Self-assembly of a double calix[6]arene pseudorotaxane in oriented channels

Article abstract of DOI:10.1002/chem.200700748

A synthetic study to disclose the more appropriate manner by which two calix[6]arene units could be connected for the construction of an extended tubular structure was undertaken. As a result, a head-to-tail double calix[6]arene having the structure of an

Full text of DOI:10.1002/chem.200700748

DOI: 10.1002/chem.200700748
Self-Assembly of a Double Calix[6]arene Pseudorotaxane in Oriented
Channels
Arturo Arduini,*[a] Alberto Credi,*[b] Giovanni Faimani,[a] Chiara Massera,[c]
Andrea Pochini,[a] Andrea Secchi,[a] Monica Semeraro,[b] Serena Silvi,[b] and
Franco Ugozzoli*[c]
Abstract: A synthetic study to disclose
the more appropriate manner by which
two calix[6]arene units could be con-
nected for the construction of an ex-
tended tubular structure was undertak-
en. As a result, a head-to-tail double
calix[6]arene having the structure of an
oriented nanotube that is about 2.6 nm
long and 1.6 nm wide was prepared
and characterized. This molecule is
able to act as a wheel-type host and
forms a supramolecular complex with
an axle-type molecule, derived from
4,4’-bipyridinium (viologen), through
very efficient self-assembly in solution.
The properties of such a pseudorotax-
ane-type complex, which is stabilized
by a combination of noncovalent inter-
actions, were investigated in solution
by UV/Vis absorption spectroscopy
and voltammetric methods. These ob-
servations provide a clue about the lo-
cation of the bipyridinium unit along
the nanotube. In the solid state, the
complex undergoes a further stage of
self-assembly, thereby initiating extend-
ed oriented tubular structures. Crystal-
lographic studies revealed that the po-
sitioning of the viologen dication in
this asymmetric wheel is addressed by
a complicated pattern of cooperative
noncovalent intermolecular interac-
tions that involve only one half of the
host, whereas the remaining (more
polar) half of the host is exploited to
create long-range structural order that
leads to a “secondary” structure of ex-
tended supramolecular channels that,
in turn, self-assemble in the lattice,
thus giving rise to a “tertiary” structure
of parallel sandwiches of nanotubes.
Keywords: calixarenes
·
electro-
chemistry
nanotubes
·
ion channels
self-assembly
·
·
·
X-ray diffraction
Introduction
The construction of systems able to generate channel-like
structures is a topic of intense research activity in several
scientific fields, such as material sciences, biochemistry, and
chemistry. Several strategies for their preparation exist; as
an arbitrary selection of examples, molecules that because
of the presence of suitable groups appended to a rigid elon-
gated skeleton are able to rearrange and initiate channels or
pores have been widely explored.[1] Alternatively, a channel-
like structure can be achieved through the self-assembly of
macrocyclic platforms.[2] Within this latter methodology, the
calix[4]arene platform has demonstrated its potential to
build up these systems.[3] In spite of its larger annulus, which
could host and possibly allow the passage (transit) of larger
species, the calix[6]arene platform has never been employed
within this aim.
[a]Prof. A. Arduini, Dr. G. Faimani, Prof. A. Pochini, Dr. A. Secchi
Dipartimento di Chimica Organica e Industriale
Università di Parma
via G.P. Usberti 17/a, 43100 Parma (Italy)
Fax : (+39)0521905472
E-mail: arturo.arduini@unipr.it
[b]Prof. A. Credi, M. Semeraro, Dr. S. Silvi
Dipartimento di Chimica “G. Ciamician”
Università di Bologna
via Selmi 2, 40126, Bologna (Italy)
Fax : (+39)0512099456
E-mail: alberto.credi@unibo.it
[c]Dr. C. Massera, Prof. F. Ugozzoli
Dipartimento di Chimica Generale e Inorganica
Chimica Analitica, Chimica Fisica
Università di Parma
via G.P. Usberti 17/a, 43100 Parma (Italy)
Fax : (+39)0521905557
E-mail: franco.ugozzoli@unipr.it
We have, however, recently shown that the triphenylurei-
docalix[6]arene 1 (Scheme 1) can act as a three-dimensional
heteroditopic receptor that can be threaded selectively from
the upper rim by suitable axles derived from the 4,4’-bipyri-
Supporting information for this article is available on the WWW
under http://www.chemistry.org or from the author.
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FULL PAPER
guest, connected through an appropriate sequence of bind-
ing sites, could meet this criterion. Herein, we report the
design, synthesis, and properties of an asymmetric covalently
linked double calix[6]arene composed of two calix[6]arene
units characterized by the very different binding properties
of their inner surfaces.
Results and Discussion
Design, synthesis, and characterization of the tubular host:
To establish the most suitable connectivity and relative posi-
tion of the binding sites within a calixarene dimer, we pre-
liminarily verified whether the three ureido NH groups that
are present in 1 are essential in the threading process of the
calix[6]arene platform by viologen salts. Therefore, wheel 1
was fully methylated at the NH groups with CH3I in the
presence of an excess of NaH in THF to give 2 in 83%
yield to block the hydrogen-bond donor groups and thus
prevent its ability to act as a heteroditopic receptor toward
ion pairs (see Scheme 1).
The main features of the 1H NMR spectrum of 2 (re-
corded in solution with CDCl3) are the signals of the two
different N-methyl groups at d=3.04 and 2.70 ppm, the
latter being overlapped with the signals of the three me-
thoxy groups. The signals of all the other protons are broad,
and the axial and equatorial methylene protons give rise to
two very broad bands at about d=4.2 and 3.2 ppm, respec-
tively, thus suggesting a more flexible conformation of 2
with respect to 1. The ability of 2 to act as a receptor for
cations was tested in solution toward dimethyl- (MV2+) and
dioctylviologen (DOV2+) tosylates and monitored through
1H NMR spectroscopic analysis in CDCl3, C6D6, and
CD3CN. We found that 2, probably because of insufficient
preorganization and the lack of three binding sites for the
recognition of the two anions, could not take up the dialkyl-
viologen salts and form supramolecular complexes in an ob-
servable amount. These data suggest that under these condi-
tions the energy gain for complex formation between the
cationic portion of the salts employed and the calix[6]arene
cavity is not sufficient to overcome the energy needed to
separate the cation from the two counteranions in the ion
pair. Quite surprisingly, negative evidence of binding was
also obtained by adding an excess of 3, which is an efficient
host for the chloride ion,[8] to a suspension of 2 and MV2+
or DOV2+ as their chloride salts in CDCl3 (see Scheme 1),
thus indicating that the cation and anion binding with dis-
tinct hosts (dual-host strategy)[9] is less efficient than that af-
forded by an heteroditopic receptor.[10] Indeed the lack of
preorganization of 2 in solution was indirectly evidenced
from its structure in the solid state (see the Supporting In-
formation for a detailed structure description).
