Supramolecular solid-gas complexes: A thermodynamic approach

Article abstract of DOI:10.1002/anie.200701666

(Table Presented) Phasing up to complex problems: A thermodynamic approach based on solution data has been proposed for the determination of the stability of gas complexes and elucidation of the selectivity of gas binding. Stability constants, reaction enthalpies, and entropies for the complexation of gaseous guests (n-alkylamines) by solid macrocyclic hosts (β-cyclodextrin, cucurbit[6]uril) were calculated by using the Born-Haber type cycle (see picture).

Full text of DOI:10.1002/anie.200701666

Angewandte
Chemie
DOI: 10.1002/anie.200701666
Gas Complexes
Supramolecular Solid–Gas Complexes: A Thermodynamic Approach
Alexander G. Grechin, Hans-Jürgen Buschmann,* and Eckhard Schollmeyer
The reversible binding of gases by solid materials is of great
interest for modern technologies concerning gas separators,
sensors, and storage devices for use in fuel cells. Since the first
between [2.2.2.]cryptand and alkali-metal salts in the solid
[
12a]
state by using the corresponding data in solution.
Danil
de Namor et al. calculated from solution data “the standard
enthalpies of coordination” corresponding to the complex-
ation process of calixarene derivatives with metal salts in
[
1]
studies of Cramer and Henglein on the complexation of
gases by a-cyclodextrin in aqueous solution, a range of
inclusion complexes with gases has been investigated.
Recently, various macrocyclic receptors were proposed as
[
12b]
which the reactants and product are in their solid state.
Herein we demonstrate and discuss the application of an
indirect “solution” approach to investigate gas inclusion in
solid macrocyclic ligands by using examples of the typical
1:1 complexation equilibria between solid cucurbit[6]uril (1),
b-cyclodextrin (2, Figure 1), and the volatile n-alkylamines
propylamine (3), butylamine (4), pentylamine (5), and hexyl-
amine (6).
[
2]
suitable molecular containers for different gases. Never-
theless, the rules governing the selectivity of gas–receptor
interactions and the thermodynamic stability of gas com-
plexes are still poorly understood. Day et al. found that
cucurbit[n]urils are able to form complexes with various gases
[
3]
at atmospheric pressure, while Atwood and co-workers
have shown the possibilities of gas encapsulation by substi-
[
4]
tuted calixarenes. The main research trends in the supra-
molecular chemistry of gases were analyzed by Rudkevich
[
2a,b]
et al.
In some cases it was possible to determine the
thermodynamic quantities of gas-inclusion processes. Cram
[
5]
[6]
et al. as well as Rebek and co-workers measured the
stability constants and reaction enthalpies for the complex-
ation of CH and Xe by self-assembling cavities in CDCl by
4
3
1
129
using H NMR and
Xe NMR spectroscopic methods.
[
7]
Gorbatchuk et al. calculated the Gibbs free energies for
the formation of a complex between solid calixarenes and
gaseous organic compounds from adsorption isotherms. The
results on the thermodynamics of the binding of molecular
II
oxygen by Co complexes and protein molecules (hemoglo-
bin, myoglobin, etc.) in solution were reviewed by Martell and
[
8]
[9]
co-workers. Although adsorption microcalorimetry and
[
10]
variable-temperature FTIR spectroscopy were successfully
applied for the study of the interactions between gases and
porous inorganic materials (zeolites etc.), it is not clear from
the literature whether such methods are suitable for the
investigation of macrocyclic ligand complexes. Furthermore,
the interpretation of the results may be difficult because of
incomplete equilibrium between gaseous and solid phases as a
result of the undefined capacity of the surface as well as the
possibility of changes in the crystal structure in the course of
Figure 1. Chemical and schematic structures of cucurbit[6]uril (1)and
b-cyclodextrin (2).
The principle of the approach is as follows: The equilibria
for the complexation in solution and at the solid–gas interface
are related to the Gibbs free energies (G), enthalpies (H), and
entropies (S) of the reaction participants in solution (solid
host, gaseous guest, and “solid–gas” complex) through a
thermodynamic cycle represented by Scheme 1.
[
8]
the reaction. To date, no direct methods seem to have been
adapted to determine of thermodynamic parameters for the
complexation of gases by solid macrocyclic receptors.
