Upper rim site lipophilic calix[4]arenes as receptors for natural terpenes and functionally related solvent molecules: Combined crystal structure and QMB sensor study

Article abstract of DOI:10.1039/c0ce00696c

Three upper rim site lipophilic calix[4]arenes 1-3 featuring different para-alkanoyl substituents at the phenolic moieties, i.e.n-hexanoyl, n-octanoyl and 3-cyclohexylpropanoyl groups, are reported to complex, aside from the common solvent molecule n-buta

Full text of DOI:10.1039/c0ce00696c

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Upper rim site lipophilic calix[4]arenes as receptors for natural terpenes and
functionally related solvent molecules: combined crystal structure and QMB
sensor study†‡
a
a
a
b
a
*
Tobias Gruber,x Conrad Fischer, Wilhelm Seichter, Petra Bombicz and Edwin Weber
Received 17th June 2010, Accepted 20th October 2010
DOI: 10.1039/c0ce00696c
Three upper rim site lipophilic calix[4]arenes 1–3 featuring different para-alkanoyl substituents at the
phenolic moieties, i.e. n-hexanoyl, n-octanoyl and 3-cyclohexylpropanoyl groups, are reported to
complex, aside from the common solvent molecule n-butanol, different keto and hydroxylic terpene
guest molecules such as (ꢀ)-menthone, (ꢀ)-menthol or (+)-carvone in the solid state. Although for all
six inclusion compounds described here, a strict 1 : 1 host : guest stoichiometry is observed,
complementary size and polarity relationships between the cone shaped calixarene cavity and the guest
molecule emerge from single crystal X-ray structural study. Dependent on the structure of the guest
molecule, the calixarenes are arranged in a head-to-head or head-to-tail orientation in the crystalline
packing, giving rise to capsular or otherwise deeply enclosed inclusion mode of the guest molecules,
respectively. Furthermore, the O-acyl atoms of the calixarenes were found to be directly involved in
host–guest interaction. In order to estimate the degree of isostructurality of the host frameworks and to
examine the influence of the guest molecules on the solid-state conformation of these calixarene
molecules, isostructurality comparison, cell similarity and molecular isometricity calculations were
carried out revealing a rare case of supramolecular morphotropism for the pair of (ꢀ)-menthone and
(ꢀ)-menthol inclusions of the n-hexanoyl substituted calixarene. Structure–inclusion property
relationship was examined by QMB measurements of thin layers of the calixarenes. Vapor studies with
different terpenes and common organic solvents show an increased affinity towards guest molecules
with polar functionalities, whereas small solvent molecules like acetone are bound considerably better
than the more bulky terpene molecules.
by different guest species.5 Aside from the conformational
mobility of the chalice the complexation behaviour seems to be
determined by the nature of the substituents attached to the
upper rim site.6 Thus, calix[4]arenes that carry lipophilic alka-
noyl residues at the upper rim should prefer a cone conformation
and are expected to form inclusion compounds with relatively
large lipophilic molecules such as biologically interesting
terpenes. However, so far this has been proven by X-ray struc-
tural analysis only in a single case where a hexanoyl substituted
calix[4]arene and (+)-carvone were used.7 On the other hand,
correspondingly modified alkanoyl calix[4]arenes were shown to
include a number of other organic guest molecules,7–11 among
them radicals,12–14 allow guest exchange,15 are capable of forming
Langmuir–Blodgett layers,16 assist topochemical photore-
actions,17 and have proven successfully as adsorbing substance
for gases.18 They have also been studied in view of the structural
effects dependent on the length of the acyl chains.19,20 Three
general types of inclusion structures were found, being addi-
tionally controlled by the size and polarity of the guest as well as
the crystallization conditions, that is an open container or head-
to-tail complex, a nanocapsular or tail-to-tail complex and
a partially self-included or interdigitated complex, where the role
of the guest is played by another calixarene molecule.10 Consid-
ering the importance of terpenes with regard to their medicinal,
cosmetic, odorous and flavouring aspects,21 a more profound
study of the host behaviour of alkanoyl substituted calix[4]arenes
toward the terpenes is worthwhile, for instance concerning their
Introduction
After three productive decades, calixarenes continue to be
a stimulating class of compounds with the calix[4]arene as the
typical example.1 One of its very attractive features is the cuplike
structure provided by this molecule both in the solid state and in
solution, from which this compound family derives the name and
which is capable of hosting neutral guest molecules of comple-
mentary size. For the well-known conformationally fixed p-tert-
butylcalix[4]arene the feature of gas-storage and diffusion in the
crystalline state has been studied in detail2–4 and some of
the resulting nonporous solids have been proven to form
a well-defined stoichiometric 1 : 1 host–guest inclusion induced
aInstitut fu€r Organische Chemie, Technische Universita€t Bergakademie
Freiberg, Leipziger Str. 29, D-09596 Freiberg, Sachs., Germany. E-mail:
edwin.weber@chemie.tu-freiberg.de; Fax: +49 3731 392386
bInstitute of Structural Chemistry, Chemical Research Center, Hungarian
Academy of Sciences, P. O. Box 17, H-1525 Budapest, Hungary
ꢀ
ꢀ
† This paper is dedicated to Prof. Alajos Kalman on the occasion of his
75th birthday, who performed prominent work in the fields of
isostructurality, morphotropy and polymorphy.
‡ Electronic supplementary information (ESI) available: Additional
structural data of the compounds studied by X-ray structural analysis
(Tables S1 and S2) and isostructural calculations (Fig. S1–S6). CCDC
reference numbers 780763–780768. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0ce00696c
x Present address: University of Oxford, Department of Chemistry,
Chemistry Research Laboratory, Mansfield Road, Oxford OX1 3TA,
UK.
