Inclusion compounds of p - Tert -butylcalixarenes: Structures, kinetics, and selectivity

Article abstract of DOI:10.1021/cg2004084

Inclusion compounds from p-tert-butylcalix[4] and [6]arenes were grown from chlorobenzene, bromobenzene, benzylamine, and ethylene diamine. The crystal structures of seven new compounds were elucidated, their host-guest ratios were established, and their respective thermal stabilities were investigated. The kinetics of desorption of four of the compounds yielded their activation energies of decomposition. We established the selectivity profile of p-tert-butylcalix[4]arene for chlorobenzene versus bromobenzene as guests.

Full text of DOI:10.1021/cg2004084

ARTICLE
pubs.acs.org/crystal
Inclusion Compounds of p-tert-Butylcalixarenes: Structures,
Kinetics, and Selectivity
Ga€elle Ramon,*,† Ayesha Jacobs,‡ Luigi R. Nassimbeni,‡ and Rodriguez Yav-Kabwit‡
†Department of Chemistry, P.D. Hahn Building, University of Cape Town, Rondebosch 7701, South Africa
‡Department of Chemistry, Faculty of Applied Sciences, Cape Peninsula University of Technology, P.O. Box 652,
Cape Town 8000, South Africa
S Supporting Information
b
ABSTRACT: Inclusion compounds from p-tert-butylcalix[4]
and [6]arenes were grown from chlorobenzene, bromoben-
zene, benzylamine, and ethylene diamine. The crystal structures
of seven new compounds were elucidated, their hostꢀguest
ratios were established, and their respective thermal stabilities
were investigated. The kinetics of desorption of four of the
compounds yielded their activation energies of decomposition.
We established the selectivity profile of p-tert-butylcalix[4]arene
for chlorobenzene versus bromobenzene as guests.
Scheme 1. Hosts and Guests Molecules: Structures, Names,
and Abbreviations
’ INTRODUCTION
The calixarenes are versatile host compounds which are read-
ily functionalized at both their upper and lower rims and contain
a hydrophobic cavity. They form complexes with cations, anions,
and neutral molecules and show considerable conformational
flexibility. Their chemistry and structural properties have been
extensively reviewed.1ꢀ4
The first crystal structure detailing the molecular arrangement
in a calixarene inclusion compound was that of p-tert-butylcalix-
[4]arene with toluene,5 and since then the Parma group has
elucidated many structures formed by calixarenes with a variety
of guest molecules.6
While the synthesis, chemical characteristics, and transport
phenomena of calixarene-based systems have been studied
extensively, little attention has been paid to the physicochem-
ical properties of their crystalline inclusion compounds, such as
their thermal stabilities, kinetics of decomposition, and gas
sorption.
We have investigated the selectivity properties of the
resorcinarene 14,16,54,56-tetrahydroxy-2,4,6,8-tetrapentyl-34,36,
74,76-tetra(p-toluenesulfonyloxy)-1,3,5,7(1,3)-tetrabenzenecyclo-
octaphane with seven pentanol isomers and elucidated their
structures and thermal properties.7 This same resorcinarene host
was employed in the separation of picoline isomers,8 in which it
was shown by competition experiments that the selectivity curve
for the guest pair 3-picoline/4-picoline is concentration depen-
dent, but, unusually, the host preferentially enclathrated the guest
of lower concentration.