The chemical reactions, which are difficult to study in an
experiment, can be described on the basis of Born–Haber-
[
11]
type thermodynamic cycles. Thus, Gox and Schneider have
obtained the thermodynamic parameters of the complexation
[
*] Dr. A. G. Grechin, Dr. H.-J. Buschmann, Prof. Dr. E. Schollmeyer
Deutsches Textilforschungszentrum Nord-West e.V.
Universität Duisburg-Essen
Adlerstrasse 1, 47798 Krefeld (Germany)
Fax : (+49)2151-843-143
Scheme 1. Thermodynamic cycle for the calculation of solid–gas
E-mail: buschmann@dtnw.de
complexation equilibria.
Angew. Chem. Int. Ed. 2007, 46, 6499 –6501
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
The thermodynamic parameters of the complexation in
The stability constants, reaction enthalpies, and entropies
for the complexation of several amines in the gas phase by
solid cucurbit[6]uril and b-cyclodextrin, as well as the
corresponding data in aqueous solution are presented in
Table 1. The thermodynamic parameters determined in acidic
solution were recalculated for the reactions of unprotonated
amines as described in the Experimental Section. The
reactions in solution and under solid–gas conditions are
related to the complexation of unprotonated amines.
solution can be determined by using calorimetric, potentio-
metric, or spectrophotometric titrations. The Gibbs free
energies of solution are calculated from solubility values
according to Equation (1).
DG ¼ À2:303RTlogS
ð1Þ
S
The enthalpies of solution may be obtained by using
solution calorimetry or from the temperature-dependence of
the solubility.
Table 1: Thermodynamic data, logK, DH, and TDS for the complexation
of n-alkylamines (Am)by cucurbit[6]uril (cuc)and b-cyclodextrin (b-CD).
The thermodynamic parameters for the complexation of a
gaseous guest by a solid host are calculated from Equa-
tion (2).
[
a]
À1
[a]
À1
Amine
log K
-DH [kJmol
]
298.15DS [kJmol ]
Cuc(6)(aq) +Am(aq),[Cuc(6)ꢁAm]
(aq)
D Y
¼ D Y À½DY ðComplexÞÀDY ðHostÞÀDY ðGuestފ
r
solidÀgas
r
aq
S
S
S
3
4
5
6
6.6Æ0.3
6.3Æ0.2
6.3Æ0.2
5.9Æ0.1
62.2Æ0.7
77.0Æ1.0
78.5Æ0.7
74.3Æ0.6
À24.5Æ1.0
À41.1Æ1.2
À42.7Æ0.9
À40.4Æ0.7
ð2Þ
0
D Y
r
is the Gibbs free energy or enthalpy for “solid–
solidÀgas
0
Cuc(6) +Am ,[Cuc(6)ꢁAm]
(
s)
(g)
(s)
gas” complexation, D Y is the corresponding parameter for
r
aq
0
3
4
5
6
5.3Æ0.4
4.9Æ0.3
4.3Æ0.3
4.4Æ0.2
118.1Æ0.9
136.0Æ1.0
140.6Æ0.8
140.1Æ0.7
À87.8Æ1.3
the reaction in solution (here in aqueous solution), and DYs
are the Gibbs free energies or enthalpies of solution of the
reactants.
The free energies and enthalpies of the amines in solution
and the solubility of cucurbit[6]uril in water at 258C were
À108.0Æ1.3
À116.1Æ1.1
À115.0Æ0.9
[
b]
b-CD( +Am(aq),[b-CDꢁAm]
aq)
(aq)
3
4
5
6
4.3Æ0.07
4.3Æ0.06
4.3Æ0.06
4.6Æ0.04
1.6Æ0.05
1.5Æ0.15
1.6Æ0.15
1.2Æ0.13
22.7Æ0.5
23.1Æ0.5
23.2Æ0.5
[
13,14]
taken from the literature.
For many inclusion processes,
an almost complete shielding of the guest (cation or
molecule) in the complex from interactions with the solvent
leads to an appropriate parity in the thermodynamic functions
of solution (transfer) of the host (ligand) and the host–guest
25.1Æ0.4
[b]
b-CD +Am(g),[b-CDꢁAm]
(
s)
(s)
3
4
5
6
4.6Æ0.1
4.5Æ0.1
4.3Æ0.2
4.4Æ0.1
57.5Æ0.1
60.6Æ0.2
63.7Æ0.2
67.0Æ0.2
À32.6Æ0.7
À35.8Æ0.8
À39.6Æ0.8
À41.6Æ0.8
[
12a]
complex [Eq. (3)]
DY ðcomplexÞ ꢀ DY ðhostÞ
ð3Þ
S
S
[
a] Mole fraction standard state. [b] Data from Ref. [15].