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the tert-butyl groups,28 and
a
subsequent Friedel–Crafts
acylation with the respective acid chloride following a literature
procedure.29
Crystallization of 1–3 in solutions of the corresponding guest,
or in a mixture with a cosolvent or from a melt of the guest, as
specified in the Experimental section, afforded the inclusion
compounds 1a, 1b, 2a, 2b, 3a and 3b, respectively (Scheme 1). A
number of other potential guest compounds, which were also
tested with reference to inclusion formation of the calixarenes 1–
3, proved ineffective. Within the scope of the naturally occurring
terpenes, they involve (+)-pulegone, (ꢀ)-fenchone and
(+)-camphor for all three calixarenes, as well as (+)-carvone for
1, (ꢀ)-menthol for 2, (ꢀ)-menthone and (ꢀ)-menthol for 3. This
indicates a distinct selectivity behaviour of the calixarenes. For
instance (+)-pulegone, not being included, is close to the
monocyclic structures of (ꢀ)-menthone or (+)-carvone, while
(ꢀ)-fenchone and (+)-camphor, also failing in the inclusion, are
more bulky bicyclic terpenes.
Crystallographic studies
The inclusion compounds (six species) of the para-alkanoyl
calix[4]arenes 1–3, namely 1$(ꢀ)-menthone (1 : 1) (1a), 1$(ꢀ)-
menthol (1 : 1) (1b), 2$(ꢀ)-menthone (1 : 1) (2a) and 2$(+)-
carvone (1 : 1) (2b), 3$(+)-carvone (1 : 1) (3a), 3$n-BuOH
(1 : 1) (3b), have been characterized by single-crystal X-ray
diffraction. A common atom labelling diagram for the calix-
arene framework including specification of the arene rings as
well as geometrical parameters describing the conformation of
the alkanoyl residues are given in Scheme 2. Crystallographic
data and selected refinement parameters of the structures are
summarized in Table 1. Perspective views of the molecular
structures are shown in Fig. 1, 3 and 6 and 7 whereas selected
crystal packings are illustrated in Fig. 2, 4, 5 and S1–S6‡. In
general, the conformation of the calixarene framework (Table
2) can be described by a set of interplanar angles which defines
the inclination of the aromatic rings A–D with respect to the
reference plane that is given by the methylene atoms C(7),
C(14), C(21) and C(28) (Scheme 2(a)). These angles correlate
with the dihedral angles between pairs of opposite aromatic
rings A/C and B/D, respectively. Corresponding data of
Scheme 1 Compounds and complexes studied in this paper (a); included
guest molecules (b).
protection and storage22,23 or chemical sensing using an appro-
priately modified calixarene.24
Here we report on the synthesis of the upper rim tetraalkanoyl
modified calix[4]arenes 1–3 (Scheme 1) of which the 3-cyclo-
hexylpropanoyl substituted derivative 3 is a new compound.
Inclusion behaviour of these lipophilically engineered calixarenes
towards naturally occurring keto and hydroxylic terpenes as
guest molecules is tested, giving rise to the formation of the
crystalline complexes 1a, 1b, 2a, 2b and 3a, the X-ray crystal
structures of which are described and comparatively discussed.
In the case of 3, a solvent inclusion with common n-BuOH is also
reported, enabling further comparison just as with the previously
known inclusion complexes of 1 and 2. To reveal the extent of
isostructurality of the present host frameworks in the solid state
influenced by the host substituents and the quality of the guest,
isostructurality analysis, cell similarity and molecular iso-
metricity calculations25 were performed. One obvious aim in this
respect was to examine how far a crystal structure can tolerate
small changes of either host or guest molecules. Moreover, we
demonstrate reversible adsorption and desorption properties of
the solid calixarenes 1–3 to both vaporous terpene and small
solvent molecules using a QMB measurement technique.26
Results and discussion
Synthesis and crystalline inclusion formation
The tetraacyl substituted calix[4]arenes 1–3 (Scheme 1) were
prepared in two steps with conventional tetra-tert-butylcalix[4]-
arene as the starting compound.27 These include dealkylation of
Scheme 2 Labelling diagram of atoms and marking of arene rings of the
calixarene core (a); specification of the torsion angles (s) of the alkanoyl
residues (b).
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Table 1 Crystallographic and structure refinement data of the compounds studied
Compound
1a
1b
2a
2b
3a
3b
Empirical formula
C52H64O8$C10H18O C52H64O8$C10H20O C60H80O8$C10H18O C60H80O8$C10H14O C64H80O8$C10H14O C64H80O8$C4H10O
Formula weight/g molꢀ1 971.28
973.29
Monoclinic
P21
1083.48
Monoclinic
P21
1079.45
Monoclinic
P21
1127.49
Monoclinic
P21
1051.40
Monoclinic
P21/n
Crystal system
Space group
Monoclinic
P21
ꢁ
a/A
15.7432(3)
22.1360(4)
15.9373(3)
90.00
15.6344(6)
22.1572(7)
15.8763(5)
90.00
15.6509(4)
25.0523(6)
15.9443(4)
90.00
15.6292(3)
23.7075(5)
16.6136(4)
90.00
15.9123(11)
24.6729(16)
16.1465(10)
90.00
15.6436(4)
23.5263(5)
15.9163(3)
90.00
ꢁ
b/A
ꢁ
c/A
a/ꢁ
b/ꢁ
g/ꢁ
V/A
Z
F(000)
90.294(1)
90.00
91.049(2)
90.00
90.258(1)
90.00
92.02(1)
90.00
90.295(4)
90.00
91.356(1)
90.00
3
ꢁ
5553.94(18)
4
2104
5498.9(3)
4
2112
6251.6(3)
4
2360
6152.0(2)
4
2344
6339.1(7)
4
2440
5856.1(2)
4
2280
Dc/Mg mꢀ3
m/mmꢀ1
1.181
0.076
1.176
0.077
1.151
0.074
1.165
0.075
1.181
0.076
1.192
0.077
Data collection
Temperature/K
No. of collected
reflections
Within the q-limit (ꢁ)
Index ranges
ꢂh, ꢂk, ꢂl
133(2)
100 570
153(2)
109 553
123(2)
112 683
103(2)
108 464
193(2)
122 207
90(2)
55 044
1.3–27.4
ꢀ20/20,
ꢀ28/28,
ꢀ20/20
12 958
1.8–26.4
ꢀ19/19,
ꢀ27/27,
ꢀ19/18
11 557
1.3–27.4
ꢀ18/20,
ꢀ32/32,
ꢀ20/19
14 540
1.2–26.3
ꢀ19/19,
ꢀ29/29,
ꢀ20/20
24 972
1.3–25.8
ꢀ16/19,
ꢀ30/30,
ꢀ19/19
12 480
1.5–26.1
ꢀ10/19,
ꢀ29/29,
ꢀ19/19
11 587
No. of unique
reflections
Refinement calculations: full-matrix least-squares on all F2
Weighting
expression wa
No. of refined
parameters
1342
1329
7972
1446
1479
1506
8208
700
No. of F values
used [I > 2s(I)]
Final R-indices
R(¼S|DF|/S|Fo|)
wR on F2
11 174
10 453
18 102
8651
0.0555
0.1542
1.057
0.0524
0.1500
1.015
0.0433
0.1126
1.040
0.0862
0.2346
1.043
0.0710
0.2093
1.065
0.0527
0.1402
1.065
S(¼goodness
of fit on F2)
ꢀ3
ꢁ
Final Drmax/Drmin/e A 0.54/ꢀ0.33
0.55/ꢀ0.34
0.35/ꢀ0.28
0.55/ꢀ0.57
0.50/ꢀ0.32
0.55/ꢀ0.44
a
P ¼ (Fo2 + 2Fc )/3.