methods have been employed in combination with X-ray diffrac-
tion to study the dynamic features of several inclusion com-
pounds of p-tert-butylcalix[4]arene.11,12
In a series of articles dealing with gas adsorption, Barbour and
Atwood have studied the properties of p-tert-butylcalix[4]arene
and its ability to carry out a single-crystal-to-single-crystal
transition when reacted with vinyl bromide,13 its polymorphic
nature,14 and its use in hydrogen recovery from gas mixtures.15
They further studied its absorption of hydrogen16 and methane17
and compared the absorption isotherms of acetylene and carbon
dioxide.18 Their work has been summarized in the form of a
The selectivity of calixarenes was also demonstrated by
capillary gas chromatography,9,10 where p-tert-butylcalix[4]arene
and p-tert-butylcalix[8]arene were employed in the separation
of dialkyl substituted benzenes and chloromethanes. NMR
Received: April 1, 2011
Published: May 10, 2011
r
2011 American Chemical Society
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|

Crystal Growth & Design
ARTICLE
Table 1. Crystal Data and Refinement Parameters of the Inclusions Compounds
C4 ClBzn
C4 BrBzn
C4 3(BzlAm)
C4 4(En) 2(H2O)
C6 ClBzn
C6 BrBzn
1.5(C6) 7(En)
3
3
3
3
3
3
3
3
molecular formula
C
50H61O4Cl
C50H61O4Br
805.94
1:1
C65H83O4N3
970.39
1:3
C52H92O6N8
925.35
1:4:2
C72H89O6Cl
1085.95
1:1
C72H89O6Br
1130.40
1:1
C113H182O9N14
1880.78
1.5:7
molecular mass/g molꢀ1
761.48
3
host/guest ratio
1:1
crystal symmetry
monoclinic
P2/n
tetragonal
P4/n
monoclinic
P21/n
monoclinic
P21/n
monoclinic
P21/n
monoclinic
P21/n
monoclinic
P21/c
space group
a/Å
12.982(3)
12.724(3)
13.044(3)
90
13.056(2)
13.056(2)
12.631(3)
90
16.170(3)
20.882(4)
17.125(3)
90
13.253(7)
18.573(9)
22.81(1)
90
17.224(3)
18.618(4)
19.778(4)
90
17.265(4)
18.591(4)
19.923(4)
90
17.406(4)
32.849(7)
20.633(4)
90
b/Å
c/Å
R/°
β/°
γ/°
90.22(3)
90
90
104.08(3)
90
94.62(2)
90
95.44(3)
90
95.92(3)
90
106.89(3)
90
90
Z
2
2
4
4
4
4
2
V/Å3
Dc /g cmꢀ3
μ(Mo-KR)/mmꢀ1
2154.6(9)
1.166
2153.0(7)
1.240
5609(2)
1.149
5597(5)
1.091
6314(2)
1.140
6361(2)
1.159
11288(5)
1.105
3
0.132
0.998
0.070
0.072
0.111
0.661
0.070
F(000)
810
849
2104
2008
2336
2352
4109
temp of collection/K
range scanned, θ /̊
index ranges (h, k, l)
173
173
173
173
173
173
173
1.6ꢀ26.4
ꢀ16/16;
ꢀ15/15;
ꢀ16/16
50416
4415
2.2ꢀ28.3
ꢀ17/17;
ꢀ17/15;
ꢀ16/16
18260
2685
2.0ꢀ28.3
ꢀ20/21;
ꢀ27/20;
ꢀ20/22
23455
2.0ꢀ26.0
ꢀ16/13;
ꢀ21/22;
ꢀ21/28
15220
1.9ꢀ26.4
ꢀ21/21;
ꢀ23/23;
ꢀ24/24
71257
1.5ꢀ25.4
ꢀ20/20;
ꢀ22/22;
ꢀ23/23
181962
11613
1.9ꢀ27.8
ꢀ22/11;
ꢀ34/43;
ꢀ27/27
65577
no. reflections collected
no. unique reflections
no. reflections with I > 2σ(I)
data/parameters refined
goodness of fit, S
11678
10517
12906
26537
3488
2172
8600
4405
9859
7485
18182
4415/288
1.07
2685/144
1.04
11678/704
1.04
10517/734
0.95
12906/847
1.02
11613/822
1.02
26537/1401
1.05
final R indices (I > 2σ(I))
largest diff peak and hole/e Åꢀ3
0.0647
ꢀ0.30/0.46
0.0619
ꢀ0.62/0.57
0.0532
ꢀ0.37/0.69
0.0690
ꢀ0.23/0.59
0.0627
ꢀ0.66/0.96
0.0839
ꢀ0.65/1.31
0.0738
ꢀ0.86/2.30
3
Figure 1. (a) Hydrogen bonds stabilizing C4 ClBzn displaying only one position of the disordered guest molecule, (b) packing of C4 ClBzn viewed
3
3
along [001] displaying antiparallel columns along [010].
tutorial review which compares the sorption properties of
calixarenes with those of metalꢀorganic frameworks.19
In this work we present the results of structures of p-tert-
butylcalix[4]arene and p-tert-butylcalix[6]arene with chlor-
obenzene, bromobenzene, benzylamine, and ethylene dia-
mine, their thermal stabilities and their kinetics of
desorption. The details of the host and guest molecules are
given in Scheme 1.