Scheme 1. Reactions and conditions for the synthesis of calix[6]arene de-
rivatives 2 and 5: a) NaH, CH3I, THF, 708C, 4 h (83%); b) PhNCO,
CH2Cl2, 3 h (75%). Graphical representation of the receptor 3 for the
chloride ion.
dinium (viologen) unit to yield oriented pseudorotaxanes.[4]
These supramolecular assemblies are endowed with a high
thermodynamic stability that derives from the inclusion of
the cationic portion of the guest inside the electron-rich
cavity and by the interaction of the two counteranions with
the three urea moieties.[5] The type of counteranions of the
bipyridinium-based threads indeed play an important role in
the formation of the complex, as they affect both the ther-
modynamic stability of the complex and the rate of the
threading process. These pseudorotaxanes can be reversibly
decomposed through electrochemical stimuli.[6] This behav-
ior suggests that the calix[6]arene skeleton could be used to
build up an extended tubular structure with a wheel as a
wall, characterized by a functioning mode that could be
traced back to that of molecular machines[7] based on pseu-
dorotaxanes.
From analysis of the solid-state structure of the pseudoro-
taxane formed by 1 and an asymmetrical axle, it emerges
that the triphenylureidocalix[6]arene wheel possesses
a
trigonal prism structure 1.65 nm in length with an internal
width of about 0.75 nm (estimated from interatomic distan-
ces).[5] We, therefore, envisaged that the ability of wheel 1 to
yield oriented pseudorotaxanes could be exploited for the
construction of oriented asymmetric channel-like structures.
However, if the calix[6]arene skeleton should be used to
build up a tubular structure, then its inner surface should be
extended to approach the length of a bilayer while maintain-
ing all the properties of an asymmetric heteroditopic wheel.
To this aim, we envisaged that the linkage of two calix[6]ar-
ene units with different hosting properties toward a viologen
To verify whether the three phenylureido binding sites
that are positioned at the upper rim of 1 maintain their
function also when they are anchored at the lower rim of
the calixarene, derivative 5, which could in principle still
behave as an heteroditopic receptor because of the presence
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A. Arduini, A. Credi, F. Ugozzoli et al.
of the calixarene cavity and the three hydrogen-bond donor
groups, was thus synthesized by treating the known
5,11,17,23,29,35-hexa-tert-butyl-37,39,41-trimethoxy-38,40,42-
tris(3-aminopropoxy)calix[6]arene (4)[11] with phenylisocya-
nate (see Scheme 1). Host 5 was submitted to complex for-
mation with MV2+ or DOV2+ ditosylate under the same ex-
perimental conditions adopted for 2. These attempts gave,
however, negative results as evidenced through NMR spec-
troscopic and ESI-mass-spectrometric analysis.
The lack of binding ability experienced by 5 and 2 toward
the same guests clearly demonstrates that not only are the
three urea binding sites essential for binding, but also that
they should be positioned in proximity to the calixarene
cavity. Thus, a double calix[6]arene should be assembled to
maintain the main structural and chemical information pres-
ent on the reference wheel 1. We, therefore, considered the
possibility of employing three urea moieties for the connec-
tion of two calix[6]arene units. From a synthetic point of
view, three possible methods exist by which two calixarenes
could be covalently connected (Figure 1).
On the basis of the aforementioned considerations, the
lower-rim/lower-rim linkage (Figure 1a) appears as the least
promising, since it bears the three ureido bridges positioned
at the lower rim of both calixarene subunits. On the other
hand, as also previously observed by us,[12] the upper-rim/
upper-rim linkage (Figure 1b) yields dimers that show quite
poor binding ability toward organic cations. The head-to-tail
connecting mode (Figure 1c) was then chosen for the syn-
thesis of a double calix[6]arene.
Figure 1. Schematic representation of the three different bridging modes
between two calix[6]arenes that afford a tubular structure.
To assign the different binding abilities of the viologen
salts toward the two calix[6]arene units of the dimer, it was
decided to focus on the hydrogen-bond-acceptor ability of
one of the two calixarene units. Therefore, the double cal-
ix[6]arene 11 was synthesized in 40% yield starting from the
triaminocalix[6]arene derivative 8 and the triisocyanate 10
(see Scheme 2). In 11, a calix[6]arene unit that bears three
ureido moieties at its upper rim is connected through a
propyl spacer to the lower rim of a second calixarene unit
that bears three electron-withdrawing nitro groups at its
1
upper rim. The H NMR spectra of 11 recorded in several
deuterated solvents at room temperature is broad; neverthe-
Scheme 2. Reactions and conditions for the synthesis of the double calix[6]arene 11: a) K2CO3, 3-(N-bromopropyl)phtalimide, CH3CN, reflux, 15 days
(30%); b) NH2NH2·H2O, EtOH, reflux, 12 h (90%); c) 10, CH2Cl2, 3 h (30%); d) triphosgene, toluene, reflux, 3 h (70%).
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less, in C6D6 at 330 K (see the Supporting Information) it is
sufficiently sharp to allow the complete assignment of all
the signals. Under these conditions, the spectrum clearly
shows the presence of two sets of signals for the two differ-
ent calixarene subunits, which both assume a cone confor-
mation on the NMR timescale. In particular, the two dou-
blets at d=4.47 and 4.62 ppm were assigned to the pseu-
doaxial methylene protons of the lower and upper subunits,
respectively. The methoxy groups of both subunits resonate
as broad singlets at d=3.1 and 3.4 ppm, respectively, thus in-
dicating that the methoxy groups of both subunits are ori-
ented inwards toward their corresponding calixarene cavi-
ties. As expected, because of the different substitutions of
the aromatic ring in the two calixarenes, the aromatic pro-
tons of the nitro-substituted unit resonate at d=8.20 ppm,
whereas those adjacent to the ureido groups resonate at d=
6.94 ppm. The six chemically different NH protons resonate
as two distinct broad signals at d=5.7 and 6.9 ppm. The
complete assignment of all the signals present in the spec-
trum of 11 was achieved through 2D NMR DQF-COSY and
ROESY techniques (see the Supporting Information).