2
Fig. 1 Perspective views of the asymmetric units of (a) the complex
1$(ꢀ)-menthone (2 : 2) (1a) and (b) the 1$(ꢀ)-menthol complex (2 : 2)
(1b). Thermal ellipsoids are drawn at 40% probability level. For clarity
only one position of the disordered parts of the alkanoyl fragments is
shown. Broken lines represent hydrogen bonds.
Fig. 2 Packing diagram of the 2 : 2 complex 1$(ꢀ)-menthone (1a)
viewed down the crystallographic c-axis. Calixarene molecules are dis-
played as wire model, menthone molecules as ball and stick model.
Broken lines represent hydrogen bonds.
Results of the isostructurality study are specified in Tables 3
and 4, and illustrated in Fig. S1–S6‡.
geometric parameters describing the conformation of the
alkanoyl branches are summarized in Table S1‡. Information
regarding hydrogen bond geometries is listed in Table S2‡.
In the present crystal structures, the calixarene skeleton itself is
perfectly ordered whereas the upper rim substituents show higher
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Fig. 3 Perspective views of the asymmetric unit of (a) the complex
2$(ꢀ)-menthone (2 : 2) (2a) and (b) 2$(+)-carvone (2 : 2) (2b). Thermal
ellipsoids are drawn at 40% probability level. For clarity only one posi-
tion of the disordered parts of the alkanoyl fragments is shown. Broken
lines represent hydrogen bonds.
Fig. 5 Packing diagram of the 2 : 2 complex 2$(+)-carvone (2b) viewed
down the crystallographic c-axis. Calixarene molecules are displayed as
wire model, carvone molecules as ball and stick model. Broken lines
represent hydrogen bonds.
Fig. 4 Packing diagram of the 2 : 2 complex 2$(ꢀ)-menthone (2a)
viewed down the crystallographic c-axis. Calixarene molecules are dis-
played as wire model, menthone molecules as ball and stick model.
Broken lines represent hydrogen bonds.
Fig. 6 Perspective view of the asymmetric unit of the complex 3$(+)-
carvone (2 : 2) (3a). Thermal ellipsoids are drawn at 40% probability
level. Broken lines represent hydrogen bonds.
displacement parameters or their terminal carbon atoms are
disordered over two positions even at low temperatures.
Considering the isostructurality investigation, the atoms with the
higher site occupation factor were selected for the respective
calculations. For the calixarene–terpene complexes the statistical
analysis of E-values indicates crystallographic inversion
symmetry which, however, is inconsistent with the presence of an
enantiomerically pure guest species. A conclusive structure
model is represented by the monoclinic space group P21. This
means that the asymmetric entity of the unit cell comprises two
crystallographically independent 1 : 1 host–guest complex units.
According to the presence of an intramolecular cyclic hydrogen
bonding system formed by the phenolic hydroxyl groups, the
calixarene molecules are fixed in the cone conformation. The
inclusion compounds of the p-hexanoyl substituted calix[4]arene
1 with (ꢀ)-menthone (1a) and (ꢀ)-menthol (1b) are characterized
by nearly identical cell parameters which suggests similarities
regarding calixarene geometry and packing behaviour. As shown
in Fig. 1, the basic structure elements of 1a and 1b consist of pairs
of calixarene molecules being arranged in a tail-to-tail fashion to
Fig. 7 Perspective view of the complex 3$n-BuOH (1 : 1) (3b). Thermal
ellipsoids are drawn at 40% probability level. Broken lines represent
hydrogen bonds.
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Table 2 Selected conformational parameters of the cone frameworks
Compound
1a
1a(0)
1b
1b(0)
2a
2a(0)
2b
2b(0)
3a
3a(0)
3b
Interplanar anglesa/ꢁ
mplab/A
mpla/B
mpla/C
mpla/D
A/C
55.7(1)
57.8(1)
57.2(1)
54.9(1)
60.5(1)
67.4(1)
62.3(1)
58.2(1)
59.7(1)
57.2(1)
58.3(1)
58.7(1)
62.0(1)
64.1(1)
50.2(1)
59.2(1)
56.0(1)
57.3(1)
59.9(1)
63.5(1)
64.1(1)
57.3(1)
60.5(1)
58.0(1)
58.1(1)
54.1(1)
61.4(1)
67.9(1)
64.3(1)
60.0(1)
58.3(1)
58.2(1)
54.1(1)
61.8(1)
67.6(1)
63.6(1)
49.5(1)
61.3(1)
49.4(1)
61.9(1)
81.1(1)
56.8(1)
83.4(2)
50.1(1)
60.2(1)
50.1(1)
62.5(1)
79.8(1)
57.3(1)
77.4(2)
50.8(1)
61.6(1)
54.4(1)
57.0(1)
74.8(1)
61.4(1)
82.8(2)
52.6(1)
59.7(1)
52.3(1)
59.8(1)
75.1(1)
60.5(1)
74.1(2)
55.1(1)
62.1(1)
56.2(1)
59.2(1)
68.7(1)
58.7(1)
57.0(1)
56.9(1)
59.9(1)
67.5(1)
63.1(1)
80.0(1)
B/D
mplac/guestd
a
Aromatic rings: ring A: C(1)/C(6); ring B: C(8)/C(13); ring C: C(15)/C(20); ring D: C(22) /C(27). b Best plane through atoms C(7), C(14), C(21)
c
d
and C(28). For (0)-molecules: best plane through atoms C(7A), C(14A), C(21A) and C(28A). Mean plane through the six-membered ring of the
terpene guest.