’ EXPERIMENTAL SECTION
Crystal Growth. Crystals of the inclusion compounds were
obtained by crystallizing saturated solutions of the host with the
respective guests. The saturated solutions were prepared by dissolving
the host in warm solvent (guest, at 70 °C) under gentle stirring. The
solutions slowly cooled and evaporated allowing the inclusion com-
pounds to crystallize.
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Crystal Growth & Design
ARTICLE
Table 2. Metrics of the Hydrogen Bonds Stabilizing the Inclusion Compounds: (A) Intramolecular Hydrogen Bonds in the
Calixarene Lower Rim and (B) Intermolecular Hydrogen Bonds
(A) Intramolecular Hydrogen Bonds in the Calixarene Lower Rim: OꢀH
O
3 3 3
A (Å)
DꢀH (Å)
H
A (Å)
D
angle DꢀH A (deg)
3 3 3
3 3 3
3 3 3
C4 ClBzn
0.91(4)
1.02(4)
0.75(4)
0.90(3)
0.95(3)
0.84(3)
1.02(5)
0.90(4)
1.13(6)
0.90(4)
0.88(4)
1.12(6)
0.97(4)
1.01(5)
0.98(7)
1.79(4)
1.67(4)
1.94(4)
1.62(3)
1.62(3)
1.94(3)
1.53(5)
1.82(4)
1.34(7)
1.72(4)
1.74(4)
1.53(6)
1.70(4)
1.58(5)
1.63(6)
2.671(2)
2.673(2)
2.672(3)
2.508(2)
2.549(2)
2.743(2)
2.545(4)
2.664(4)
2.457(4)
2.610(2)
2.621(2)
2.639(2)
2.646(2)
2.590(2)
2.605(2)
2.648(4)a
2.607(4)a
2.588(4)a
2.626(4)a
2.608(4)a
2.642(4)a
2.585(2)
2.620(2)
2.564(2)
2.599(3)
2.422(2)
2.655(3)
162(4)
165(4)
162(4)
171(3)
166(3)
160(2)
173(4)
157(3)
172(6)
170(3)
176(4)
172(5)
167(3)
179(5)
178(8)
3
C4 BrBzn
3
C4 3(BzlAm)
3
C4 4(En) 2(H2O)
3
3
C6 ClBzn
3
C6 BrBzn
3
1.5(C6) 7(En)
(H1)
(H1)
(H1)
(H1)
(H2)
(H2)
0.86(4)
0.92(3)
0.86(4)
0.95(4)
1.14(4)
0.87(4)
1.73(4)
1.71(3)
1.71(4)
1.65(4)
1.29(4)
1.81(4)
172(4)
169(4)
172(3)
172(4)
167(4)
165(4)
3
(B) Intermolecular Hydrogen bonds
DꢀH A type
DꢀH (Å)
H
A (Å)
D
A (Å)
angle DꢀH A (deg)
3 3 3
3 3 3
3 3 3
3 3 3
C4 3(BzlAm)
(G1)NꢀH N(G3)
0.85(3)
0.86(3)
1.03(3)
0.93(2)
0.96(2)
0.89(5)
0.82(5)
0.90(7)
1.02(5)
1.08(6)
1.14(5)
0.99(4)
0.91(5)
0.91(8)
1.06(5)
0.97(9)
0.89(4)
0.92(3)
1.01(5)
2.44(3)
2.45(3)
1.70(3)
1.85(2)
1.84(2)
2.34(5)
2.22(5)
2.00(7)
2.10(5)
1.88(6)
1.63(5)
2.05(4)
2.08(5)
2.01(8)
1.83(5)
1.88(9)
2.07(4)
1.92(3)
2.14(5)
3.171(3)
3.062(2)
2.720(3)
2.781(2)
2.788(2)
3.062(5)
3.023(6)
2.895(5)
3.101(6)
2.934(7)
2.753(6)
2.913(6)
2.972(6)
2.889(7)
2.881(6)
2.816(6)
2.959(4)
2.842(4)
3.095(5)
145(2)
129(2)
177(3)
174(2)
168(2)
138(4)
168(4)
177(9)
168(4)
166(5)
168(5)
145(3)
166(5)
163(7)
170(3)
161(8)
174(3)
178(3)
158(4)
3
3 3 3
(G1)NꢀH O(Host)
3 3 3
(G2)NþꢀH N(G1)
3 3 3
(G2)NþꢀH O(Host)
3 3 3
(G2)NþꢀH O(Host)
