Synthesis and recognition properties of calix[4]arene semitubes as ditopic hosts for: N -alkylpyridinium ion pairs

Article abstract of DOI:10.1039/c6ce00268d

A series of calix[4]arene semitubes (4a-c) having alkyl bridging spacers of different lengths and flexibilities on their lower rim was synthesised and characterised in solution and in the solid state by ¹H NMR spectroscopy and X-ray crystallogr

Full text of DOI:10.1039/c6ce00268d

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Synthesis and Recognition Properties of Calix[4]arene Semitubes
as Ditopic Hosts for N-alkyl-pyridinium Ion Pairs
Received 00th January 20xx,
Accepted 00th January 20xx
Francesco Vita, Michela Vorti, Guido Orlandini, Valeria Zanichelli, Chiara Massera, Franco
Ugozzoli,* Arturo Arduini, and Andrea Secchi*
DOI: 10.1039/x0xx00000x
A series of calix[4]arene semitubes (4a-c) having alkyl bridging spacers of different length and flexibility on their lower rim
was synthesised and characterised in solution and in the solid state by 1H NMR spectroscopy and X-ray crystallography.
The binding properties of these novel ditopic hosts toward a series of N-alkyl pyridinium ion pairs were investigated in low
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1
polar media by H NMR and UV-Vis spectroscopy and compared with those of the corresponding calix[4]arene monotopic
host 5. These studies showed that in low polar solvents the calix[4]arene semitubes behave as homoditopic hosts giving
rise with N-alkyl pyridinium ion pairs to supramolecular adducts of 1:2 host-guest stoichiometry.
(viologen) ion pairs giving rise to 2:1 host-guest self-assembled
capsules in solution of low polar solvents23,24 and in the solid
state.25 Viologen-based guests are endowed with interesting
Introduction
Molecular self-assembly is a process in which molecules
spontaneously form, through non-covalent interactions,
ordered aggregates whose structure, in equilibrium conditions,
is determined by the chemical information present in the
components of the assembly.1,2 The clever exploitation of self-
assembly can in principle give access to a range of complex
supramolecular entities endowed with specific physical and
chemical properties which otherwise would be considered
impossible to construct.3 Enticing examples of these type are
represented by supramolecular polymers (SP), which are
ensembles of suitable monomer units held together by
reversible non-covalent intermolecular interactions.4–7 These
assemblies are frequently composed of two or more
components endowed with complementary structural features
and chemical information. They are attracting an increasing
attention by the scientific community because they combine
part of the properties of conventional polymers with those
conferred by the weak and reversible interactions among its
components.8-10
redox properties,26 and it has been already shown as their
affinity for the calixarene cavity27-31 can be turned on and off
depending on their oxidation state.27 Supramolecular adducts
of this type can thus represent the prototype for the
development of novel guest-driven supramolecular polymers
and stimuli-responsive materials.32-37 The pursuing of this aim,
however, necessarily implies the design of suitable ditopic
hosts capable of linking difunctional guests in
a linear
structural motif.38-40 Herein we report on the synthesis of a
series of novel ditopic calix[4]arene-based hosts and their
complexation properties in solution of weakly polar media
towards a series of N-alkyl pyridinium ion pairs as the guests.
Xꢀ
N+
NMPX
Xꢀ = TsO, Br,
OH
OH
O
HO
n
O
I, (CF3SO2)2N
(
)
(
)
n
O
O
HO
Whitin the plethora of synthetic macrocyclic compounds
employed for the preparation of SP,11-13 the use of calixarene-
based hosts14 is increasing over the years.15–22 These
Xꢀ
N+
NOPX
Xꢀ = Br, I
4a n = 1
4b n = 4
4c n = 10
componds are indeed characterised by a π-rich aromatic cavity
that, if properly functionalised and rigidified, can be exploited
for self-assembly processes through the recognition of neutral
or charged species.14 We have recently verified that both
calix[6]arene and calix[4]arene-based hosts having N-
phenylurea groups at their larger (upper) rim may interact with
ditopic guests based on N,N′-dialkyl-4,4'-bipyridinium
N
S
NH
HN
S
NH
S
NH
OH
NH
HN
HO
O
O
(
)
n
(
)
n
6
5 n = 6
Chart 1. Calix[4]arene-based ditopic (4a-c) and monotopic (5) hosts; N-alkyl pyridinium
(NMPX and NOPX) guests; and the anion host 6 employed in this study.
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Results and discussion
Synthesis of the ditopic hosts.
The design of the ditopic hosts was based on the analysis of
the literature,41-44 which showed that the bridging of two
calix[4]arene macrocycles through their lower rim with two
alkyl spacers, would result in so-called calix[4]arene
"semitubes",45 whose binding sites, i.e. the two
π-rich
aromatic cavities, have the proper geometrical arrangement to
yield SP. In particular, we tackled the synthesis of a series of
novel calix[4]arenes semitubes having bridging units of three
Scheme 1. Synthesis of calix[4]arene semitubes 4a-c.