Figure 2. Visible absorption spectra of
[1ꢀDOV](TsO)2 (full line) and [11ꢀDOV]
CH2Cl2. The inset shows the absorption changes recorded at 460 nm
upon addition of DOV
(TsO)2 to a 1.2”10À5 m solution of 11 in a cell
with an optical path length of 5 cm. The curve shows the fitting of the ex-
perimental data according to the formation of a 1:1 complex.
a
solution of the complexes
ACHTREUNG
A
AHCTREUNG
The addition of the colorless DOV
A
Similar behavior was observed in the case of a 1:1 mixture
to a solution of 11 in apolar media (CDCl3 or C6D6) yielded
orange–red solutions from which it was not possible to es-
tablish the structure of the species that had formed by using
NMR spectroscopic techniques because of the extensive
overlapping and broadening of several signals.
of double calixarene 11 and DOVAHCTRE(UNG TsO)2, whose absorption
spectrum exhibits a new tail centered at around 440 nm
(Figure 2). Spectrophotometric titrations (Figure 2, inset) in-
dicate that a 1:1 complex is formed with a stability constant
of (1.1Æ0.3)”106 mÀ1.
UV/Vis absorption experiments: We recorded the absorp-
As expected, no absorption changes were observed for a
tion spectra of DOV
A
1:1 mixture of host 12 and DOVACHTRE(UNG TsO)2 in comparison with
calix[6]arenes 1 and 12 in CH2Cl2 at room temperature. The
latter two macrocycles can be taken as models for the elec-
tron-rich (lower half) and electron-poor (upper half) por-
tions of the double calixarene 11, respectively (see
Scheme 2). All these compounds show intense absorption
bands in the near-UV region (see the Supporting Informa-
tion) and no luminescence.
the sum of the spectra of the isolated components. The lack
of complexation of 12 towards DOV2+ may be related to
the fact that the former does not possess an electron-donat-
ing cavity (in fact 12 is an electron acceptor). Moreover, 12
is more flexible relative to 1 or 11 and hence poorly preor-
ganized as a host, as evidenced by its 1H NMR (CDCl3)
spectra, in which, for example, the twelve bridging methyl-
ene protons resonate as a singlet at d=4.0 ppm.
The absorption spectrum of a 1:1 mixture of calixarene 1
and DOV
G
The above results are consistent with the inclusion of the
separated species. Besides the changes in the UV absorption
bands of the molecular components, a new broad, weak ab-
sorption band of lmax =465 nm shows up in the absorption
spectrum of the mixture (Figure 2). Spectrophotometric ti-
trations point to the formation of a 1:1 complex with a sta-
bility constant of (2.0Æ0.5)”106 mÀ1. The absorption band
typical of the supramolecular complex is attributed to
charge-transfer (CT) interactions between the electron-rich
DOV2+ axle into the cavity of nanotube 11. The similarity
between the stability constants of the [1ꢀDOV]
ACHTREU(NG TsO)2 and
[11ꢀDOV](TsO)2 complexes indicates that the extension of
AHCTREUNG
the host inner surface in 11 has not changed the binding effi-
ciency toward viologen salts and, taken together with the
lack of complexation of DOV2+ by 12, suggests that the vi-
ologen portion of the axle is surrounded by the electron-rich
calix[6]arene unit of 11 (the lower half in Scheme 2). All
these observations are in full agreement with the structure
diphenylureido units of 1 and the viologen unit of DOV2+
as shown by control experiments carried out on mixtures of
DOV2+ and N,N-diphenylurea. In fact, we previously
showed[5] that 1, and a very similar triphenylureidocalix[6]ar-
observed for the [11ꢀDOV]
state (see below). It should be noted that the CT band of
the [11ꢀDOV](TsO)2 pseudorotaxane is displaced to higher
(TsO)2 species
(Figure 2), presumably because in the double calixarene 11
the three N,N-diphenylureido moieties present in 1 are re-
placed by less electron-donating phenylureido units.
ACHTRE(UNG TsO)2 complex in the solid
AHCTREUNG
ene,[13] forms
a
1:1 pseudorotaxane-type complex with
energy relative to that of the [1ꢀDOV]
ACHTREUNG
DOV2+ in apolar solvents stabilized by 1) CT interactions
between the aromatic rings of the host and bipyridinium
guest, 2) hydrogen bonding between the counteranions of
the viologen dication and ureidic groups of the wheel, and
possibly 3) solvophobic effects on account of the very low
solubility of viologen salts in apolar solvents.
We also measured the rate constant for the threading of
DOV
ACHTRE(UNG TsO)2 into 11 by stopped-flow absorption spectrosco-
py. The absorption changes recorded over time (see the Sup-
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101

A. Arduini, A. Credi, F. Ugozzoli et al.
porting Information) could be fitted according to a bimolec-
ular reaction model, thus yielding a second-order rate con-
stant of (1.4Æ0.1)”103 mÀ1 sÀ1 at 293 K. This process is
much slower than that corresponding to the threading of
idation processes. Calixarenes 1 and 12 and double calixar-
ene 11 exhibit several irreversible oxidation processes at po-
tentials higher than E=+1.0 V, assigned to the oxidation of
their electron-rich aromatic units.[16] Calixarene 1 shows no
reduction processes in the potential window examined,
whereas 11 and 12 exhibit irreversible processes at E<
À1.2 V, which we assigned to the reduction of their nitro-
substituted aromatic units.[16] For the sake of the discussion
and because of the irreversibility of the redox processes of
the hosts, we will consider only the reduction processes of
the viologen unit in the various compounds.