3 3 3
C4 4(En) 2(H2O)
NꢀH O(Host)
3
3
3 3 3
NꢀH O(Host)
3 3 3
(H2O)OꢀH O(Host)
3 3 3
NꢀH
N
3 3 3
NþꢀH
NþꢀH
N
3 3 3
3 3 3
N
NþꢀH O(H2O)
3 3 3
NꢀH O(H2O)
3 3 3
(H2O)OꢀH
(H2O)OꢀH
(H2O)OꢀH
N
N
N
3 3 3
3 3 3
3 3 3
1.5(C6) 7(En)
NþꢀH
NþꢀH
NþꢀH
N
N
N
3
3 3 3
3 3 3
3 3 3
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Crystal Growth & Design
ARTICLE
Table 2. Continued
(B) Intermolecular Hydrogen bonds
DꢀH A type
3 3 3
DꢀH (Å)
H
A (Å)
D
A (Å)
angle DꢀH A (deg)
3 3 3
3 3 3
3 3 3
NþꢀH
N
1.14(4)
0.98(6)
1.02(4)
0.84(4)
0.98(4)
1.07(5)
1.67(4)
1.95(5)
1.85(4)
1.97(4)
1.70(4)
2.03(5)
2.811(5)
2.849(6)
2.862(4)
2.788(3)
2.686(6)
3.090(3)
173(3)
151(5)
172(3)
165(3)
179(6)
168(4)
3 3 3
NꢀH
N
3 3 3
NþꢀH
N
3 3 3
NþꢀH O(Host)
3 3 3
NꢀH O(Host)
3 3 3
NꢀH O(Host)
3 3 3
a In cases where the hydroxyl hydrogen atoms could not be found, only the O O is reported.
3 3 3
Figure 2. (a) Hydrogen bonds stabilizing C4 BrBzn displaying only one position of the disordered guest molecule for clarity. (b) Packing of C4 BrBzn
3
3
viewed along [010] displaying antiparallel columns along [001].
Figure 3. The hydrogen bonds stabilizing C4 3(BzlAm) structure are shown. (a) The asymmetric unit displays one host and three guest molecules;
3
(b) the hydrogen bond network around the protonated benzyl amine is displayed with the weakly hydrogen bonded guest omitted.
Structure Analysis. In all cases the hostꢀguest ratios were
confirmed by thermal gravimetry (TG). Cell dimensions were estab-
lished from the intensity data measurements on a Nonius Kappa CCD
diffractometer using graphite-monochromated MoꢀKR radiation. The
strategy for the data collections was evaluated using COLLECT
software.20 For all structures, data were collected by the standard
phi- and omega-scan techniques. They were scaled and reduced using
DENZO-SMN software.21 The structures were solved by direct meth-
ods using SHELX-8622 and refined by least-squares with SHELX-9723
refining on F2. The program X-Seed24,25 was used as a graphical interface
for the structure solution and refinement using SHELX, as well as to
produce the packing diagrams.
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Crystal Growth & Design
ARTICLE
In each of the structures, the positions of all non-hydrogen host
atoms were obtained by direct methods and the non-hydrogen guest
atoms were located in the difference electron density maps. All non-
hydrogen atoms were refined with anisotropic temperature factors.