The pattern of the aromatic signals, the two overlapped
doublets at ca. 7.1 ppm, integrating for 16 protons, and two
(
4a), six (4b) and twelve (4c) methylene groups, respectively
(see Chart 1). We chose to study alkyl spacers of different
length in order to evaluate how the bridge flexibility and the
distance between the two aromatic cavities may affect the
recognition of two positively charged guests. The use of doubly
bridged bis-calix[4]arene host systems rather than the tetra-
bridged ones46 was prompted by the following considerations:
i) lower rim partially alkylated calix[4]arene hosts have a highly
well-separated triplets at
δ = 6.89 and 6.64 ppm, each
integrating for 4 protons, is consistent with calix[4]arene
macrocycles adopting a flattened cone conformation on the
NMR time-scale. As expected, considering the symmetry of
this bis-calix[4]arene host, the bridging methylene groups give
rise to a typical AX system of two doublets with geminal
coupling of 13 Hz at δ = 4.46 and 3.48 ppm. The two propyl
chains, linking head-to-head the calix[4]arene macrocycles, are
pre-organised
π-donor cavity suitable for the recognition of
charged25 and small neutral47–50 organic species in weakly
polar solvents, and ii) the free phenolic groups may promote
ion-pairs recognition by acting as hydrogen-bond donor
binding sites.
responsible for a multiplet integrating for 8 protons at ca. 4.4
ppm and a quintuplet integrating for 4 protons at δ = 3.17
ppm. Besides the signals arising from their longer C6 and C12
1
linking alkyl chains, the H-NMR spectra of 4b and 4c, taken in
In a one-pot reaction, calix[4]arene
refluxing acetonitrile in the presence of K2CO3 and the
appropriate ditosylate linker 2a-c), using 1:2:2
3 was reacted in
CDCl3, show a NMR signals pattern very similar to that of 4a
(see Fig. 3g).
(
a
stoichiometric ratio between the reagents (see Scheme 1). In
these conditions, the “short” calix[4]arene semitube 4a was
recovered in 60% yield, while the more flexible hosts 4b and 4c
were isolated in 20 and 10% yield, respectively, after
chromatography separation. All hosts were characterised
through NMR spectroscopy, elemental analysis and mass
spectrometry. Unlike 4b and 4c, the 1H NMR spectroscopic
characterisation of 4a in solution was accomplished in CD2Cl2
because of its poor solubility in chloroform. The 1H-NMR
spectrum of 4a recorded at 300 MHz is depicted in Fig. 1, and
Solid state studies. Crystals suitable for the X-rays structure
analysis of calix[4]arenes 4a and 4b were obtained from the
slow evaporation of their solution in dichloromethane and
chloroform, respectively. 4a crystallizes with two symmetry-
independent molecules in the unit cell. The two molecules (see
Fig. 2a), although slightly different from each other in size and
conformation, are both centrosymmetric and the
conformation of their calix[4]arene baskets are blocked by
intramolecular O-H···O hydrogen bonds between phenolic OH
groups (see Table
1 for the geometrical parameters).
it shows a singlet at δ = 8.89 ppm, which was assigned to the 4
equivalent phenolic protons of the macrocycles lower rims.
2 | J. Name., 2012, 00, 1-3
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Fig. 1. 1H-NMR (300 MHz) of calix[4]arene semtitube 4a in CD2Cl2 (resonances assignment has been indicated with symbols on the calix[4]arene sketch).
Fig. 2. a) Stick view of the X-ray structure of the two independent molecules present in the unit cell of calix[4]arene semitube 4a and b) ball and stick view of their
self-assembly in the crystal lattice. Only the relevant hydrogen atoms are reported for clarity, while the intramolecular hydrogen bonds and CH/π interactions were
highlighted with black dashed and continuous green lines, respectively; c) stick view of the self-assembly of the mutual inclusion motif between adjacent
calix[4]arenes sub-units in calix[4]arene semitube 4b in the crystal lattice (colours: C grey, O red, Cl green H white; centroids of benzene rings, green.; centre of
symmetry, orange).
In the ABCD macrocycle (and in its centrosymmetric one)
Table 1. Geometrical parameters for intramolecular hydrogen bonds in calix[4]arene
the calix[4]arene is blocked in a “flattened” cone conformation
semitubes 4a and 4b (D and A denote donor and acceptor atoms, respectively)
by the two hydrogen bonds O1A-H···O1D and O1C-H···O1B,
H-bonds
D∙∙∙A (Å)
2.755
2.909
2.765
2.689
2.727
2.707
2.852
2.893
H∙∙∙A (Å)
1.823
1.982
1.898
1.834
1.782
1.722
1.978
1.927
D-H∙∙∙A (°)
169.8
167.5
159.3
158
whereas in the EFGH macrocycle (and in its centrosymmetric
one) the phenolic oxygen O1H accepts contemporarily the two
hydrogen bonds donated by the OH groups at the phenolic
rings E and G. In both macrocycles, a CH2Cl2 molecule is
anchored to two adjacent benzene rings through attractive
CH/π interactions.51,52 All the C(CH2Cl2)∙∙∙Centroid(CT)
separations (C∙∙∙CT-B 4.136 Å, C∙∙∙CT-C 3.670 Å e C∙∙∙CT-G 3.738
Å, C∙∙∙CT-H 4.031 Å) lie in the range where the binding energy
between benzene and dichloromethane is attractive.53 In the
crystal lattice, 4a and CH2Cl2 self-assemble giving rise to layers
of CH2Cl2 molecules intercalated between layers of
calix[4]arene semitubes parallel to the crystallographic ac
plane (Fig. 2b).
O1B-H∙∙∙O1A
O1D-H∙∙∙O1A
O1F-H∙∙∙O1E
O1H-H∙∙∙O1G
O1A-H∙∙∙O1D
O1C-H∙∙∙O1B
O1E-H∙∙∙O1H
O1G-H∙∙∙O1H
4a
4b
155.9
162.2
144.5
164.86
In the molecular structure of 4b (see Fig. 2c), the two
calix[4]arene sub-units are still blocked in the flattened cone
conformation by four strong intramolecular hydrogen bonds:
the phenolic oxygen O1A in the ABCD macrocycle acts as
“bifurcated” acceptor of the two hydrogen bonds O1B-H···O1A
and O1D-H···O1A, whereas the two simple O1F-H···O1E and
O1H-H···O1G hydrogen bonds block the mutual orientations
on the phenolic rings in the EFGH calix[4]arene (see Table 1).