DOVACHTREUNG(TsO)2 into a triphenylureidocalix[6]arene very similar
to 1 under the same conditions, for which the rate constant
was found to be (1.7Æ0.3)”106 mÀ1 sÀ1.[6] Previous NMR
spectroscopic studies indicate that the insertion of the violo-
gen axle occurs from the upper rim of 1 in apolar solvents.[4a]
We also pointed out[6] that the dissociation of the tight ion
pairs between the cationic axle and counteranions and the
complexation of the latter by the ureidic units of the host
play a major role in determining the self-assembly rate. By
The electrochemical behavior of the [1ꢀDOV]
ACHTREUNG(PF6)2
pseudorotaxane on reduction (see the Supporting Informa-
tion) is nearly identical to that previously observed[6] for a
very similar system. The cyclic voltammetric wave for the
first reduction of the viologen unit in the complex is charac-
terized by a large separation between the cathodic and
anodic peaks. The cathodic wave (Ep =À0.63 V at a scan
rate of 0.3 VsÀ1) is negatively shifted by more than 330 mV
hypothesizing a similar process between DOV
11, it could be reasonable to assume that the slower self-as-
(TsO)2 derives from the fact
ACHTREU(NG TsO)2 and
sembly observed for [11ꢀDOV]
ACHTREUNG
that the ureidic anion receptors are not located nearby the
entrance of the cavity in the double calixarene and thus
cannot efficiently assist the dissociation of the DOVACHTRE(UNG TsO)2
ion pairs. The displacement of the viologen axle inside the
tubular host, that is, from the upper to lower calixarene moi-
eties, could not be observed in our experiments, most likely
because the displacement is too fast on the stopped-flow
timescale. Further speculations would be unsafe because we
did not investigate the self-assembly kinetics in sufficient
detail (e.g., the energy barrier was not determined).
with respect to that of DOV
sponding anodic wave (Ep =À0.23 V) occurs at nearly the
same potential as that of free DOV(PF6)2. The second re-
duction process is reversible and takes place at a potential
(E1/2 =À0.82 V) identical to that of the free axle, thus indi-
cating that a one-electron reduction of the viologen unit
causes the dethreading from the host. The behavior ob-
ACHTRE(UNG PF6)2 alone, whereas the corre-
AHCTREUNG
Voltammetric experiments: Electrochemical techniques
are particularly useful for investigating the behavior of the
axle unit inside the calixarene wheels as a result of the val-
uable redox properties of the viologen salts.[14] It should be
noted that since the electrochemical experiments are carried
out in the presence of a large excess of tetrabutylammonium
hexafluorophosphate, used as a supporting electrolyte, the
served for the first reduction process of [1ꢀDOV]
ACHTRE(UNG PF6)2 can
be accounted for by the fact that 1) the viologen dication is
stabilized by inclusion into the calixarene wheel and 2) the
(re)threading of the electrochemically regenerated DOV2+
axle into 1 is slow on the timescale of the voltammetric ex-
periment.[6]
The [11ꢀDOV]CAHTRE(UNG PF6)2 pseudorotaxane exhibits, as expect-
cationic species are present in solution as ion pairs with the
ed, the two monoelectronic reductions of its viologen axle
(E1/2 =À0.27 and À0.82 V; Figure 3). The fact that the violo-
gen unit is associated with the double calixarene host[17] is
reflected by the decrease in the current intensity of the vol-
tammetric waves, because of the smaller diffusion coefficient
of the complex. In contrast with the behavior of the mono-
calixarene pseudorotaxane described above, the first reduc-
tion process is reversible and occurs at a potential value
almost equal to that of the same process in the free axle.
The second reduction wave is affected by adsorption phe-
nomena; its potential appears to be not shifted relative to
the same wave in the free DOV2+ axle. The lack of a poten-
tial shift in the first reduction wave (Figure 3) is a somewhat
surprising result because the absorption data indicate that
the viologen unit is involved in CT interactions with the
host. Moreover, while we found that encapsulation of the vi-
ologen unit into a triphenylureidocalix[6]arene wheel causes
a decrease in the heterogeneous electron-transfer rate con-
stant,[6] the first reduction wave of DOV2+ complexed by 11
is electrochemically reversible under our conditions, thus in-
dicating a relatively fast heterogeneous electron-transfer
process.
À
PF6 ions. DOV
A
monoelectronic processes (E1/2 =À0.28 and À0.82 V versus
the saturated calomel electrode (SCE); Figure 3) and no ox-
Figure 3. Cyclic voltammetric curves for the reduction of DOV2+ alone
(full line) and in the presence of one equivalent of the double calixarene
11 (dashed line). Conditions: CH2Cl2, 2.2”10À4 m, 0.02 m tetrabutylam-
monium hexafluorophosphate, 0.3 VsÀ1, glassy carbon electrode. The re-
versible wave at +0.46 V versus SCE is ferrocene used as an internal ref-
erence.
These observations can be accounted for by the fact that
the viologen unit may exhibit some mobility along the tubu-
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Calixarene Nanotubes
FULL PAPER
lar host, thereby spending some of its time close to the
region of connection of the two calixarene halves. Inspection
of molecular models (see also Figure 4) shows that in such a
position the viologen unit 1) is no longer surrounded by
tends outside the dimer through the portals of the equatorial
plane of 11, probably because the cavity of the second cal-
ix[6]arene unit GHILMN is partially obstructed by the me-
thoxy groups of rings L and N and by the highly polar char-
acter of the inner surface of this latter macrocycle. Two ad-
ventitious water molecules are also present in the host,
while, as expected, the two tosylate anions are confined out-
side 11 in proximity to the three urea moieties. All compo-
nents are held together by the simultaneous cooperation of
strong, non-directional electrostatic interactions between the
charged species and weak, highly directional (and thus selec-
tive) intermolecular interactions, such as hydrogen bonds
and CH–p interactions. The geometrical parameters for the
weak attractive interactions in the complex are listed in the
Supporting Information.
A set of multiple weak attractions between the calixarene
ABCDEF and viologen units are responsible of the asym-
metric position of the guest within the cavity. The strongest
À
attraction is the hydrogen bond C9V H···OlD (O1D···H=
2.207(9); C9V···O1D=3.156(9) Š). Contemporarily, the hy-
drogen atom on C7V points toward the centroid of the phe-
nolic ring A, thus giving rise to a CH–p interaction with the
hydrogen atom at 2.658(9) Š over the centroid of phenolic
ring A. This value is significantly shorter than the value of
2.84 Š calculated by Tsuzuki et al. in the high-level ab initio
calculation of the benzene/methane interaction in the gas
Figure 4. X-ray structure of the complex [11ꢀDOV]
(TsO)2 (the DOV2+
ACHTREUNG
component is yellow). a) Partial stick view; b) partial space-filling view.