Table 3 Cell similarity indices (p) calculated for 1a–3b
in hydrogen bonding. Irrespective of some differences regarding
the conformation of the alkanoyl substituents, the packing mode
of the calixarene molecules in 1a (Fig. 2) and 1b is nearly iden-
tical, which will be discussed in detail in the Isostructurality
studies section. The crystal structures are both stabilized by
a close network of intermolecular interactions with the carbonyl
oxygen atoms acting as acceptor sites for ordinary and bifurcated
1b
2a
2b
3a
3b
1a
1b
2a
2b
3a
0.00674
—
—
—
—
0.05022
0.05658
—
—
—
0.02926
0.03577
0.02206
—
0.05144
0.05779
0.00129
0.02338
—
0.01774
0.02432
0.03306
0.01173
0.03552
—
ꢁ
C–H/O hydrogen bonds [d(C/O) 3.239(4)–3.568(4) A (1a),
3.314(4)–3.501(5) (1b)].
form molecular capsules. A structural feature worth mentioning
is the asymmetric arrangement of the guest molecules within the
cavity of the calixarene dimer. In one of the 1 : 1 complex units,
the isopropyl residue of the terpene is included into the aromatic
cavity of the calixarene whereas in the second complex unit the
methyl group is the included residue. Similar structural features
are also found in the related (+)-carvone complex of 1.7 Within
the capsular complex, the guest molecules are aligned in a head-
to-tail mode along the crystallographic b axis, showing different
degrees of inclination with respect to the reference plane of the
calixarene.
In order to elucidate in which way an increase of the alkanoyl
chain length influences solid-state structures in general, we were
able to determine the crystal structures of 2a and 2b obtained
from the octanoyl substituted calixarene 2 with (ꢀ)-menthone
and (+)-carvone, respectively. In each of the 1 : 1 host–guest
entities of 2a, the methyl group of the terpene molecule is located
near the aromatic core of the calixarene whereas its isopropyl
part is directed out of the cavity (Fig. 3(a)). The host–guest
complex is stabilized by C–H/p(arene) contacts31 and relatively
strong C–H/O hydrogen bonds30 (3.087(6), 2.587(5) A) with the
ꢁ
carbonyl oxygens O(1G) and O(1H) acting as acceptors. Also in
the complex structure of 2b, which is depicted in (Fig. 3(b)) the
asymmetric unit comprises two non-equivalent 1 : 1 host–guest
complexes, which are similar in terms of the molecular confor-
mation.
Unlike complex 1a, which lacks directed non-covalent host–
guest interactions and thus seems to be primarily stabilized by
van der Waals forces, the donor–acceptor nature of the menthol
molecules in 1b allows host–guest binding via O–H/O [d(O/O)
ꢁ
ꢁ
3.138(4), 3.187(4) A] and C–H/O [d(C/O) 3.371(4) A]
hydrogen bonds.30 However, maximum saturation of hydrogen
bond formation is fulfilled only by one of the menthol molecules
whereas in the second one only the hydroxyl hydrogen takes part
In both inclusion compounds of 2, the calixarene molecules are
packed in a columnar fashion along the b axis accompanied by
an elongation of the cell in this direction compared to the
complexes of 1 (Table 1). In contrast to the menthone inclusion
Table 4 Molecular isometricity values I(m) (%)
Same host, different guests
Same guest, different hosts
Compared compounds
Number of atoms fitted
I(m) (%)
Compared compounds
Number of atoms fitted
I(m) (%)
1a(1)–1a(2)
1b(1)–1b(2)
1a(1)–1b(1)
1a(2)–1b(2)
2a(1)–2a(2)
2b(1)–2b(2)
2a(1)–2b(1)
2a(2)–2b(2)
3a(1)–3a(2)
3a(1)–3b(1)
3a(2)–3b
60
60
60
60
68
68
68
68
72
72
72
63.17
27.48
24.68
72.13
21.93
48.99
21.50
19.54
68.97
82.78
60.40
1a(1)–2a(1)
1a(1)–2a(2)
1a(2)–2a(1)
1a(2)–2a(2)
2b(1)–3a(1)
2b(2)–3a(1)
2b(1)–3a(2)
2b(2)–3a(2)
60
60
60
60
48
48
48
48
53.24
16.59
77.54
40.37
63.93
70.60
69.53
63.42
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2a, adopting the classical head-to-tail mode (Fig. 4), the respec-
tive carvone including structure 2b shows a closer packing of
calixarene molecules because of interpenetrated branches of the
long octanoyl residues—probably due to the unsaturated char-
acter of carvone with its lack in four hydrogen atoms in
comparison to menthone and adopting a more planar geometry
(Fig. 5).
inclusion structures of compound 1 with respect to the tail-to-tail
assembly mode (Fig. S6‡).
Isostructurality studies
In principle, the crystal structures of the present inclusion
compounds exhibit pronounced similarities, but also distinct
differences. All synthesized inclusion crystals have similar unit cell
dimensions (Table 1) and the cell similarity indices (p)32 are low
indicating a high level of metric correspondence (Table 3).
Although the crystallographic host : guest stoichiometry 2 : 2 and
the space group P21 are identical for 1a, 1b, 2a, 2b and 3a even so
only some of them are homostructural, e.g. isostructural con-
cerning the host framework. The main difference in the packing
arrangements (Fig. S1–S6‡) is that the two crystallographically
independent calixarene molecules in the asymmetric unit can be
found in the same (1a and 1b) or in different (2a, 2b and 3a)
twofold screw axes. In the first case, the head-to-head arrange-
ment forms capsular dimers, the crystallographically independent
host molecules are alternating along the crystallographic b-axis.
In the second case, along the crystallographic b-axis head-to-tail
columnar arrangement is realized: one column consists of one of
the crystallographically independent molecules only.
