced by the a-meth-
ylene hydrogen atoms of di-n-hexylammonium 3·H+ and n-
hexylammonium 5·H+ ions, upon inclusion into 1 (Dd=0.61
and 3.89 ppm, respectively), suggests that a di-n-alkylammo-
nium cation is able to penetrate more deeply into the calix-
arene cavity with respect to an n-alkylammonium one
(Table S1 in the Supporting Information). Data for 3·H+ꢀ1
are indeed consistent with the a-CH2 group of the guest sit-
ting at the periphery of the shielding cone of the aromatic
moieties. Threading of 3·H+ into 1 is additionally substanti-
ated by the opposite (downfield) CIS detected on the a’-
CH2 resonance (Dd=0.74 ppm
Table 1. Percentages of formation[a] and conditional binding constants[19a]
of pseudorotaxanes [2·H+ꢀ1]XÀ and [3·H+ꢀ1]XÀ, determined by
1H NMR spectroscopy (500 MHz, 258C, 5 mm, (CDCl2)2).
XÀ
2·H+ꢀ1 [%] ([mÀ1])
3·H+ꢀ1[c] [%]
([mÀ1])
ACHTUNGTRENNUNG
À[b]
PF6
88 (1.2ꢃ104)
57 (6.1ꢃ102)
<5 (<10)
69 (1.4ꢃ103)
39 (2.1ꢃ102)
<5 (<10)
PicÀ
ClÀ
[a] Values derived from the average of three independent measurements.
Standard error ꢁ10%. [b] Samples prepared as solutions in (CDCl2)2/
CD3OD (9:1, v/v), evaporated to dryness and then taken up in (CDCl2)2
(see the Supporting Information). [c] Equilibration time 24 h.
show different behavior of the two cationic guests, along
with a strong dependence on the ion-pairing tendency of the
salts used.[19] Addition of 2·H+ or 3·H+ to calix[5]arene 1 re-
sulted in the formation of slow exchanging (on the NMR
timescale), endo-cavity inclusion complexes, unambiguously
confirmed by the appearance of high-field resonances (d=
À1.10 to À2.65 and À0.14 to À2.50 ppm, respectively), for
the methylene hydrogen atoms of the included dialkylam-
monium guest experiencing the shielding effects of the calix-
arene aromatic units (Figure 1).
Threading of 3·H+, at room temperature, was found to
reach equilibrium after 24 h from the addition (Figure S9 in
the Supporting Information),[20] whereas 2·H+, was able to
thread 1 within a few minutes. Most likely, the higher
with respect to the free guest),
which indicates that the second
n-alkyl chain of the guest (the
exo-cavity one, residing among
the narrow rim substituents) is
placed in the deshielding region
of the aromatic rings.
Additional evidence on the
above-mentioned NMR spec-
troscopy structural assignment
was provided by the compari-
son of the calculated equilibri-
um geometries[22] of pseudoro-
taxane 3·H+ꢀ1 and the model
complex 5·H+ꢀ1 obtained from
1 and 5·H+ (Figure 3). DFT cal-
culations at the B3LYP/6-
31G(d) level of theory showed
that in 5·H+ꢀ1[23] the calix[5]ar-
ene adopts a Cs symmetric con-
formation (Figures S13 and S14
Figure 1. 1H NMR Spectra (500 MHz, 258C, 5 mm, (CDCl2)2) of a) 1; b) [2·H+ꢀ1]PicÀ; c) [3·H+ꢀ1]PicÀ. *Re-
in the Supporting Information),
with one of the five aryl groups
sidual solvent peak.
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Chem. Eur. J. 2010, 16, 2381 – 2385

Calix[5]arene Threading
COMMUNICATION
Figure 3. Details of the calculated structures of 5·H+ꢀ1 (top) and 3·H+
ꢀ1 (bottom), highlighting the penetration of (di)alkylammonium ions
into the cavity. The top and bottom lines represent the mean planes gen-
erated by the bridging methylene carbon atoms and the phenolic oxygen
atoms, respectively. The oxygen atoms of the calixarene and the nitrogen
atom of the guests are shown in ball style. Wide and narrow rim substitu-
ents not involved in complexation are omitted for clarity.
Figure 2. Section of the ROESY spectrum of [3·H+ꢀ1]PicÀ (500 MHz,
258C, 5 mm, (CDCl2)2).
2·HPF6, which resulted in 88% complex formation (Ka =
1.2ꢃ104 mÀ1 in (CDCl2)2).
tilted outwards to allow its pendant carbonyl group to hy-
drogen bond to the ammonium ion. In the pseudorotaxane
3·H+ꢀ1, on the other hand, calix[5]arene 1 is forced to
stand in a more regular C5v conformation (Figures S13 and
S14 in the Supporting Information), as a result of the pres-
ence of the second hexyl group filling the pocket created by
Dethreading of the [2]pseudorotaxane 3·H+ꢀ1 was ach-
ieved by applying a number of chemical stimuli of a differ-
ent nature, acting either on the host (through addition of a
competitive guest), on the guest (through a base) or on the
counterion (through a tighter ion-pairing anion). In all in-
stances, dethreading was found to proceed with slow kinet-
ics, on a timescale similar to (or even slower than) that ob-
served for the formation of the pseudorotaxane (Figure 4).