Only the hydrogen atoms involved in weak, attractive intermolecular in-
teractions are shown. The hydrogen bonds and CH–p interactions are de-
noted by green lines.
phase,[19] when the methane C H bond is orthogonal to the
À
À
electron-rich aromatic moieties, 2) is not effectively encap-
sulated because of the presence of relatively large portals
between the spacers that link the two calixarene halves,[18]
and 3) may come in contact with external molecules (solvent
or anions). Therefore, it can be expected that in such a
supramolecular conformation the electrochemical properties
of the viologen unit may not be affected too much by the
host (apart from the decrease in current intensity as a result
of the smaller diffusion coefficient). The fate of the complex
between 11 and the viologen axle upon reduction of the
latter is difficult to assess on the basis of the redox potential
values.
benzene centroid. A third (weaker) hydrogen bond C19V
H···OlC (O1C···H=2.489(9); C19V···O1C=3.310(9) Š) in-
volves one hydrogen atom at the terminal aliphatic chain of
the guest and the phenolic oxygen atom of ring C. These in-
teractions are superimposed onto the electrostatic attrac-
tions that occur between the doubly charged guest and the
two tosylate anions. These latter species are also involved in
multiple weak attractive interactions involving simultane-
ously the host and the two water molecules inside the host
cavity and play different roles. In particular, anion Q forms
four hydrogen bonds with the urea chains of the host: two
of them through O3Q, which acts as acceptor of the two hy-
À
X-ray structure of the supramolecular complex: Crystals
suitable for X-ray analysis were obtained from the slow
evaporation of the reddish solution obtained by mixing 11
drogen
bonds
O3Q···H N1Y
(O3Q···H=2.058(9);
À
O3Q···N1Y=2.968(9) Š) and O3Q···H N2Y (O3Q···H=
2.032(9); O3Q···N2Y=2.942(9) Š); the other two oxygen
atoms O1Q and O2Q act as acceptors of the two hydrogen
and DOVACHTREUNG(TsO)2 in benzene/methanol.
À
In the solid state, four chemical species, namely, the neu-
tral host 11, DOV2+, the two tosylate anions, and two water
molecules, self-assemble in a supramolecular complex (see
Figure 4) that belongs to the pseudorotaxane class and has
the shape of a trigonal prism of about 2.6”1.6 nm. In the
complex, the two aromatic domains are characterized by dif-
ferent hydrogen-bond-acceptor abilities and are also slightly
different in size and shape (see the Supporting Information
for an unequivocal description of the calix[6]arene confor-
mations).
bonds O1Q···H N2Z (O1Q···H=1.993(9); O1Q···N2Z=
À
2.937(9) Š)
and
O2Q···H N1Z
(O2Q···H=1.928(9);
O2Q···N1Z=2.846(9) Š). In any case the role of the urea
NH groups is crucial for the binding of the two tosylate
anions through hydrogen bonding. The tosylate Q simulta-
neously interacts with the urea groups of chains Y and Z
(Figure 4a), whereas the anion P interacts with the NH
groups of chain X.
The role of the second anion P is quite different and more
important in stabilizing the supramolecular architecture of
the complex. In fact, the oxygen atom O2P links the host
The dicationic thread is positioned within the ABCDEF
cavity through a system of weak intermolecular attractive
interactions, and one of its octyl chains protrudes from the
lower rim of this latter subunit. The other octyl chain ex-
À
through the single hydrogen bond O2P···H N2X (O2P···H=
1.978(9); O2P···N2X=2.927(9) Š). At the same time, the
oxygen atom O3P connects with the anion P, the two water
Chem. Eur. J. 2008, 14, 98 – 106
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www.chemeurj.org
103

A. Arduini, A. Credi, F. Ugozzoli et al.
molecules within the cavity, and the guest dication. In fact,
this oxygen atom accepts one hydrogen bond from the host,
the plane orthogonal to the axis of the tubules, thus giving
rise to a sandwich superstructure of segregated bilayers (of
about 19.5 Š in thickness, calculated as interatomic distan-
ces) of nanotubes. Although each bilayer is electrically un-
charged, the two surfaces of each bilayer are strongly polar
À
O3P···H N1X
(O3P···H=1.961(9);
O3P···N1X=
2.879(9) Š), and contemporarily shows a hydrogen-bond
contact with oxygen atom O1W of one of the two water
molecules, which in turn is linked to the host and dication
through two hydrogen-bond contacts: one with N1V of the
dication (O1W···N1V=3.170(9) Š) and a second with the
second water molecule O2W within the host cavity
(O1W···O2W=2.68(1) Š). The stability of the complex is
further enhanced by another hydrogen-bond contact be-
tween O2W and O1Z with a donor–acceptor distance of
2.952(8) Š. Through this way, the role of the two water mol-
ecules within the host cavity is only limited, thus enhancing
the stability of the complex and appearing unable to influ-
ence the position of the dioctylviologen within the host
cavity.
À
as a result of the presence of the external SO3 groups.
Conclusions
We have shown that a tubular molecular host containing
predetermined chemical functionalities can be obtained by
appropriately linking two calix[6]arene units. The length of
this calixarene dimer is comparable to the thickness of a bi-
layer membrane, while its diameter is suitable for the encap-
sulation of aromatic rings. In fact, we have shown that this
compound is able to function as a dissymmetric heteropoly-
topic host, thus forming a fairly stable pseudorotaxane-type
complex with dioctylviologen in apolar solvents. In the solid
state, the supramolecular complexes self-assemble to yield a
“secondary” arrangement of extended channels, which in
turn give rise to a “tertiary” structure of parallel sandwiches
of nanotubes.
These results show that our approach is promising for the
construction of functional self-assembling structures at the
nanoscale. More specifically, pseudorotaxane-type com-
plexes based on extended and dissymmetric (oriented) tubu-
lar hosts may constitute a promising platform for the reali-
zation of artificial molecular channels that can be controlled
by external stimuli. Studies are undergoing in our laborato-
ries to exploit this approach and for the preparation of new
working tubular devices.