One chloroform solvent molecule is linked via strong CH/π
interactions to the aromatic ring A. The geometrical
parameters of the CH/π interactions, C∙∙∙CT 3.392 Å, H∙∙∙CT
2.495 Å, C-H∙∙∙CT 155.8°, are almost identical to the values
found by Tsuzuki et al.,54 in the high level ab-initio quantum
chemical calculations of the intermolecular interactions in the
benzene-CHCl3 complex.
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A second chloroform molecule fills the voids of the crystal
lattice. More interesting is the self-assembly of calixarenes and
NMR binding studies. The binding properties of
CHCl3 solvent molecules in 4b: two adjacent calix[4]arene are calix[4]arene semitubes 4a-c were preliminary evaluated
mutually included one into another giving rise to the toward a series N-alkyl pyridinium salts (NMPX and NOPX, see
centrosymmetric dimers shown in Fig. 2c. The structure- Chart 1) characterised by: a) anions of different coordination
directing factor which leads to the self-assembly in strength (from the lowest to the highest X = (CF3SO2)2N, I, Br,
centrosymmetric dimers is the network of the six CH/π and TsO) and b) different bulkiness of the alkyl substituent on
interactions evidenced in Fig. 2c with green lines. Contrary to the iminium nitrogen. This choice was based considering that
4a, in the X-ray crystal structure of 4b a CHCl3 molecule is in weakly polar solvents these species are present as tight ion
bound outside the cavity thus favouring, probably as pairs whose binding is usually driven by cation/
consequence of the larger flexibility of the longer alkyl bridging between the portion of the N-alkyl pyridinium cation with the
π
interactions56
chains, the self-assembly of two calix[4]arene subunits.
highest positive charge density and the
of the calix[4]arene host.25,55
π-rich aromatic cavity
Table 2. Geometrical parameters for CH/π interactions in calix[4]arene semitube 4b.a
The NMR titrations were carried out in CDCl3 solution
using, for solubility reasons, only hosts 4b and 4c. In a typical
titration experiment, progressive amounts of a 10−2 M solution
C-H∙∙∙CT
H∙∙∙CT (Å)
3.087(6)
3.058(7)
2.908(6)
C ∙∙CT (Å)
3.942(7)
3.761(7)
3.810(7)
C-H∙∙∙CT (°)
149.0(4)
131.3(4)
157.1(5)
C3E-H∙∙∙CTH’
C4E-H∙∙∙CTG’
C5E-H∙∙∙ CTF’
3
of the host were added to a 10− M solution of the pyridinium
ion pair. In all titrations experiments the binding process was
fast on the NMR timescale and the host addition induced a
progressive up-field shift of the pyridinium ring resonances.
The stack plot of Fig. 3 depicts some of the 1H-NMR spectra
collected during the titration of NMPI with 4c. The large up-
field shift endured by the pyridinium ring resonances upon
addition of the host solution (cf. spectra a and f of Fig. 3)
confirmed that the recognition of the ion-pair occurs between
the NMP cation and the host aromatic cavities. More intriguing
was the observation that the aromatic resonances of the NMP
experience an up-field shift almost as twice as the one of the
corresponding N-methyl resonance. The negligible shifts
endured by the resonances of the hosts during the titrations
were explained with a non significative conformational change
of the calixarene scaffold upon guest binding. The down-field
shift of ca. 0.15 ppm experienced by the calix[4]arene phenolic
protons (cf. spectra b and g of Fig. 3) could be reasonably
ascribed to such conformational changes rather than a real
involvement of these groups in hydrogen bonding with the
ion-pair counter anion
a
CT indicates the centroid of the aromatic rings; the three H atoms of the
phenolic ring E act as donor groups to the three aromatic rings of the phenolic
rings F′ G′ H′ of the centrosymmetric calixarene unit and contemporarily the
three H atoms of the phenolic ring E′ act as donor groups to the three aromatic
rings of the phenolic rings F G H.
The unexpected different complexing mode experienced
by 4a and 4b suggests a possible different behaviour as ditopic
hosts also for organic ion pairs. This prompted us to
investigate the binding abilities of 4a-c towards the series of N-
methyl pyridinium salts (NMPX) illustrated in Chart 1. It is
indeed known that the electron-poor NMP cation can
efficiently interact with the
π-rich aromatic cavity of
preorganised rigid monotopic calix[4]arene hosts, giving rise to
inclusion complexes in the solid state.23,55 Unfortunately, the
several attempts so far accomplished to crystallise the
inclusion complex between these pyridinium salts and 4a-c
,
from the corresponding 2:1 mixtures in dichloromethane,
failed.¥ Therefore, the binding properties of these ditopic
hosts have been evaluated in weakly polar solvents through
NMR and UV-vis spectroscopies.
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Fig. 3. 1H-NMR stack plot (400 MHz, CDCl3) for the titration of NMPI with 4c: a) free NMPI; b) [NMPI]/[4c] = 2; c) [NMPI]/[4c] = 1.6; d) [NMPI]/[4c] = 1.1; e) [NMPI]/[4c
]
= 0.9; f) [NMPI]/[4c] = 0.5; g) free 4c. The resonances of the NMP cation were designed as follow: Py+-Me ( ), Py+-H ortho ( ), Py+-H meta ( ) and Py+-H para (
• ♦ ■ ▼);
while the OHs resonances in the calix[4]arene semitube as (◊).
Table 3. Apparent binding constants (logK11), theoretic chemical shift of the guest resonance in the 100% complex formed (δ∞) and observed up-field shift (Δδobs, ppm) for
the complexation of NMPX and NOPX ion pairs with monotopic (5 .