Hydroxyl and amino hydrogen atoms were located in the difference
electron density maps and refined independently with simple bond
length constraints and independent temperature factors. All other
hydrogen atoms were placed with geometric constraints and refined
with isotropic temperature factors. All structures were determined at
low temperature (173 K). Crystal data and refinement parameters are
detailed in Table 1.
In cases where the molecules were disordered, the hydrogen atoms
were modeled as far as possible.
Thermal Analysis and Kinetics. Both differential scanning
calorimetry (DSC) and thermal gravimetry (TG) were performed on
a Perkin-Elmer 6 series system. Finely powdered, air-dried specimens
(2ꢀ5 mg) were placed in crimped, vented aluminum pans or open
Figure 4. Packing of the structure of C4 3(BzlAm) viewed along [010]
Figure 6. Asymmetric unit of C6 ClBzn displaying the pinched cone
3
3
displaying the different positions of the guest molecules in the structure:
G1 (orange) is situated in the cavity of the host, while G2 (pink) and G3
(green) are situated outside of the host cavity.
conformation of the host molecule in which a disordered guest
resides. The host molecule is stabilized through intramolecular
hydrogen bonds.
Figure 5. C4 4(En) 2H2O: (a) View of the hydrogen bond network, (b) view of the solvent channels along [100] with the solvent molecules in van der
3
3
Waals radii, (c) structure viewed along [100] with the host molecules in van der Waals radii and the solvent molecules omitted.
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Crystal Growth & Design
ARTICLE
Figure 8. Asymmetric unit of C6 BrBzn displaying the pinched cone
3
conformation of the host molecule in which a disordered guest
resides.
250 °C, detector temperature: 250 °C, carrier gas: nitrogen (flow rate:
0.5 mL/min).
Before each set of competition experiments, the GC analyzer was
calibrated by analyzing the starting mixture of known mole fraction of
the guests.
’ RESULTS AND DISCUSSION
Structure Analysis. The inclusion compound C4 ClBzn
3
crystallizes in the space group P2/n with Z = 2. Both host and
guest molecules are located on a diad at Wyckoff position e. The
guest molecule is disordered over two positions with site
occupancies 0.43 and 0.57. Because of the disorder, the hydrogen
atoms of the guest molecule were not modeled. The calixarene is
in the cone conformation and is stabilized by intramolecular
hydrogen bonding of the hydroxyl groups. The guest molecule
lies within the cone of the calixerene, the chlorine atom pointing
out of the cavity. The guest molecule is stabilized through weak
(Host)CꢀH π(Guest) interactions between the methyl
Figure 7. (a) Single layer of host molecules viewed along [010].
3 3 3
The packing of C6 ClBzn is constituted of such layers which
groups of the calixarene and the benzene ring of the guest
molecule (Figure 1a). The metrics of the hydrogen bonding
for this and the subsequent structures are given in Table 2. The
packing displays antiparallel columns of imbricated inclusion
compounds along [010] (Figure 2b).
3
intercalate along [010], (b) packing diagram of C6 ClBzn viewed
3
along [100] displaying the disordered guest molecules situated in
cavities.
aluminum TG sample pans. Dry nitrogen was used as purging gas at a
flow rate of 20 mL/min. All experiments were carried out over
temperature ranges of 30ꢀ360 °C at a constant heating rate of
10 °C/min.
The structure of C4 BrBzn crystallizes in the space group
3
P4/n with Z = 2. Both the host and guest molecules are situated
on a tetrad at Wyckoff position c (origin 2). The hydrogen atoms
of the guest molecule were not modeled. The stabilization
The kinetics of desorption were studied by carrying out a series
of non-isothermal TG runs, performed over the temperature range
30ꢀ250 °C with a varying heating rate ranging from 2 to 32 °C/min.
The crystals were crushed to limit the effect of particle size on the
results.
patterns observed previously in the C4 ClBzn inclusion com-
pound are similar in this structure. Both the intramolecular
3
hydrogen bonds and (Host)CꢀH π(Guest) bonds are present.