In the crystal, the asymmetry of the complex is responsi-
ble for the formation of a “secondary” structure that results
in the formation of an oriented “channel-like” structure. In
fact, the aliphatic chain that protrudes from the calixarene
ABCDEF of one complex acts as a dominant structure-di-
recting factor by entering into the calixarene GHILMN of
another complex, thus filling the space in the NO2 region
(Figure 5a). The “pitch” along a single channel is 22.424 Š,
and each adjacent parallel channel is shifted in that direc-
tion by one half of the pitch (11.212 Š).
Experimental Section
General: All the reactions were carried out under nitrogen, and all the
solvents were freshly distilled under nitrogen and stored over molecular
sieves for at least 3 h prior to use. All the other reagents were of reagent-
grade quality as obtained from commercial suppliers and were used with-
out further purification. Column chromatography was performed on
silica gel (63–200 mesh). The NMR spectra were recorded in CDCl3
unless otherwise indicated. Mass spectra were determined in ESI mode.
The melting points are uncorrected. The elemental analyses were carried
out at the Laboratory of Microanalysis, Dipartimento Farmaceutico, Uni-
versity of Parma. Compounds 1,[20] 5,11,17,23,29,35-hexa-tert-butyl-
37,39,41-trimethoxy-38,40,42-tris(3-aminopropoxy)calix[6]arene
5,17,29-tri-tert-butyl-11,23,35-trinitro-37,39,41-trimethoxycalix[6]arene
(4),[11]
(6),[20]
5,17,29-tri-tert-butyl-11,23,35-triamino-37,39,41-trimethoxy-
38,40,42-tris(2-ethoxyethoxy)calix[6]arene (9),[20] and 5,17,29-tri-tert-
butyl-11,23,35-trinitro-37,39,41-trimethoxy-38,40,42-tris(2-ethoxyethoxy)-
calix[6]arene (12)[20] were synthesized according to literature procedures.
Figure 5. a) Self-assembly in a columnar arrangement of the supramolec-
ular complexes in the nanotube “secondary” structure. Hydrogen atoms
have been omitted for clarity. b) Self-assembly of the nanotubes in the
crystal lattice give rise to a “tertiary” structure of sandwiched bilayers of
nanotubes (a bilayer is evidenced between the two horizontal yellow
lines).
N,N’-Hexamethyltriphenylureidocalix[6]arene (2): A solution of calix[6]-
arene 1 (0.23 g, 0.16 mmol) and NaH (60% in oil, 0.075 g, 1.88 mmol) in
dry THF (20 mL) was poured in a thick-walled glass autoclave and
heated at 708C for 30 min. The solution was cooled to room temperature,
and CH3I (0.26 g, 1.88 mmol) added. The resulting mixture was kept at
708C for a further 4 h, cooled to room temperature, diluted with ethyl
acetate (50 mL), and quenched with a 10% aqueous solution of HCl
(25 mL, CAUTION!). The separated organic phase was washed to neu-
trality with brine and dried over anhydrous Na2SO4. The solvent removed
A third structural motif (a “tertiary” structure) is ob-
served in the self-assembly of the channels (Figure 5b) in
104
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 98 – 106

Calixarene Nanotubes
FULL PAPER
to dryness under reduced pressure. The crude brownish oily residue was
purified by column chromatography (hexane/ethyl acetate 1:1, then 4:6)
to afford 2 (0.20 g, 83%) as a yellowish solid. M.p. 108–1108C; 1H NMR
CDCl3): d=1.19 (t, 3J
(CH3)3), 2.89 (s, 9H, ArOCH3), 3.55 (q, JACHTRE(UNG H,H)=7 Hz, 6H, OCH2CH3),
3.6–4.2 (m, 24H, OCH2CH2O, ArCH2Ar), 6.48 and 7.16 ppm (2 s, 12H,
Ar(calix)-H); 13C NMR (75 MHz, CDCl3): d=15.2, 30.3, 31.5, 34.2, 60.0,
66.6, 69.6, 72.3, 123.5, 125.5, 127.2, 128.4, 132.7, 136.0, 146.3, 152.1,
154.4 ppm; ESI-MS: m/z (%): 1028 (20) [M+Na+].
ACHTREU(GN H,H)=7 Hz, 9H, OCH2CH3), 1.31 (s, 27H, C-
3
A
(300 MHz, C6D6): d=1.14 (t, 3J
27H, C
3.14 (s, 9H, PhNCH3), 3.2 (bs, 6H, ArCH2Ar, equatorial), 3.43 (q, 3J-
(H,H)=7 Hz, 6H, OCH2CH3), 3.61 (t, 3J
(H,H)=5 Hz, 6H, Ar-
OCH2CH2O), 3.82 (t, 3J
(H,H)=5 Hz, 6H, ArOCH2CH2O), 4.6 (bs, 6H,
ArCH2Ar axial), 6.3–6.8 and 7.4 ppm (m and bs, 27H, Ar(calix)-H and
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
Biscalix[6]arene (11): A solution of 8 (0.21 mg, 0.18 mmol) in CH2Cl2
(100 mL) was added to a solution of 10 (0.2 g, 0.18 mmol) in CH2Cl2
(200 mL) with vigorous stirring. The resulting mixture was stirred at
room temperature for a further 3 h, and then the solvent was removed
under reduced pressure. Purification of the residue by column chroma-
tography (dichloromethane/ethyl acetate 4:6) afforded 11 (0.13 g, 30%)
ACHTREUNG
AHCTREUNG
Ph-H); 13C NMR (75 MHz, C6D6) d=15.6, 30.3, 31.9, 34.4, 38.9, 39.3,
60.7, 66.9, 70.1, 72.9, 124.4, 125.0, 125.9, 126.9, 127.2, 133.6, 135.0, 141.8,
146.0, 146.5, 151.6, 155.1, 160.5 ppm; ESI-MS: m/z (%): 797.5 (100) [M+
2Na+]/2, 1572.0 (15) [M+Na+].