) and ditopic (4b-c) calix[4]arenes a,b
c
d
Guest
NMPTsO
NMPBr
logK1:1
δ
∞ (ppm) c
3.62(6)
3.79(4)
3.54(2)
3.73(4)
3.37(4)
4.39(2)
4.29(2)
ꢀ
δ
obs (ppm) c
0.8
logK1:1
δ
∞ (ppm) d
6.8(1)
ꢀδ
obs (ppm) d
1.7
2.76(7)
3.07(6)
3.08(3)
3.17(7)
3.21(7)
3.18(8)
3.13(4)
2.78(5)
3.01(4)
3.04(2)
3.18(7)
3.24(6)
3.15(7)
3.17(4)
0.9
7.19(6)
6.89(4)
7.19(7)
6.89(7)
7.40(8)
7.21(4)
1.8
NMPI
1.0
2.0
NMPTf2N
NMPBr + (6)
NOPBr
0.9
1.9
1.2
2.1
0.6
1.7
NOPI
0.6
1.8
a
1
Determined by H NMR spectroscopic titrations at T = 300 K in CDCl3 (standard deviations in parenthesis); initial concentration of the guest ion pair c =
10−3 M, concentration of the titrant c = 10−2 M; b Δδobs = δfree–δguest where δfree (ppm) is the chemical shift of the resonance of the free guest: NMPTsO, 9.24
(Horto) and 4.71 (N-CH3); NMPBr, 9.43 (Horto) and 4.83 (N-CH3), 6 + NMPBr, 9.23 (Horto) and 4.72 (N-CH3); NMPI, 9.35 (Horto) and 4.77 (N-CH3) ppm; NMPTf2N, 9.41 (Horto
)
and 4.76 (N-CH3); NOPBr, 9.39 (Horto) and 5.03 (N-CH2-) ppm; NOPI, 9.34 (Horto) and 5.00 (N-CH2-), while δguest is the chemical shift of the guest resonance at the end of
the titration when the [6]/[NMPX] ratio is 3.57; c calculated by monitoring the chemical shift variation of the pyridinium ring ortho proton; d calculated by monitoring
the chemical shift variation of the N-CH3 (NMPX) or N-CH2- (NOPX).
The previous findings suggest a preferential binding mode form 1:1 host-guest adducts and the non-linear fitting of the
in which the two almost parallel aromatic rings of the flattened chemical shift variation experienced by the ion-pairs
π
-rich cavity of the host envelope the electron poor pyridinium resonances can be easily carried out with the equation devised
ring of the cation. This was indirectly confirmed by the by Wilcox57 to yield the apparent 1:1 binding constants
evidence that also during the complexation of 4b and 4c with logK1:1.† Among all the NMPX and NOPX resonances enduring
the series of the more sterically demanding NOPX ion-pairs, an appreciable chemical shift variation upon addition of the
very similar results were found. For comparison, the series of host solution, the logK1:1 were calculated through the non-
NMPX and NOPX ion pairs was also titrated using the 1,3- linear fitting of the shifts experienced by the resonance of the
dioctyloxy-calix[4]arene
5 (see Chart 1), which can be pyridinium ortho protons (Py-H ortho) as well as those of the
considered as the monotopic equivalent of the ditopic hosts 4b N-methyl (NMPX, see Fig. 4) and N-CH2- (NOPX) protons.‡
and 4c. This was accomplished to estimate the binding affinity
for the N-alkyl pyridinium cation of a single aromatic cavity. In
fact, differently from ditopic hosts 4b and 4c, host
5 can only
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K11
(1)
(2)
(3)
⊂
G HꢀH
HꢀH + G
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
NMPBr
K12
β
G⊃HꢀH + G
HꢀH + 2G
G⊃HꢀH⊃G
NMPI
G⊃HꢀH⊃G
NMPTsO
NMPTf2N
K'11
(4)
2HꢀH + 2G
HꢀH⊃G…G⊂HꢀH
NMPB ⊂ 6 (1:1)
where H-H describes a generic ditopic host and G a generic
guest. The overall binding process is described by equation (3).
To verify the correct stoichiometry of the binding process
the method of continuous variation61,62 was applied in selected
cases involving 4b as the ditopic host and NMPI and NOPI as
representative examples of N-alkylpyridinium ion pairs. Two
different experiments were devised (see experimental): by
3
mixing in different ratios 5
×
10− M solutions of 4b and NOPI,
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[5]/[NMPX]
we obtained the smooth Job’s plot denoted in fig. 5 by white
circles. This plot reaches its maximum for molar fraction of the
Fig. 4. Plots of the chemical shift variation endured by N-CH3 NMR resonance of a series
of NMPX ion pairs upon addition of host 5 in CDCl3 solution at T = 300 K (for the
structure of the ion pairs and of the hosts see Chart 1).
guest XNOPI
≈ 0.6 (see Fig. 5, white circles). When the reactants
2
starting concentration was doubled (10− M) the Job’s plot
becomes sharper and its maximum shifted in correspondence
of a guest molar fraction of XNOPI = 0.5 (see Fig. 5, black circles).
These results suggest that at lower concentration, the
equilibria described by equations (1) and (2) are coexistent in
solution, while a 1:1 binding process is dominating when the
NOPI concentration is increased. Similar results were found
with NMPI. This totally unexpected result can be tentatively
explained considering that this type of ion-pairs may give rise
to large aggregates in weakly polar solvents, which represent
the actual guest specie present in solution. It is thus
reasonable to assume that when a host lacks of anion ancillary
binding sites the complexation process may occur as described
by equation (4), i.e. the ion pair is complexed as a dimer giving
rise to an apparent 1:1 binding stoichiometry.§§ On the other
hand, the fast exchange conditions observed in the 1H NMR
titration experiments do not help to reveal the nature of this
possible aggregation. Therefore, to verify the aggregation of
the NMPI ion pairs, we devised a simple dilution experiment
which was investigated by UV-vis spectroscopy. A series of
absorption spectra of chloroform solution of NMPI at
increasing concentration was collected in Fig. 6.