3 3 3
The packing diagram also features antiparallel columns of enclath-
ratedguestbutalong[001] (Figure2). Thepackingoftheinclusion
compounds of C4 ClBzn and C4 BrBzn are isostructural.
Competition Experiments. Mixtures of the two guests were
prepared in vials in which the mole fraction of a given guest varied from
0 to 1 in steps of 0.2. A fixed amount of host compound was added and
dissolved by heating and stirring. The ratio of the host compound
to the total guest was at least 1:20 to ensure that there was enough
of the guest of lower mole fraction should the host have 100%
preference for it. The composition of the crystals obtained was
analyzed using gas chromatography (GC). These analyses were carried
out using an Agilent 6890 gas chromatograph equipped with a
CP-1177 split/splitless injector, an FID detector, a Varian fused HP-5 silica
column (30 m, 320 μm, 0.25 μm). The chromatograms obtained were
recorded and analyzed using the program package ChemStation.26 The
method used for all the measurements was the following: injection
volume: 1 μL, injector temperature: 150 °C, column temperature:
3
3
The compound C4 3(BzlAm) crystallizes in the space group
3
P21/n with Z = 4. The three guest molecules occupy different
positions which have different stabilities as further demonstrated
by the thermal analysis (Figure 3).
The guest molecule (G1 in orange) is the one that resides in
the cavity of the host molecule and is stabilized by two
(Host)CꢀH π(G1) bonds (Figure 3a). It is also hydrogen
3 3 3
bonded to the second guest molecule (G2 in pink) and weakly
hydrogen bonded to a second host molecule (Figure 3b). The
amine group of the G2 is protonated by the loss of one hydroxyl
hydrogen of the calixarene and the resulting ionic guest bonds to
the guest in the host cavity via (G2)NþꢀH N(G1). This
3 3 3
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Crystal Growth & Design
ARTICLE
Figure 9. The two calixarene hosts of the asymmetric unit are represented when viewed down the cavity and when viewed from the side. The host H1
(a) is situated in general positions and the host H2 (b) is situated about a center of inversion.
second guest is also stabilized in the structure via two
The packing diagram of C6 ClBzn is constituted of layers
3
(G2)NþꢀH O(Host) (Figure 3b). The third guest molecule
(Figure 7a) which intercalate along [010] while the packing
viewed along [100] displays the position of the guest molecules
in “cavities” (Figure 7b).
3 3 3
(G3 in green) is stabilized by a weak hydrogen bond
(G1)NꢀH N(G3). The packing viewed down b shows that
3 3 3
C6 BrBzn is isostructural with C6 ClBzn and displays similar
both G2 and G3 lie outside the cavity of the host molecule
(Figure 4).
3
3
characteristics. The hydroxyl hydrogens of the calixarene mole-
cule could not be found in the difference electron density map.
Contrary to C6 ClBzn, the guest molecule in C6 BrBzn is
The inclusion compound obtained from calix[4]arene, C4 4-
3
(En) 2H2O crystallizes in P21/n with Z = 4. One of the methyl
3
3
3
disordered over three positions and the hydrogen atoms could
not be modeled. The site occupancies for the three positions of
the bromobenzene molecule were 0.40 (G1-green), 0.29 (G2-
orange), and 0.27 (G3-pink). G1 and G2 have their bromine
atoms pointing out of the cavity, while G3 lies almost within the
cone of the host molecule (Figure 8).
groups of the host molecule is disordered over two positions. On
the lower rim of the calixarene, one hydroxyl function is
deprotonated which results in only three hydrogen bonds
stabilizing the cone conformation of the host. As a result of this
proton transfer, the amine group of the guest shown in yellow is
protonated. All guest molecules and water molecules are in-
volved in an intricate network of hydrogen bonds (Table 2,
Figure 5).
The compound 1.5(C6) 7(En) crystallizes in the space group
3
P21/c with Z = 2. One host molecule (H1) lies in a general
position, and a second one (H2) is located on a center of
inversion at Wyckoff position d. All guests are in general
positions. One methyl of the host molecule H2 is disordered
over two positions. The calixarene molecules are both in the
1,2,3-alternate conformation, but their respective shapes are
different (Figure 9).