This suggests that the rate-limiting event is the dethreading
the narrow rim ester substituents, thus preventing the forma-
+
À
tion of the additional C=O···H N bond seen with 5·H . Fur-
thermore, the NH3 group of 5·H+ lies above the mean
+
plane generated by the phenolic oxygen atoms, whereas the
secondary ammonium moiety of 3·H+ is coplanar to the
same plane, mirroring the behavior of ammonium ions/
crown ether complexes.[15] It is also worth noting that, when
compared with 5·H+ꢀ1, upon inclusion of 3·H+ the narrow
rim of the cavity of 1 opens up from approximately 5 to
6 ꢀ.[24]
The counterion chosen for the dialkylammonium compo-
nents was found to play a significant role on the effective-
ness of the threading process (Table 1). As predicted by the-
ory,[13b,18,19] ion-pairing effects hamper the complexation of a
À
charged species by a neutral receptor. On going from PF6 ,
to PicÀ, and finally to ClÀ, a progressive decrease of the per-
centages of complexation was observed (from 69–88% to
<5%, respectively), with a consequent drop of the condi-
tional binding constant. Overall, efficiency was highest for
Figure 4. Time course of threading/dethreading for the [2]pseudorotaxane
3·H+ꢀ1 determined by 1H NMR (500 MHz, 258C, (CDCl2)2). a) Forma-
tion of [3·H+ꢀ1]PicÀ; dethreading of [3·H+ꢀ1]PicÀ upon addition of
b) nBu4N+ClÀ; c) Et3N; d) nBuNH3+PicÀ (4·HPic).
Chem. Eur. J. 2010, 16, 2381 – 2385
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M. F. Parisi, S. Pappalardo et al.
of the guest molecule and not any of the rapid chemical
transformations the cation undergoes upon release from the
calixarene (i.e., recombination of the original salt, deproto-
nation, or formation of a tighter ion-paired salt).
Acknowledgements
The authors are grateful to Dr. D. Garozzo and A. Messina (CNR-ICTP,
Catania, Italy) for ESIMS spectra. MiUR is gratefully acknowledged for
financial support of this research.
Typically, addition of 10 equiv of nBu4N+ClÀ[25] to a 5 mm
solution of the pseudorotaxane [3·H+ꢀ1]PicÀ in (CDCl2)2
results in complete decomplexation after 24 h (Figure S11 in
the Supporting Information), as a consequence of the forma-
tion of a tighter ion pair between 3·H+ and the chloride
ion.[26] Similarly, the addition of 10 equiv of triethylamine to
a solution of [3·H+ꢀ1]PicÀ produces deprotonation of the
thread with consequent disassembly of the pseudorotaxane,
although, in this case, the system was found to retain the
presence of a minute amount (<3%) of the initial pseudor-
otaxane after equilibration for 4 days (Figure S10 in the
Supporting Information). Lastly, the addition of 10 equiv of
solid nBuNH3+PicÀ (4·HPic) results in the complete replace-
ment of the dialkylammonium guest over a 3 day period
(Figure S12 in the Supporting Information). Dethreading
was also carried out by physical methods: heating of a solu-
tion of 3·H+ꢀ1 in (CDCl2)2 causes the progressive extrusion
of the di-n-hexylammonium thread from the calix[5]arene
cavity, reaching complete decomplexation at 1008C (Fig-
ure S8 in the Supporting Information).
Keywords: calixarenes · density functional calculations · ion
pairs · NMR spectroscopy · pseudorotaxanes
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Experimental Section
Calix[5]arene 1 was synthesized according to a literature procedure.[17]
Dialkylammonium salts (2·HX and 3·HX) were conveniently prepared by
treating a solution of the amine (2 or 3) in MeOH with an equimolar
aqueous solution of the appropriate acid (HCl, HPic, or HPF6). Routine-
ly, residues obtained after solvent removal under reduced pressure were
triturated with Et2O to afford solid materials, which were then collected
by suction filtration.
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Calix[5]arene Threading
COMMUNICATION
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[16] Upon encircling the dibutylammonium guest, dibenzo[24]crown-8
assumes a pinched elliptical conformation. Cavity size was estimated
to be around 6 ꢀ by taking into consideration the narrowest portion
of the macrocycle. See reference [15].
[21] ROESY experiments carried out on the inclusion complexes of cal-
ixarene 1 with primary alkylammonium guests (n-butylammonium
4·H+ and n-hexylammonium 5·H+), showed a behavior similar to
3·H+ꢀ1 (Figures S6 and S7 in the Supporting Information).
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[24] Calculated complexation energies are also in agreement with the ob-
served affinity of calix[5]arene 1 for primary over secondary alky-
lammonium cations (5·H+ vs. 3·H+). See the Supporting Informa-
tion for a more detailed discussion.
[25] Tetrabutylammonium ions are too bulky to be included inside the
cavity of a calix[5]arene.
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[20] Heating of the solution did not result in an increase of pseudorotax-
ane yield, indicating that threading does not occur according to the
classical slippage mechanism. See reference [1a].
Received: October 23, 2009
Published online: January 28, 2010
Chem. Eur. J. 2010, 16, 2381 – 2385
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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