Analysis of the results gathered in Table 3 evidences as, for
each ion pair, the value of the logK1:1 is independent of the
resonance used for their determination. Nevertheless, the
observed up-field shift (ꢀδobs) is generally higher for the Py-H
ortho resonance rather than that of N-CH3 and N-CH2- protons
(see Table 3 and Fig. 4). These findings seem to indicate an
interaction between the ion pair and the calix[4]arene
aromatic cavity with
a preferential orientation of the
pyridinium cation through its aromatic portion. This is
probably due to steric hindrance induced by the anion which
“sits” nearby the positively charged iminium nitrogen. A
second and most important observation is that, as expected,
the value of logK1:1 increases as the coordination strength of
the anion does decrease (see e.g. 3.17 for NMPTf2N vs. 2.76
for NMPTsO). Indeed, the highest value of logK1:1 was
calculated for the titration of NMPBr in the presence of an
equimolar amount of a tri-n-butylthioureido derivative of tren
(6
, see Chart 1). This tris-urea-based compound,58 is a host
with a large affinity for spherical halide anions. Such “dual host
approach” was previously used by us to favour the loosing of
tight ion pair in weakly polar solvents.59 The enwrapping of the
anion by
6, indeed, frees the NMP cation that can now more
easily interact with the cavity of
5
. As a result the δobs
ꢀ
determined for this titration experiment were the highest: 1.2
and 2.1 for the Py-H ortho and N-Me resonances, respectively
(see Table 3). Such behaviour was also evidenced by the
comparison of the N-Me resonance chemical shift variation in
the series of NMPX ion pairs (see Fig. 4).
The Wilcox equation (see experimental) allows the
calculation of binding constants from NMR titration data
resulting from the formation of host/guest adducts with a 1:1
binding stoichiometry. Unfortunately, this powerful equation
cannot handle the chemical shift variation deriving from
multiple and concomitant equilibria such as those reasonably
involving ditopic hosts 4b and 4c which were designed to
interact simultaneously with two N-alkyl pyridinium ion pairs
thus giving rise to supramolecular adducts having 1:1 and 1:2
binding stoichiometry as shown by equilibria (1) and (2):60
Fig. 5. Job’s plots relative to the binding process between ditopic host 5 and NOPI. The
experiments were carried out in CDCl3 at T = 300 K by mixing continuously (the total
amount of moles in solution was kept constant) solutions of the interacting species of 5
3
× 10− M (○) and 10−2 M (●), respecꢀvely.
6 | J. Name., 2012, 00, 1-3
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ARTICLE
alkylpyridium guests is strongly affected by aggregation of the
ion pair to be bound. To reduce this undesired phenomenon,
the binding experiments were carried out with more diluted
solutions, using UV-vis spectroscopic titrations as the only
reliable tool to calculate binding constants involving several
concomitant equilibria in solution.64
In a typical optical titration experiment, carried out in
dichloromethane solution rather than in chloroform to
3
improve the solubility of 4a, increasing amounts of 10−
M
4
solutions of the hosts were added to 10− M solutions of the
ion pairs (see experimental). UV-vis spectra of 4a-c in CH2Cl2
are characterised by a strong absorption band centred at λ
∼
285 nm, which was never appreciably shifted upon
complexation with either NMPX or NOPX ion pairs. UV-vis
spectra of the NMPX and NOPX ion pairs sharing the same
anion are identical. However, only the two bromides (NMPBr
and NOPBr) and iodides (NMPI and NOPI) present in this
solvent a non-hidden charge-transfer (CT) band exploitable for
the titration experiments.63 Indeed, upon addition of the host
solution, such band usually underwent an intensity damping
accompanied by a slight hypsochromic shift. As an example,
the two collections of spectra recorded during the titration of
NMPI and NMPBr with host 4b have been gathered in Fig. 7.
The blue-shift experienced by the CT bands of these salts,
centred at λ = 365 and 320 nm for NMPI and NMPBr,
respectively, is accompanied in both cases by the formation of
Fig. 6. UV-Vis spectra of solutions of NMPI in CHCl3 at increasing concentration (the
absorbance profile of the 10-4 M solution has been multiplied five times to evidence the
band at 325 nm).
The spectrum of the 10−3 M solution (Fig. 6b) evidences
two charge transfer (CT) bands centred at λ = 370 and 290 nm.
Increasing the ion-pair concentration, the maximum position
2
of both bands shifts to higher energy. At concentration of 10−
M, the highest energy band (denoted with an asterisk in Fig.
6d) is barely recognised as a “shoulder” of an intense
absorption due to the
π
-
π
* transition of the pyridine ring.63
4
Contrariwise, both bands are red-shifted upon dilution to 10−
M. Such shift at lower energy was explained as an ion-pair
aggregation lessening. The results of the previous experiments
indirectly confirmed that the stoichiometry of the binding
between the ditopic hosts 4b-c and the series of N-
an isosbestic point at λ
≈ 300 nm (see Fig. 7).
Fig. 7. Collection of UV-vis spectra (275 – 450 nm expansion) taken during the titrations in CH2Cl2 solution of a) NMPI and b) NMPBr with calix[4]arene semitube 4b
.