These form columns of solvent which are stabilized in the
structure through hydrogen bonds with the host molecules.
These solvent molecules are situated in channels running along
[100]. One guest (G1, in orange) is protruding from the solvent
channel but stabilizes the structure by being situated in the host
cavity (Figure 5b).
The compound C6 ClBzn crystallizes in the space group
From the calixarene H1, we found only four hydroxyl hydro-
gens and from the half calixarene H2 we found two hydroxyl
hydrogens. This implies that three of the ethylene diamine guests
are protonated. However, after inspection of the difference
electron density map, only two guest moieties were successfully
refined with protonated nitrogen, while in two cases, no amino
hydrogen atoms could be located and were therefore omitted
from the final model. The solvent molecules are stabilized by
hydrogen bonding (Figure 10a)
3
P21/n with Z = 4. The host molecule is in the pinched cone
conformation stabilized by intramolecular hydrogen bonds on
the lower rim and the guest molecule resides in one of the cavities
which this conformation offers. Two methyl groups of the host
molecule are disordered over two positions and have been
modeled successfully. The guest molecule is disordered over
two positions and the hydrogen atoms were partially modeled.
The site occupancies for the two positions of the chlorobenzene
molecule refined to 0.42 (G1-green) and 0.58 (G2-orange)
(Figure 6).
The solvent molecules are stabilized within the structure
through hydrogen bonding with the host molecules. The hydrogen
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Figure 10. In 1.5(C6) 7(En): (a) Hydrogen bonding in the guests, (b) ribbons of alternating host and guest molecules, (c) hydrogen bonding
3
connecting the guest molecules to both host molecules (H2 in pink).
Table 3. Thermal Analyses
C4 ClBzn
C4 BrBzn
C4 3(BzlAm)
C4 4(En) 2(H2O)
C6 ClBzn
C6 BrBzn
1.5(C6) 7(En)
3
3
3
3
3
3
3
3
TG Analysis
theoretical
14.8
19.5
33.1
29.9
10.4
13.9
22.4
mass loss (%)
experimental
mass loss (%)
TG curve
15.0
20.3
32.7
34.5
11.0
13.8
25.2
neat, multiple
step mass loss
neat, multiple
step mass loss
neat, multiple
step mass loss
continuous weight
loss from room
temperature
neat, single
neat, single
continuous
step mass loss
step mass loss
weight loss from
room temperature
DSC Analysis
Tonset peak A (°C)
Tpeak A (°C)
Tpeak B (°C)
171.7
172.7
270.5
107.1
110.6
286.4
72.4
68.1
217.1
242.1
216.0
236.2
94.3
84.6
102.7
197.8
104.2
176.9
136.0
Tpeak C (°C)
303.6
Tpeak-host (°C)
Tb (guest) (°C)
Tonset/Tb ꢀ temp in K
DSC curve
342.4
132.0
1.10
342.9
193.0
0.82
342.6
342.4
374.8
132.0
1.21
374.6
193.0
1.05
360.9
185.0
118.0
118.0
0.75
0.87
0.94
neat
neat
multiple
overlapping peaks
multiple
overlapping peaks
neat
neat
multiple
overlapping peaks
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inclusion compound being fairly unstable once removed from the
mother liquor. This precluded the blot-drying of the crystals,
hence the slightly enlarged experimental mass loss values.
As a measure of the inclusion compounds thermal stability, we
calculated the ratio of the onset of desolvation of the guest
(Tonset) and the normal boiling point of the guest (Tb). Inclusion
compounds with Tb/Tonset > 1 (namely, C4 ClBzn, C6 ClBzn,
3
3
and C6 BrBzn) are more stable as they release the guest
3
they entrap at a higher temperature than the guest normal
boiling point.
Kinetics. The data obtained from the non-isothermal TG runs
were converted into Rꢀtemperature curves then into plots
displaying log β versus 1/T where β is the heating rate and the
temperature T is expressed in Kelvin. The slopes of the linear
plots yielded the activation energies, Ea. In general, parallel slopes
are only obtained at the beginning of the desorption reaction,
indicating a single mechanism of decomposition. We therefore
report the range of activation energies for the mass loss in the
0ꢀ15% range.