In 2b, two alkyl chains of each calixarene molecule are inserted
into the cavities of calixarene molecules of adjacent columns, so
that the complex 2b can be considered as being self-included—
the interdigitation of calixarene molecules in 2b hardly affects the
location of guest molecules within the calixarene cavity and the
mode of host–guest interaction.
The structure of the host–guest complex of the p-(3-cyclo-
hexylpropanoyl) substituted calix[4]arene 3 with (+)-carvone (3a)
(Fig. 6) resembles those of 1a and 1b. However, an appreciable
difference is given by the way the carvone molecules are
accommodated inside the calixarene cavity. In both 1 : 1 complex
units of 3a, the carbonyl group of the guest molecule points out
of the calixarene cavity, so that the complex is stabilized by weak
C–H/p(arene) contacts31 formed between the isopropenyl part
of the carvone molecules and aromatic rings A and C (C–H/
ꢁ
centroid 2.80–2.92 A). These findings remarkably differ from the
crystal structure of the related 1$(+)-carvone complex described
in a previous report7 which is composed of capsular calixarene
dimers with a unique orientation of the included guests.
Furthermore, molecular isometricity [I(m)] calculations25 were
performed in order to examine the effect of the change in the host
substituents and the nature of the guest molecules (Table 4). The
molecular geometry of the two crystallographically independent
calixarene molecules in the respective asymmetric unit changes
less in 1a and 3a compared to all the others. As expected, the
largest difference occurs in the case of the n-octanoyl host
substitution—the longer alkyl chain has more freedom of
motion. In contrast, the chalice in the 3-cyclohexylpropanoyl
substituted compound is the most rigid one among the three
types of hosts. Its molecular geometry changes the least with the
change of the inclusion guest molecule. Comparing the molecular
geometry of the (ꢀ)-menthone inclusion complexes 1a and 2a,
there is a considerable difference in the host conformation in
spite of the relatively small difference in the length of the n-
hexanoyl or n-octanoyl substituents. The molecular geometry of
the host is more similar compared to n-octanoyl and 3-cyclo-
hexylpropanoyl substitution in the (+)-carvone inclusion
complexes (2b and 3a, respectively) which is also reflected in the
packing arrangement.
Thus, modification of the alkanoyl substituents as in 3a
induces significant changes in the complex structure and prevents
formation of molecular capsules. Similar to the menthone
inclusion 2a, the crystal structure of 3a is characterized by
a columnar packing of the calixarene complexes in the head-to-
tail mode along the b-axis (Fig. S5‡). The equidistance of the
guest oxygens O(1G) and O(1H) to the phenolic oxygens of the
ꢁ
neighbouring calixarene molecule [d(O/O) 2.987(6)–3.010(6) A]
suggests that the hydroxyl hydrogens participate in host–guest
binding. Interestingly, although the complexes 2b and 3a both
contain (+)-carvone as the guest molecule, their packing struc-
tures reveal different modes of interaction between the hydro-
phobic alkyl residues.
Using n-BuOH as a solvent, the calixarene 3 yields as
colourless crystalline blocks crystallizing in the monoclinic space
group P21/n with the asymmetric unit containing one host–guest
complex of 3b, the structure of which is displayed in Fig. 7. In the
complex 3b, the calixarene adopts a highly unsymmetrical
geometry with one alkanoyl residue being in a fully extended
conformation (torsion angles s1–s3 anti) whereas other two are
considerably distorted around their acyl function (torsion angle
s1 gauche). The alcohol molecule is coordinated to one of the
carbonyl oxygens by a weak conventional hydrogen bond
At first sight, the packing arrangements of the host mole-
cules in 1a and 1b look rather similar. However, there is
ꢁ
ꢁ
[O(1G)–H(1G)/O(5) 2.36 A, 141.4 ].
Host–guest aggregates in a 2 : 2 ratio with the calixarenes
being arranged in a tail-to-tail fashion represent the basic
structure motif of the crystal. In the capsular structure of 3b, the
calixarene molecules are shifted such that the cyclohexyl part of
one of their alkanoyl branches is located above that of the
respective opposite calixarene molecule thus creating a nearly
closed calixarene cavity. The packing structure of 3b is stabilized
by a variety of intermolecular C–H/O]C hydrogen bonds of
Fig. 8 The space groups P21 and P21/n, respectively, viewed along the
crystallographic c-axis indicating the existing symmetry elements and the
virtual twofold axes which organize the morphotropic relation between
the morphotropic pairs.
ꢁ
different strengths [d(H/O) 2.29–2.60 A] and is similar to the
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a substantial change in the placement of the molecules in the
unit cell thus in packing relations demonstrated by the
presentations of the cells from the crystallographic c-axis
(Fig. S1 and S2‡): every second layer is slightly moved to
ꢀa (1a) or +a (1b) direction. The two structures cannot be
transformed to each other by crystallographic symmetries
determined by the space group without an enantiomeric
change. Structure 1a could be transferred to the arrangement
of 1b, if the two molecules in the asymmetric unit were rotated
3
by a virtual twofold axes parallel to b in a and c at ¼ and ⁄4
(Fig. 8). However, there is no twofold axis in the space group
P21. Thus, the host positions in 1a and 1b complexes are
related by a pseudo-twofold axis, i.e. the host frameworks have
a morphotropic relation to each other.33,34
Enhancing the supramolecular aspects of polymorphy, the
present pair of (ꢀ)-menthone and (ꢀ)-menthol inclusions of the
n-hexanoyl substituted calix[4]arene is the second reported
example of supramolecular morphotropism. The first described
example was the upper rim dinitro substituted and lower rim
ethyl ester annexed calix[4]arene molecule crystallized from
selected polar protic and aprotic solvents: acetone, DMF,
DMSO and n-BuOH as guests.35 All these four inclusion
compounds have similar cell parameters and identical space
group symmetries (P21/n). Nevertheless, the inclusion of the
protic n-BuOH molecule does not only affect the host confor-
mation, but it also leads to a substantial change of the packing
relations. The location of the calix[4]arene molecule in the unit
cell of the previous n-BuOH inclusion compound is related to the
host position in all other three complexes by a pseudo-twofold
axis in the space group P21/n (Fig. 8). Obviously, the mode of
guest recognition seems to seriously affect the crystal structures.