Another barely visible isosbestic point is found λ
≈
365 nm for
The families of spectra collected during the titrations were
NMPBr (see Fig. 7b) and at λ = 460 nm (not shown) for NMPI, fitted with the SpecFit/32 software.66 In first instance, singular
evidencing that more than one binding equilibrium is present value decomposition (SVD) analysis was performed in order to
in solution. The shift of the CT band at higher energy can be estimate the number of species present in solution; then, a
explained considering that the interaction of the pyridinium non-linear fitting of the optical data with the binding equilibria
cation with the host aromatic cavity contributes to a loosening illustrated in equations (1) – (3) was used to calculate the
of the ion pairing and a general stabilization of the more polar binding constants. The results are gathered in Table 4 and it is
ground state of the charge transfer interaction generating this worth to observe that a good fitting of the optical data was
band.65
obtained only when the formation of 1:2 host-guest adducts
was considered (equation 3).Among the series of ditopic hosts
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Journal Name
4a-c, the highest values of log
β
were always calculated for 4b
,
on Bruker Avance instruments operating at 300 and 400 MHz
followed by 4c. Except in the case of NMPBr binding by 4c, the for 1H and 100 MHz for 13C. Chemical shift reported are
efficiency of these flexible hosts is significantly higher than the referred to residual solvent resonances. ESI-MS spectra were
rigid 4a. These results were explained considering that the recorded with Infusion Waters Acquity Ultra Performance LC
rigidity of 4a and the spatial proximity of its calix[4]arene sub- instrument. UV-Visible spectra were recorded with a Lambda
units negatively affects the simultaneous binding of two Bio 20 instrument. Melting points are uncorrected, and were
positively charged NMP cations. This hypothesis was further recorded with Electrothermal instrument. Silica gel layers
supported by the observation that also the logβ calculated for (SiO2, MERCK 60 F254) were used for thin layer
the longer and more flexible hosts 4b and 4c are never as chromatography (TLC). 60 Å silica gel (MERCK, 0.04-0.063 mm,
twice as the value of the constants calculated for the 230-240 mesh) were used for column chromatography. NMPX
. These results show that the and NOPX salts,25 compounds 2a 67 2b 68 3 69 5 70 658 were
corresponding monotopic host 5 , , , ,
formation of the 1:2 host – guest adduct is affected by a synthesised according to reported procedures. All other
negative cooperative effect, i.e. the binding constant reagents were of reagent grade quality as obtained from
governing the equilibrium (2) is always smaller than the one commercial suppliers and were used without further
governing equilibrium (1) (logK1:2 < logK1:1). Finally, it is worth purification.
to mention that even using the bulkier N-octyl pyridinium salts
Synthetic procedures
(NOPBr and NOPI) as guests, the calculated binding constants
are, within the experimental errors, very similar to those General procedure for the synthesis of calix[4]arene
determined for the NMPX series (see Table 4).
semitubes 4a-c. A solution of calix[4]arene 3 (3 g, 0.007 mol)
and K2CO3 (2 g, 0.014 mol) in CH3CN (150 mL) was refluxed
under vigorous stirring for 10 minutes. After this period, the
appropriate ditosylate 2a-c (0.014 mol) was added. The
resulting mixture was refluxed for further 3 days, cooled to
room temperature and evaporated to dryness under reduced
pressure. The residue was taken up with 50 mL of a 10% v/v
solution of HCl and CH2Cl2 (200 mL). The separated organic
Table 4. Overall binding constants (logβ and logK1:1) for the complexation of NMPX and
NOPX (with X = Br and I) ion pairs in CH2Cl2 solution with ditopic (4a-c) and monotopic
(5) calix[4]arene hosts.a
Host
4a
4b
4c
NMPBr
6.2(7)
8.1(3)
6.8(1)
4.3(1)
NMPI
5.0(1)
8.4(2)
8.4(2)
4.2(2)
NOPBr
5.0(1)
7.7(1)
7.2(1)
3.5(7)
NOPI
6.4(7)
8.2(2)
7.4(2)
4.1(1)
phase was washed thrice with water (3×25ml), dried over
anhydrous Na2SO4, and evaporated to dryness under reduced
5
a
Determined at room temperature through UV-vis spectrophotometric
pressure.
titrations.
Calix[4]arene semitube 4a: purification of the solid residue by
trituration with chloroform afforded 4a in 60% yield as a white
Conclusions
solid compound. m.p.>300 °C; 1H NMR (CD2Cl2, 300 MHz)
δ
In summary, we have synthesised three head-to-head
calix[4]arene semitubes (4a-c) characterised by two linking
alkyl chains of different length. The X-rays crystal structures of
two of these homoditopic receptors show different binding
properties towards one of the two solvent used for their
crystallization. The crystal structure of 4b, in which the two
calix[4]arene subunits are connected through two hexyl
chains, evidences the potential of these new ditopic hosts for
the construction supramolecular polymers. The binding
properties of these hosts have been evaluated in solution in
weakly polar media toward a series of pyridinium ion pairs.
NMR and UV-vis spectroscopy measurements show that hosts
are able to recognise simultaneously two guest ion-pairs.
Studies to insert on these hosts ancillary binding sites for the
simultaneous recognition of both the cation and its counter
anion as guests are undergoing in our laboratories.
(ppm): 8.90 (s, 4H, -OH); 7.11 (d, 8H, J = 7.5 Hz, ArH); 7.09 (d,
8H, J = 7.4 Hz, ArH); 6.89 (t, 4H, J = 7.5 Hz, ArH); 6.65 (t, 4H, J =
7.4 Hz, ArH); 4.46 (d, 8H, J = 12.8 Hz, Ar-CH2-Ar axial); 4.39 (t,
8H, J = 8 Hz, OCH2CH2-); 3.51 (d, 8H, J = 12.8 Hz, Ar-CH2-Ar
equatorial); 3.16 (q, 4H, J = 8 Hz, OCH2CH2CH2 ); 13C NMR
/
(CD2Cl2, 100 MHz): δ (ppm) 153.6; 151.9; 134.9; 129.6; 129.0;
129.0; 126.1; 120.0; 73.0; 32.2; 31.2; ESI-MS (m/z) = 951
[M+Na]+; elemental analysis for C62H56O8, calcd.: C 80.17, H
6.03; obs.: C 80.35, H 6.28.