as
330 K): d=1.09 (s, 27H, Ar(calixdown)-C
9H, OCH2CH3), 1.44 (s, 27H, Ar(calixup)-C
OCH2CH2CH2N), 3.09 (s, 9H, Ar(calixup)-OCH3), 3.35 (d, 2J
6H, ArCH2Ar(up) equatorial), 3.5 (bs, 15H, Ar(calixdown)-OCH3 and
OCH2CH2CH2N), 3.55 (q, 4J
(H,H)=7 Hz, 6H, OCH2CH3), 3.6 (bs, 6H,
OCH2CH2CH2N), 3.7–3.8 (m, 12H, OCH2CH2O and ArCH2Ar(calixdown)
equatorial), 4.0 (bs, 6H, OCH2CH2O), 4.48 (d, 2J
(H,H)=15 Hz, 6H,
ArCH2Ar(calixup) axial), 4.69 (d, 2J
(H,H)=15 Hz, 6H, ArCH2Ar(calixdown)
a
white solid. M.p. 1508C (decomp); 1H NMR (300 MHz, C6D6,
(CH3)3), 1.25 (t, 3J
(H,H)=7 Hz,
(CH3)3), 1.9 (bs, 6H,
(H,H)=15 Hz,
Calix[6]arene (5): A solution of calix[6]arene 4 (0.48 g, 0.41 mmol) and
phenylisocyanate (0.24 g, 2 mmol) in CH2Cl2 (25 mL) was stirred at room
temperature. After 3 h, the solvent was completely evaporated under re-
duced pressure. Purification of the solid residue by column chromatogra-
A
ACHTREUNG
AHCTREUNG
AHCTREUNG
phy (hexane/ethyl acetate 6:5) afforded 5 (0.47 g, 75%) as a white solid.
ACHTREUNG
1
M.p. 205–2088C; H NMR (300 MHz, CDCl3): d=0.93 (s, 27H, C
A
1.25 (s, 27H, C
OCH3), 3.41 (bs, 6H, OCH2CH2CH2NH), 3.46 (d, 6H, 2J
ArCH2Ar equatorial), 3.9 (bs, 6H, OCH2CH2), 4.47 (d, 6H, 2J
15 Hz, ArCH2Ar axial), 5.7 (bt, 3H, CH2NH), 6.80 (s, 6H, Ar(calix)-H),
6.96 (t, 3H, 3J
(H,H)=7 Hz, Ph-H(para)), 7.04 (s, 3H, PhNH), 7.13 (s, 6H,
H
ACHTREUNG
T
ACHTREUNG
N
axial), 6.9 (bs, 6H, Ar(calixdown)-H), 6.99 (s, 6H, Ar(calixdown)-H), 7.46 and
8.19 ppm (2 s, 12H, Ar(calixup)-H); 13C NMR (75 MHz, CDCl3): d=15.5,
31.2, 31.7, 34.3, 60.4, 61.2, 66.7, 70.1, 72.4, 72.8, 124.8, 125.6, 125.8, 125.9,
129.3, 132.1, 136.1, 144.2, 146.6, 147.2, 151.5, 153.8, 157.4, 160.2 ppm;
ESI-MS: m/z (%): 1193 (100) [M+2Na+]/2, 2363 (15) [M+Na+]; ele-
mental analysis calcd (%) for C138H171N9O24·H2O (2357.9): C 70.29, H
7.40, N 5.35; found: C 70.05, H 7.42, N 5.29.
ACHTREUNG
Ar(calix)-H), 7.2–7.3 ppm (m, 12H, Ph-H(ortho,meta)); 13C NMR (75 MHz,
CDCl3): d=30.1, 30.3, 31.4, 31.7, 34.3, 34.5, 37.2, 60.8, 72.8, 120.5, 124.0,
125.8, 127.5, 129.1, 133.1, 133.5, 139.5, 146.4, 146.7, 152.0, 153.6,
168.5 ppm; ESI-MS: m/z (%): 1566 (100) [M+Na+]; elemental analysis
calcd (%) for C99H126N6O9 (1544.1): C 77.01, H 8.22, N 5.44; found: C
76.87, H 8.40, N 5.41.
UV/Vis absorption spectroscopy: Measurements were carried out at
room temperature (ca. 295 K) in air-equilibrated (Merck Uvasol) solu-
tions in CH2Cl2 in the concentration range 1”10À5–2”10À4 m. The UV/
Vis absorption spectra were recorded on a Perkin–Elmer l40 spectropho-
tometer. Reaction kinetic profiles were collected for air-equilibrated sol-
utions in CH2Cl2 at 293 K with an Applied Photophysics SX18MV stop-
ped-flow spectrophotometer interfaced to a PC. Under the conditions
employed, the time required to fill the observation cell (1-cm path
length) was experimentally determined to be <1.3 ms (based on a test re-
action). The concentration of the reactants after mixing was on the order
of 1”10À5 m. The absorption titration and kinetic curves were analyzed
with the SPECFIT software.[21] Experimental errors: wavelength values:
Æ1 nm; absorption coefficients: Æ10%. For more details, see ref. [6].
Voltammetric experiments: Cyclic voltammetric (CV) and differential-
pulse voltammetric (DPV) experiments were carried out in argon-purged
CH2Cl2 (Romil Hi-Dry) with an Autolab 30 multipurpose instrument in-
terfaced to a PC. The working electrode was a glassy carbon electrode
(Amel; 0.07 cm2), the counter electrode was a Pt wire (separated from
the solution by a frit), and an Ag wire was employed as a quasireference
electrode. Ferrocene was present as an internal standard. The concentra-
tion of the compounds examined was on the order of 2”10À4 m; tetrabu-
tylammonium hexafluorophosphate (0.02m) was added as a supporting
electrolyte. Under these conditions, the observed potential window
ranged from À2.0 to +1.6 V versus SCE. Cyclic voltammograms were
obtained at sweep rates of 0.02–1 VsÀ1. Differential-pulse voltammo-
grams were obtained at a sweep rate of 0.02 mVsÀ1, with a pulse height
of 75 mV and a duration of 40 ms. The IR compensation implemented
within the Autolab 30 was used, and every effort was made throughout
the experiments to minimize the resistance of the solution. In any in-
stance, the full electrochemical reversibility of the voltammetric wave of
ferrocene was taken as an indicator of the absence of uncompensated re-
sistance effects. For reversible processes, the half-wave potential values
were calculated from an average of the CV and DPV experiments, where-
as the redox potential values in the case of irreversible processes were es-
timated from the DPV peaks. Experimental errors: potential values:
Æ10 mV for reversible processes, Æ20 mV for irreversible processes. For
more details, see ref. [6].