As in the presently reported series of the inclusion compounds,
also the n-BuOH inclusion complex 3b crystallizes in the different
space group P21/n while all others crystallize in P21. In other
words, some molecular changes (either host, guest or host–guest)
can be tolerated by the crystal structure. When the tolerance has
reached the limit, packing motifs may still be kept but the motifs
are moved related to each other. This morphotropic link can be
described by non-crystallographic motions.33,34 The non-crys-
tallographic movements are the possible outcome of the
requirement of close packing, since rotation or migration of the
common motif may transform one specific pattern into another
with denser packing.
Fig. 9 Relative change of QMB sensor mass for compounds 1–3 against
the flow of selected terpenes (a) and solvents (b).
DFmax ꢀ DFref
Dm ð%Þ ¼
ꢃ 100%
DFref
All experiments involving treatment of the calixarenes with the
common solvent vapours show a considerably higher mass
increase than for the terpenes. The highest vapour uptake of all
was found for the para-octanoyl calixarene 2 with acetone (17%).
Excepting n-butanol, dioxane and limonene (both enantiomers),
the total amount of guest uptake is always the largest for the
long-chain calixarene 2. The compound 1 is most efficient in the
adsorption of n-butanol. On the other hand, the bulky cyclohexyl
residues at the upper rim site of the calixarene 3 suggest distinct
limitation of the guest uptake with exceptions for n-butanol and
limonene which are rather equally or even better adsorbed with 3
compared to 2, respectively. With reference to the polarity of the
studied analyte molecules, there is an evident preference of the
calixarene 2 featuring long alkyl chains for the adsorption of
the apolar hydrocarbons cyclohexane and cyclohexene while
Organic vapour sorption
In view of a potential use of the present lipophilic calixarenes for
vapour sensing of terpenes, the vapour adsorption properties of
the solid calixarenes were studied by means of a quartz micro-
balance device.26,36 The results obtained with different terpenes
(Fig. 9(a)) were compared with a similar QMB experiment using
the common organic solvents, cyclohexane, cyclohexene,
acetone,37 dioxane and n-butanol, which feature functionalities
and sizes comparable to the particular terpenes (Fig. 9(b)). In
order to better demonstrate the amount of guest uptake, the
maximum frequency change after loading (DFmax) was converted
to the relative mass increase of the gold sensor following the
dependency:
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Table 5 Binding stoichiometry (S) and host affinity (ah) of compounds
1–3 with selected terpenes and solvents
dependent on the nature of the involved guest species than on the
constitution of the host. The crystal structures of the hexanoyl–
calixarene inclusion compounds with the ketone (ꢀ)-menthone
and the alcohol (ꢀ)-menthol, 1a and 1b, as well as the earlier
published (+)-carvone inclusion,7 reveal slight deviations of the
capsular structure which, however, hardly influence the packing
modes. On the other hand, the arrangement of the 1 : 1
host : guest complexes seems indeed affected by the structure and
length of the alkanoyl branches: in both inclusion compounds of
the octanoyl substituted host 2, one containing menthone and the
other carvone, the calixarene molecules are packed in a columnar
fashion along the b-axis, which is also true for the carvone
inclusion of 3 (3a) with even more bulky upper rim substituents.
A remarkable structural feature is given in the inclusion structure
of 3 with n-BuOH (3b), in which the small guest component
occupies only a part of the calixarene cavity, with the cantilev-
ered in parts cyclic alkyl groups strongly bent towards the
calixarene axis in order to reduce the cavity volume. Moreover,
the calixarenes comprising the molecular capsule are displaced
such that one of their terminal cyclohexyl fragments caps the
cavity of the respective opposite calixarene. Both these features
could explain the preference of capsular dimers despite the
presence of the particularly large upper rim substituents. As
shown by this study, calixarenes, bearing hexanoyl groups at the
upper rim, prefer the formation of dimeric capsules but octanoyl
and longer substituents seem to favor a columnar packing
behaviour, which is only valid for the inclusion of guests which
fill the entire host cavity. Of special interest for future work in
this respect could be the examination of calixarenes substituted
with unsaturated (e.g. u-alkenoyl, cyclo-hexenylpropanoyl)
residues.
S
ah
1
2
3
1
2
3
(S)-Menthol
(S)-Carvone
(S)-Limonene
(R)-Limonene
Cyclohexane
Cyclohexene
Acetone
0.2
0.2
0.2
0.2
0.1
0.4
1.4
1.0
1.7
0.3
0.2
0.1
0.1
1.5
1.5
2.2
1.2
1.2
0.1
0.1
0.1
0.1
0.1
0.2
0.5
0.2
1.4
0.09
0.21
0.11
0.13
0.02
0.06
0.17
0.44
0.11
0.21
0.20
0.12
0.15
0.03
0.06
0.08
0.40
0.34
0.09
0.13
0.01
0.06
0.09
0.09
0.19
0.51
0.16
Dioxane
n-Butanol
otherwise calixarenes 1 and 3 show a moderately higher affinity
towards the vapour molecules with polar functionality.
Including the molar weights of host and guest, the binding
stoichiometry can be calculated as follows:
ꢀ
ꢁꢀ
ꢁ
DFmax
DFref
Mhost
S ¼
Mguest
The calculated S values in Table 5 again indicate a limited
uptake particularly of the larger terpene molecules in the case of
the calixarene 3 featuring the bulky terminal cyclohexyl groups.
Corresponding to the increase of mass, the small molecules
acetone and n-butanol also show the highest binding stoichio-
metry.