Calix[4]arene semitube 4b: purification of the solid residue by
column chromatography (eluent: CH2Cl2/hexane
= 6/4)
afforded 4b in 30% yield as a white solid compound. m.p.> 300
1
°C; H NMR (CDCl3, 300 MHz):
δ (ppm) 8.45 (s, 4H, -OH); 6.99
(d, 8H, J = 7.5 Hz, ArH); 6.91 (d, 8H, J = 7.5 Hz, ArH); 6.74 (t, 4H,
J = 7.5 Hz, ArH); 6.62 (t, 4H, J = 7.4 Hz, ArH); 4.23 (d, 8H, J =
12.9 Hz, Ar-CH2-Ar axial); 4.03 (t, 8H, J = 6.7 Hz, OCH2(CH2)4-);
3.21 (d, 8H, J = 12.9 Hz, Ar-CH2-Ar equatorial); 2.9-2.8 (m, 8H,
OCH2CH2(CH2)2-); 2.0-1.9 (m, 8H, OCH2CH2(CH2)2-); 13C NMR
Experimental
(CDCl3, 100 MHz):
δ (ppm) 153.3; 151.7; 133.6; 128.8; 128.4;
General
128.0; 125.3; 118.8; 75.9; 31.2; 30.1; 24.9; ESI-MS (m/z) = 1035
[M+Na]+; m/z = 1052 [M+K]+; elemental analysis for C68H68O8:
calc. C 80.60, H 6.76; obs. C 80.91, H 7.13.
All reactions were carried out under nitrogen; all solvents were
freshly distilled under nitrogen and stored over molecular
sieves for at least 3h prior to use. NMR spectra were recorded
8 | J. Name., 2012, 00, 1-3
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Calix[4]arene semitube 4c: purification of the solid residue by where δobs
View Article Online
DOI: 10.1039/C6CE00268D
ARTICLE
,
δfree
,
δ
∞, [H]0, and [G]0 are the observed chemical
column chromatography (eluent: hexane/ethyl acetate = 7/3) shift of the proton under investigation in fast exchange
afforded 4c in 10% yield as a white solid compound. m.p.> 300 condition, the chemical shift of same proton in the non-bound
1
°C; H NMR (CDCl3, 300 MHz):
δ (ppm) 8.41 (s, 4H, -OH); 7.07 “free” salt and in the complex (guessed), the initial
(d, 8H, J = 7.5 Hz, ArH); 6.96 (d, 8H, J = 7.5 Hz, ArH); 6.78 (t, 4H, concentration of the guest and of the host for each titration
J = 7.5 Hz, ArH); 6.67 (t, 4H, J = 7.5 Hz, ArH); 4.34 (d, 8H, J = point, respectively. The conditions of the Weber parameter p
12.9 Hz, Ar-CH2-Ar axial); 4.07 (t, 8H, J = 6.3 Hz, OCH2(CH2)11-); ([conc. of the complex]/[maximum possible conc. of the
3.40 (d, 8H, J = 12.9 Hz, Ar-CH2-Ar equatorial); 2.2-2.0 (m, 8H, complex]) were verified for each titration experiment
OCH2CH2(CH2)8-); 1.8-1.7 (m, 8H, OCH2CH2CH2(CH2)6-); 1.6-1.2 according to the calculated binding constant.57 If necessary the
(4m, 24H, OCH2CH2CH2(CH2)6-); 13C NMR (CDCl3, 100 MHz):
δ concentrations of the two reactants were adjusted and the
(ppm) 153.3; 151.7; 133.6; 128.8; 128.4; 128.0; 125.3; 118.8; NMR titration experiment repeated to explore the proper p
75.9; 31.2; 30.6 (2 res.); 30.3 (2 res.); 25.0; ESI-MS (m/z) = 1204 range (0.2–0.8).
[M+Na]+; m/z = 1220 [M+K]+; elemental analysis for C80H92O8:
calc. C 81.32, H 7.85; obs. C 81.01, H 7.97.
UV-vis titrations. Optical spectroscopic titrations were carried
out in a quartz cuvette (path length = 1 cm), maintained at 300
K through an external thermostat, by adding small aliquots of a
Binding Studies
1H-NMR continuous variation methods (Job’s plot). Aliquots dichloromethane solution (c = 1 × 10–4 M) of the calix[4]arene
of stock solutions in CDCl3 of the calix[4]arene host and of the host to a dichloromethane solution (c = 2 × 10–5 M) of the
corresponding N-alkyl pyridinium organic salt were added to NMPX or NOPX (X = Br, I) ion pair. The spectral data were
several 5-mm NMR tubes in different ratios maintaining the collected in the 265–900 nm wavelength range. The binding
total molar concentration constant. In this way, 12-14 samples constants were calculated selecting different binding models
in which the mole fraction (x) of the interacting species was with the Specfit/32 software.66 The fitting of the spectral data
continuously varied from 0.1 to 1 were prepared. A 1H NMR was carried out considering the optical variation in the
spectrum was recorded for each sample. The corresponding sampled wavelength range. As for the NMR titrations, the
Job plot was obtained plotting a property proportional to the Weber parameter p was checked against the calculated
complex concentration, in this case (δobs-δfree)×[guest]0, versus binding constant.
the mole fraction of the salt. δobs and δfree represent the
chemical shift (ppm) of a resonance of the pyridinium cation in
the complex and in the free salt, respectively. The
stoichiometry of binding is obtained from the value of the
mole fraction in abscissa that yields the maximum of the plot.