Calix[6]arene (7): K2CO3 (1 g, 7.14 mmol) was added to a solution of cal-
ixarene
6 (1 g, 1.02 mmol) and N-(3-bromopropyl)phtalimide (2.5 g,
9.2 mmol) in acetonitrile (50 mL). The resulting heterogeneous mixture
was heated under reflux for 15 days, and then the solvent was evaporated
to dryness under reduced pressure. The solid residue was taken up with
CH2Cl2, and the resulting organic phase washed with a 10% aqueous so-
lution of HCl and water until neutral. The separated organic layer was
dried over Na2SO4, and the solvent evaporated to dryness under reduced
pressure. Purification of the solid residue by column chromatography
(hexane/ethyl acetate 6:4) afforded 7 (0.47 g, 30%) as a yellowish solid.
1
M.p. 257–2608C; H NMR (300 MHz, CDCl3): d=1.31 (s, 27H, C
A
2.25 (bs, 6H, OCH2CH2CH2N), 2.89 (s, 9H, ArOCH3), 3.5 (bs, 6H,
ArCH2Ar equatorial), 3.9 (bs, 12H, OCH2CH2CH2N), 4.3 (bs, 6H,
ArCH2Ar axial), 7.2 and 7.6 (2bs, 12H, Ar(calix)-H), 7.6–7.8 ppm (m, 12H,
Ar-H(phtal)); 13C NMR (75 MHz, CDCl3): d=29.4, 30.9, 31.4, 34.2, 35.2,
59.9, 71.3, 123.2, 127.4, 131.9, 132.0, 133.9, 135.9, 143.8, 146.9, 159.3,
168.2 ppm; ESI-MS: m/z (%): 1565 (100) [M+Na+], 1581 (45) [M+K+].
Calix[6]arene (8): N2H4·H2O (0.93 g, 18 mmol) was added to a suspension
of 7 (0.56 g, 0.36 mmol) in absolute ethanol (50 mL), and the resulting
mixture was refluxed overnight. The solvent was then removed under re-
duced pressure, and the solid residue taken up in CH2Cl2 (20 mL). The
resulting organic solution was washed with water and dried over Na2SO4.
The solvent was evaporated under reduced pressure to afford 8 (0.37 g,
90%) as a yellow solid. M.p. 162–1658C; 1H NMR (300 MHz, CDCl3):
d=1.24 (s, 27H, C
ACHTREUNG
A
3J
3J
ACHTREUNG(H,H)=6 Hz, 6H, OCH2CH2CH2NH2), 3.87 (s, 12H, ArCH2Ar), 7.11
and 7.73 ppm (2 s, 12H, Ar(calix)-H); 13C NMR (75 MHz, CDCl3): d=30.7,
31.3, 33.5, 34.2, 38.8, 60.0, 71.7, 123.5, 127.0, 132.0, 135.9, 143.6, 146.8,
154.0, 160.1 ppm; ESI-MS: m/z (%): 1153 (60) [M+H+].
Calix[6]arene (10): A solution of calix[6]arene 9 (0.12 g, 0.11 mmol) in
dry toluene (50 mL) was dropwise added over a period of 1 h to a solu-
tion of bis(trichloromethyl)carbonate (0.04 g, 0.14 mmol) in dry toluene
(100 mL). The resulting mixture was refluxed for 3 h, and then the sol-
vent was completely evaporated under reduced pressure. The crude resi-
due was triturated with hot hexane (5 mL), and the solid suspension fil-
tered off. Removal of the solvent filtrate under reduced pressure afford-
ed 10 (0.09 g, 70%) as a white solid. M.p. 75–778C; 1H NMR (300 MHz,
X-ray crystallographic studies: Intensity data were collected using MoKa
radiation (l=0.71073) on a Bruker AXS Smart 1000 single-crystal dif-
fractometer, equipped with a CCD area detector at 293(2) K. The struc-
Chem. Eur. J. 2008, 14, 98 – 106
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www.chemeurj.org
105

A. Arduini, A. Credi, F. Ugozzoli et al.
ture was solved by direct methods using SIR97[22] and refined by full-
matrix least-squares methods using the SHELXL-97 program.[23] The
data reduction was performed using SAINT[24] and SADABS.[25] All the
non-hydrogen atoms were refined with anisotropic atomic displacements,
with the exclusion of the carbon atoms C21A and C22A at one lower
rim, two tert-butyl groups, and two terminal carbon atoms at the lower
rim chains, which were disordered over two different orientations with
occupancy factors of 0.5 and refined with isotropic atomic displacements.
The hydrogen atoms were included in the refinement at idealized geome-
tries and refined “riding” on the corresponding parent atoms with
common isotropic atomic displacements, which were refined, and with
isotropic temperature factors 1.5-fold of their parent atoms. Geometric
calculations and molecular graphics were performed with the PARST97
program.[26] Crystallographic data and experimental details for 2 and
[7]a) V. Balzani, M. Venturi, A. Credi, Molecular Devices and Ma-
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pseudorotaxane [11ꢀDOV]
A
[10]a) A. Arduini, E. Brindani, G. Giorgi, A. Pochini, A. Secchi, J. Org.
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[12]a) A. Arduini, L Domiano, L. Ogliosi, A. Pochini, A. Secchi, R.
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[14]P. M. S. Monk, The Viologens: Physicochemical Properties, Synthesis
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Acknowledgements
This research was partly supported by MIUR (Sistemi supramolecolari
per la costruzione di macchine molecolari, conversione dellꢁenergia, sens-
ing e catalisi) and Regione Emilia-Romagna (Nanofaber NetLab). The
authors thank the Centro Interdipartimentale di Misure “G. Casnati” for
NMR measurements.
[15]R. Ballardini, A. Credi, M. T. Gandolfi, C. Giansante, G. Marconi,
S. Silvi, M. Venturi, Inorg. Chim. Acta 2007, 360, 1072–1082.
[16]C. K. Mann, K. K. Barnes, Electrochemical Reactions in Nonaqueous
Systems, Dekker, New York, 1970.
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[17]The addition of the supporting electrolyte did not affect the stability
of the [11ꢀDOV]2+ complex under our conditions.
[2]See for example: a) I. Tabushi, Y. Kuroda, K. Yokata, Tetrahedron
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Received: May 16, 2007
Revised: August 8, 2007
Published online: September 26, 2007
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