As demonstrated by previous studies,38 depending on size and
interaction of the analyte molecules, the regeneration step does
not always return the sensor frequency to the starting value. This
kind of host affinity [ah] towards the different guest uptakes can
be expressed by the ratio 1 ꢀ (DFmax/DFred), where DFred is the
reduced frequency change observed after incomplete regenera-
tion38 and DFmax the respective value after complete regeneration
achieved by purging with saturated CH2Cl2 vapour. The calcu-
lated ah values are also given in Table 5. Except the dioxane
uptake of all calixarenes and the adsorption of carvone and
menthol for 1 and 2 as well as the adsorption of n-BuOH of
compound 2, the rate of regeneration for all other experiments is
greater than 80%, i.e. the host affinity for vapours of these
molecules is very low. In reverse, this points to a significantly
higher binding affinity towards dioxane, carvone and menthol
which is in agreement with the obtained inclusion structures 1b
and 2a. Furthermore, it shows that the calixarene coating is thin
and enables fast and reversible guest diffusion.38
An important aspect in the studies of supramolecular aggre-
gation is the observation that crystal forces alone are often
sufficient to influence intramolecular torsion angles of flexible
molecules. As an approach to understand the interactions within
crystals, calixarenes as semirigid molecules offer ideal conditions
to deal with a more limited number of independent variables such
as non-crystallographic rotations or translations. In general,
a crystal structure is able to tolerate a certain degree of molecular
changes and even when the extent of tolerance is reached,
common packing motifs may still be kept, although moved, but
nevertheless related, to each other. Such a morphotropic link
could be shown for the (ꢀ)-menthone (1a) and (ꢀ)-menthol (1b)
inclusion of the n-hexanoyl substituted calixarene (1): the change
of the guest molecule is inducing a different packing arrangement
and thus, this particular case is a rare example of a guest induced
supramolecular morphotropism.25
Due to their extended and flexible alkanoyl chains, host
compounds 1 and 2 exhibit in some degree disorder in the solid
state, but allow on the other hand a moderate inclusion of
terpene and solvent vapours. Regarding the cyclohexyl equipped
calixarene 3, the discoidal upper rim residues more or less hinder
an effective vapour uptake but develop no noteworthy disorder
and so provide a formidable platform for future investigation on
the inclusion of lipophilic compounds. QMB measurements
demonstrate the predominantly reversible inclusion of both small
solvent molecules and terpenes, which is the highest for the
calixarene 2 possessing the long and flexible n-octanoyl chains.
Polar guest molecules and especially those with donor
Conclusions
The introduction of extended alkanoyl substituents attached to
the upper rim of a calix[4]arene framework enlarges the cavity
volume and thus enables complex formation with terpenes, but
also allows inclusion of smaller functionally related guest species
of ketone, alcohol and ether type. In this respect, 1 : 1 host–guest
complexes represent the basic molecular entities of the present
crystal structures, which can be arranged in a tail-to-tail
(capsular) or head-to-tail (columnar) fashion. Unexpectedly, the
occurrence of capsular host–guest aggregates is in principle less
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functionalities show moderately higher host affinities and are
exchanged using a special procedure, whereas thin layers of
calixarene inclusions with low polar molecules are regenerated in
an easy and fast way by purging with air. Following this specific
reversible guest binding, all examined para-acylcalix[4]arenes
potentially establish a new platform for vapour sensing as well as
fragrance storage.
198.7 (C]O). Anal. calcd for C64H80O8: C, 78.65; H, 8.25;
found: C, 78.40; H, 8.24%.
Vapour sorption experiments
The experimental setup of the quartz crystal microbalance
consists of two 10 MHz standard electronic quartzes with gold
electrodes (FOQ Piezo Technik, Germany). One of them is
uncoated and used as a reference; the other one is coated with the
receptor (solid calixarene). Both quartzes are located in
a thermostatted metal block (controlled to 25 ꢁC by a water
thermostat). The measurements are carried out with a constant
flow of synthetic air. The resonance frequencies of the quartzes
are measured by a multichannel frequency counter (HKR sensor
systems Munich, Germany) with a resolution of 1 Hz. The
frequency data can be read by the computer via a serial interface.
The coating of the quartzes was performed by dipping the quartz
for 30 s in a 0.1 molar solution of the respective calixarene in
methylene chloride and afterwards drying via air stream for
30 min. This coating gives a constant decrease of the oscillator
Experimental
Melting points were determined on a microscope heating stage
PHMK Rapido (VEB Dresden Analytik) and are uncorrected.
IR spectra were measured on a Nicolet FT-IR 510 as KBr pellets.
NMR spectra were recorded on a Bruker Avance DPX 400
spectrometer at 400 MHz (1H NMR) and 100.6 MHz (13C
NMR), respectively, in CDCl3 solution. Chemical shifts d are
reported in ppm relative to the internal reference TMS.
Elemental analyses were performed on a Heraeus CHN-Rapid
Analyzer. The starting compounds 5,11,17,23-tetra-tert-butyl-
25,26,27,28-tetrahydroxycalix[4]arene and 25,26,27,28-tetrahy-
droxycalix[4]arene were prepared according to the literature.27,28
frequency of about ꢄ2200 Hz, which was choosen as DFref
.
Together with the Sauerbrey-constant Sf of the quartz crystal
(0.12 ꢂ 0.02 Hz cm2 ngꢀ1, 10 MHz, TSM, polished gold elec-
trodes) and the mean density of the calixarene inclusions (rcal),
calculated from the X-ray data, the average thickness of the host
layer (da) was estimated to be ꢄ150 nm using the equation:
Synthesis of host compounds 1–3: general procedure
A mixture composed of aluminium trichloride (118.0 mmol), the
respective acid chloride (91.8 mmol) and nitrobenzene (140 cm3)
under an atmosphere of argon was stirred for about 15 min until
dissolution. 25,26,27,28-Tetrahydroxycalix[4]arene (15.3 mmol)
was added, the solution stirred for 4 h, then poured onto ice and
kept for 1 h. The organic phase was separated and the aqueous
phase extracted twice with chloroform (250 cm3). The combined
organic phases were washed with 1 M hydrochloric acid (2 ꢃ
270 mL), a mixture of 1 M brine and 1 M aqueous sodium acetate,
and water (2 ꢃ 250 cm3, each time) in this sequence. The chlo-
roform was evaporated under reduced pressure and the nitro-
benzene removed by steam distillation. Recrystallization from
acetone/chloroform (1 : 1) yielded the pure products. Specific
details of the compounds are given below. For calixarenes 1 and 2
we were unable to reproduce the melting points reported in the
literature,29 which we explain with the behaviour of poly-
morphism occurring in this particular class of calixarenes.39
DFref ꢃ 10ꢀ2
daðnmÞ ¼
Sf ꢃ rcal
According to the literature,38 such a thin film meets the
demands regarding fast diffusion and reversibility of the binding
event. In a typical QMC sensor experiment, after 300 s equili-
bration time, the saturated headspace vapour of a guest
substance in an upstreamed flask was channeled via a constant
air stream across the sensor cell for 600 s. After this loading time,
a reequilibration time of further 300 s was added to return to the
starting frequency. The frequency difference and increase of mass
were determined with an error of 5%.