For x = 0.5, the stoichiometry of binding is 1; for x = 0.66 the
stoichiometry of binding is H:G = 1:2
1H-NMR titrations. Solutions (500 μl) in CDCl3 of the organic
salt (usually c = 10–3 M) were prepared in a 5 mm NMR tube
and small aliquots of a tenfold concentrated solution of the
calix[4]arene host (usually c= 10-2 M) in CDCl3 were added. A 1H
NMR spectrum was recorded after each addition. Considering
a 1:1 host-guest binding equilibrium, the apparent binding
constant is expressed by the following equation:
where [H], [G], and [HG] are the concentrations of the
calix[4]arene receptor, of the organic salt and of the complex
at the equilibrium, respectively. The binding constants K were
calculated through the non-linear fitting of the chemical shift
variation of the protons of the organic salt using the equation
devised by Wilcox:57
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Table 5. Crystal data and experimental details for data collection and structure
Notes and references
refinement for 4a and 4b
.
¥ The lack of complexing properties in the solid state for ditopic
hosts 4b and 4c failures was tentatively explained considering
that the high flexibility of their bridging alkyl spacers should not
favour the formation of ordered units in the crystal lattice. For
the intrinsically more rigid host 4a, its poor solubility even in
dichloromethane, is reasonably the main cause of such failures
(see also NMR binding studies).
† The binding constants (logK1:1) are considered as apparent
because in the equation used for their calculations the reactants’
concentration is inserted in place of the more correct “activity”.
‡ The up-field shift of these resonances was never hidden by the
resonances of the host throughout the titrations.
§ In a previous work it has been shown that a cationic complex
between NMP and a monotopic calix[4]arene host self-assemble
in the solid state giving rise to clusters of six calixarene-NMPI
units in a columnar arrangement. In this structure the iodide
anions are interdigitated between the cationic complexes along
the same crystallographic axis (see Ref. 25).
§§ An apparent 1:1 binding stoichiometry could be also the
result of higher isodesmic stoichiometry of binding, such as: 2:2,
3:3 and so on.
Compound
Empirical formula
Formula weight
Crystal size [mm]
Crystal system
Space group
a [Å]
4a
C62H56O8•CH2Cl2
1098.985
0.2x0.3x0.3
Triclinic
4b
C68H68O8•2CHCl3
1252.036
0.3x0.4x0.3
Triclinic
P-1
P-1
10574(1)
15.785(1)
17.417(1)
88.866(2)
89.179(2)
75.592(2)
2815.0(5)
2
15.114(1)
19.254(1)
12.036(1)
103.11(1)
104.06(1)
102.42(1)
3170.8(5)
2
b [Å]
c [Å]
α
β
γ
[°]
[°]
[°]
V [Å3]
Z
ρ
(calcd.) [g/cm3]
F(000)
1.297
1.311
1152
1312
T [K]
298
298
λ
[Å]
.71069
1.54178
2.915
ꢁ
[mm-1]
0.266
1
2
3
G. M. Whitesides and B. A. Grzybowski, Science, 2002, 295,
2418–2421.
Index ranges
-13 ≤ h ≤13
-20 ≤ k ≤ 20
-22 ≤ l ≤ 22
36740
-18 ≤ h ≤ 8
-18 ≤ k ≤ 23
-14 ≤ l ≤ 14
12648
G. M. Whitesides and M. Boncheva, Proc. Natl. Acad. Sci.,
2002, 99, 4769–4774.
Refl. collected
Independent. refl.
Obs. reflections
parameters
R1
12371 (Rint = 0.045)
6573 Fo≥4σ(Fo)
689
11963 (Rint = 0.154)
6108 Fo≥4σ(Fo)
762
J. W. Steed, D. R. Turner and K. Wallace, Core Concepts in
Supramolecular Chemistry and Nanochemistry, John Wiley
& Sons, Ltd, Chichester (UK), 2007.
0.0998
0.109
4
5
6
A. Ciferri, Ed., Supramolecular Polymers, CRC Press, Boca
Raton, 2005.
wR2
0.341
0.35
Goodness-of-fit on [a]
1.112
1.119
[a] R1
=
2
Σ || F0|
-
| Fc||
/
Σ | F0|
,
wR2 = [
Σ
w(F0
-
Fc2)2/
Σ
w
F0
]
1/2. Goodness-of-fit = [
the number of
Σ
2
4
A. Harada, Ed., Supramolecular Polymer Chemistry, Wiley-
VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2011.
w(F0
-
Fc2)2/ (
n
-p
)] 1/2, where
n
is the number of reflections and
p
parameters.
T. F. A. De Greef, M. M. J. Smulders, M. Wolffs, A. P. H. J.
Schenning, R. P. Sijbesma and E. W. Meijer, Chem. Rev.,
2009, 109, 5687–5754.
Crystallography. Crystallographic data for 4a and 4b were
collected on a Siemens AED diffractometer using graphite
7
L. Brunsveld, B. J. B. Folmer, E. W. Meijer and R. P.
Sijbesma, Chem. Rev., 2001, 101, 4071–4098.
monochromatic CuKα radiation and were summarised in Table
5. The intensities were corrected for Lorentz, polarization and
absorption effects. The structures were solved by direct
methods using SIR200471 and refined on F2 by full-matrix least-
squares methods, using SHELXL-97.72 All the non-hydrogen
atoms were refined with anisotropic atomic displacements.
For 4b the structure contains two CHCl3 solvent molecules for
each calix[4]arene semitube sub-unit. All the non-hydrogen
atoms were refined with anisotropic atomic displacements.
The hydrogen atoms were included in the last cycles of the
refinement at idealised geometry (C-H and O-H 0.96 Å) and
refined in the “riding” model with isotropic atomic
displacements 1.2 times their Ueq their parent atoms.
Geometric calculations were performed with the PARST97.73
8
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Acknowledgements
The authors thank the Centro Interdipartimentale di Misure of
the University of Parma for NMR and MS measurements. This
work was supported by the Italian MIUR (2010CX2TLM) and by
the University of Parma.
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