For C4 ClBzn, the activation energy was in the range of 67 to
3ꢀ1
83 kJ mol . This is similar to that obtained with C4 BrBzn
3
3
(75ꢀ78 kJ molꢀ1) and C6 BrBzn (75ꢀ80 kJ molꢀ1).
3
3
3
3
The decomposition of C4 3(BzlAm) is unusual in that the log
β vs 1/T curves remain parallel over a wide range. In addition, the
R-temperature curves clearly show a two-step reaction. By
appropriate treatment of the results, we obtained the activation
energy for the first step, with R = 0 to 0.7, as ranging from 61.8 to
62.5 kJ molꢀ1, followed by the second step, with R = 0.7 to 0.9,
3
yielding Ea = 69.5ꢀ71.1 kJ molꢀ1. The decay curves, extent of
3
reaction, and concomitant semilogarithmic graphs are displayed
in Figure 11aꢀc. We surmise that the activation energy obtained
for the first step corresponds to the desorption of the two guest
molecules situated outside the cavity of the host molecule and are
thus less tightly held, while the higher activation energy obtained
for the second step correspond to the desorption of the guest
molecule situated in the cavity of the host molecule.
The desorption of the inclusion compounds containing ethy-
lene diamine as guest, were not analyzed because their inherent
instability yielded inconsistent initial stoichiometries or complex
multistep decay curves.
Competition Experiment. When the host molecule is exposed
to two (or more) guest molecules, the thermodynamic selectivity
is defined as the ratio of the equilibrium constants,27 and
selectivity can be regarded as a differential form of molecular
recognition. To monitor this, we have adopted the concept of a
selectivity coefficient KA:B first enunciated by Ward:28
Figure 11. For the inclusion compound C4 3(BzlAm), plots of (a)
3
the TG curves recorded at different heating rates (2ꢀ32 °C/min), (b)
the TG curves converted to R-temperature curves, and (c) log β versus
1/T where β is the heating rate and the temperature T is expressed in
Kelvin.
KA:B ¼ ðKB:AÞꢀ1 ¼ ZA=ZB XB=XA
ꢁ
bonding creates ribbons along [001] where host molecules H1
and solvent molecules alternate (Figure 10b). The solvent
molecules are also stabilized through hydrogen bonding with
the host molecule H2, which alternates alongside the ribbons,
entrapping the solvent molecules (Figure 10c).
Thermal Analysis. All inclusion compounds were analyzed by
thermal analyses (TG and DSC) and the host/guest ratios were
confirmed by comparing the experimental and the calculated
mass loss (Table 3). All experimental results were within 1% from
the calculated values. For the inclusion compounds where the
guest was ethylene diamine, the TG curves display a constant
mass loss from room temperature and the TG results obtained
are less accurate than the others. This can be explained by the
For the two guests A and B, XA, XB are their mole fractions in
the liquid mixture and ZA, ZB are the mole fractions of the guests
in the crystal.
For this study, we chose two inclusion compounds with similar
crystal structures: C4 ClBzn and C4 BrBzn. The chlorobenzene
3
3
versus bromobenzene competition experiment resulted in
chlorobenzene being preferentially enclathrated by the p-tert-
butylcalix[4]arene host over the whole concentration range.
The results of the selectivity experiments are displayed in
Figure 12.
We calculated KClBzn:BrBzn and obtained a value of 2.80 over 0
< XClBzn < 1.
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Crystal Growth & Design
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Figure 12. Competition experiments when growing inclusion compounds from mixtures of chlorobenzene and bromobenzene utilizing p-tert-
butylcalix[4]arene as the host molecule.
’ CONCLUSION
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Corresponding Author
*E-mail: gaelle.ramon@uct.ac.za. Fax: þ27 21 650 5768. Tel:
þ27 21 650 5184.
’ ACKNOWLEDGMENT
The authors thank the NRF (Pretoria) and the Cape Penin-
sula University of Technology for funding.
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