X-Ray diffraction‡
The crystals suitable for X-ray investigations were obtained as
follows: crystallization of 1 from a solvent mixture of (ꢀ)-men-
thone/EtOH (9 : 1) and from a melt of (ꢀ)-menthol yielded the
respective inclusion compounds 1a and 1b (Scheme 1). Slow
evaporation of a solution of 2 in (ꢀ)-menthone/EtOH (9 : 1)
solvent mixture afforded suitable crystals of the (ꢀ)-menthone
inclusion 2a. The inclusion compounds 2b and 3a were obtained
via crystallization from solutions of 2 and 3 in (+)-carvone.
Similarly, 3b was prepared from 3 and n-BuOH as guest solvent.
The intensity data were collected on a Bruker APEX II
5,11,17,23-p-Tetrahexanoyl-25,26,27,28-tetrahydroxycalix[4]-
arene (1). Yield 47%; mp 165–166 C (lit.,29 183 C). The other
ꢁ
ꢁ
analytical data correspond with the literature.
5,11,17,23-p-Tetraoctanoyl-25,26,27,28-tetrahydroxycalix[4]-
arene (2). Yield 53%; mp 177–178 C (lit.,29 145 C). The other
ꢁ
ꢁ
analytical data correspond with the literature.
5,11,17,23-p-Tetrakis(30-cyclohexylpropanoyl)-25,26,27,28-tet-
rahydroxycalix[4]arene (3). Yield 86%; mp 167–169 ꢁC; IR (KBr)
3229, 3043, 2924, 2852, 1683, 1600, 1445, 1326, 1316, 1284, 1166,
788; 1H NMR (CDCl3) 0.88, 1.23, 1.55, 1.67 (4 m, 48H, AlkylH),
2.18 (s, 4H, CH), 2.84 (t, 8H, COCH2), 3.73 (s, br, 4H,
ArCH2Ar), 4.28 (s, br, 4H, ArCH2Ar), 7.76 (s, 8H, ArH), 10.20
(s, 4H, OH); 13C NMR (CDCl3) 26.2, 26.5, 31.7, 31.9, 33.2, 35.8,
37.3 (AlkylC), 127.7, 129.8, 132.00 (ArC), 152.6 (ArC–OH),
ꢁ
diffractometer with MoKa radiation (l ¼ 0.71073 A) using u-
and 4-scans. Reflections were corrected for background, Lorentz
and polarization effects. Preliminary structure models were
derived by application of direct methods40 and were refined by
full-matrix least squares calculation based on F2 for all reflec-
tions.41 An empirical absorption correction based on multi-scans
was applied by using the SADABS program.42 With the
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18 G. S. Ananchenko, I. L. Moudrakovski, A. W. Coleman and
J. A. Ripmeester, Angew. Chem., Int. Ed., 2008, 47, 5616–5618.
19 M. Pojarova, G. S. Ananchenko, K. A. Udachin, F. Perret,
A. W. Coleman and J. A. Ripmeester, New J. Chem., 2007, 31,
871–878.
exception of the phenolic hydrogens in 3a all other hydrogen
atoms were included in the models in calculated positions and
were refined as constrained to bonding atoms.
20 G. S. Ananchenko, K. A. Udachin, M. Pojarova, S. Jebors,
A. W. Coleman and J. A. Ripmeester, Chem. Commun., 2007,
707–709.
21 E. Breitmaier, Terpenes. Flavors, Fragrances, Pharmaca, Pheromones,
Wiley-VCH, Weinheim, 2006.
€
22 D. Bockmuhl, R. Breves, M. Weide and J. Boy, PCT Int. Appl., 2005,
Acknowledgements
Financial support from the German Federal Ministry of
Economics and Technology (BMWi) under grant No. 16IN0218
‘ChemoChips’ is gratefully acknowledged. This work was also
supported by the Hungarian Research Fund T049712.
120, 229.
23 M. Weide, D. Bockmuhl, T. Gerke, A. Bolte, R. Breves and
€
S. Doring, PCT Int. Appl., 2004, 110, 147.
€
24 T. Gruber, C. Fischer, M. Felsmann, W. Seichter and E. Weber, Org.
Biomol. Chem., 2009, 7, 4904–4917.
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acetone from a mixture of acetone–chloroform (1 : 1). In this
special case the host molecules are arranged in a tail-to-tail fashion
similar to compound 3b. Unfortunately, because of the low quality
of the crystals we were unable to solve the structure to a sufficiently
high level.
38 L. S. Yakimova, M. A. Ziganshin, V. A. Sidorov, V. V. Kovalev,
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12 G. S. Ananchenko, M. Pojarova, K. A. Udachin, D. M. Leek,
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13 G. S. Ananchenko, K. A. Udachin, A. W. Coleman,
D. N. Polovyanenko, E. G. Bagryanskaya and J. A. Ripmeester,
Chem. Commun., 2008, 223–225.
€
14 D. N. Polovyanenko, E. G. Bagryanskaya, A. Schnegg, K. Mobius,
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15 G. S. Ananchenko, K. A. Udachin, A. Dubes, J. A. Ripmeester,
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39 S. Shinkai, T. Nagasaki, K. Iwamoto, A. Ikeda, G.-X. He,
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40 G. M. Sheldrick, SHELXS-97: Program for Crystal Structure
€
Solution, University of Gottingen, Germany, 1990.
41 G. M. Sheldrick, SHELXL-97: Program for Crystal Structure
16 P. Shahgaldian, M. Cesario, P. Goreloff and A. W. Coleman, Chem.
Commun., 2002, 326–327.
€
Refinement, University of Gottingen, Germany, 1997.
17 G. S. Ananchenko, K. A. Udachin, J. A. Ripmeester, T. Perrier and
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€
42 G. M. Sheldrick, SADABS, University of Gottingen, Germany, 2004.
This journal is ª The Royal Society of Chemistry 2011
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