We have recently found that the Gibbs free energies of b-
cyclodextrin–amine complexes in solution are nearly identical
[
15]
with that of b-cyclodextrin—amine complexes. The cucur-
bit[6]uril–amine complexes are somewhat more soluble than
that of the free host, and the assumption in Equation (3) in
terms of the Gibbs free energies is not applicable. For these
complexes the stability constants are calculated from Equa-
tion (2).
As has previously been accepted, the enthalpies of the
host (here cucurbit[6]uril) and corresponding amine com-
plexes in solution are assumed to be in accordance with
Equation (3). This equation is valid only for the inclusion
complexes in which all the solvating sites of the guest
molecule are coordinated by the functional groups of the
host. At the same time, it allows the enthalpies and entropies
for the solid–gas complexation to be estimated when appro-
priate experimental data on solution enthalpies (and entro-
pies) of the complexes are not available.
In the reactions of solid hosts with gaseous guests, any
solvation effects are lacking and the interaction forces, as
indicated by the enthalpy changes, become more favorable
upon transfer of the reactions from solution to solid–gas
conditions. At the same time, the reaction entropies are
changed from favorable (positive) to negative values for
b-cyclodextrin complexes and to more negative values for
cucurbit[6]uril complexes (Table 1).
In summary, we have demonstrated that the thermody-
namics for the complexation of gaseous substances by solid
macrocyclic receptors can be studied on the basis of the data
obtained in solution. We intend to apply this concept to the
complexation of different gases and to developing new
effective gas-binding systems.
[
15]
As all the experiments were performed in solution, any
adsorption effects at the host surfaces are lacking. Incomplete
complexation or possible changes in the crystal structure
during the course of the reaction (as may happen by direct
measurements in heterogeneous systems) were also excluded
in the study. Elemental analysis of the obtained solid
complexes shows an exactly 1:1 composition.
Experimental Section
All amines were of the highest commercially available purity (Fluka).
[
16]
Cucurbit[6]uril was prepared as described previously.
Distilled
deionized water was used throughout the experiments. pH titrations
were performed by using a GLpKa analyzer (Sirius Analytical
Instruments, Forest Row, UK). During the titrations the ionic strength
I was kept constant with KCl at I = 0.15m. At least a tenfold excess of
6
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Angewandte
Chemie
the macrocyclic ligand was used for the titrations to ensure the
complete formation of a complex. The potentiometric titration curves
were analyzed by using software package Refinement Pro (ver-
sion V1.114, Sirius Analytical Instruments, Forest Row, UK). Stability
Keywords: amines · gas-phase reactions · host–guest systems ·
macrocycles · thermodynamics
.
constants Kaq,H
+
determined in acidic solution for the reaction
[
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a
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(
aq)
unprotonated amine (Cuc( + Am(aq)Ð[CucꢁAm](aq)) according to
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[
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a
ðamineÞ
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[
Table 2: Experimental data, pK [K in lmolÀ1], ÀDH [kJmol ], and
À1
a
a
À1
S [moll ] for cucurbit[6]uril–amine complexes.
n-propyl
amine
n-butyl
amine
n-pentyl
n-hexyl
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10.61
10.67
10.72
10.78
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a
pK (complex)9.39
Æ0.03 9.09Æ0.08 9.29Æ0.07 9.60Æ0.05
a
[
a]
ÀDH (amine)
57.4
58.7
59.5
60.6
a
ÀDH (complex)9.4 Æ0.3
9.0Æ0.5
4.71Æ0.1
26.8
4.7Æ0.1
8.4Æ0.3
4.84Æ0.1
27.4
4.3Æ0.1
8.4Æ0.2
4.76Æ0.1
22.1
4.2Æ0.1
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a
log K
ÀD H
+
5.4Æ0.2
aq,H
[
b]
14.2
r
aq,H+
4
S(complex)ꢀ10
4.2Æ0.1
[
a] Data from Ref. [17]. [b] Data from Ref. [18]
Similarly, the reaction enthalpies in acidic solution DHaq,H were
+
recalculated for the reaction of unprotonated amine by using
Equation (5).
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1
D H ¼ D H
þ
À½DH ðcomplexÞÀDH ðamineފ
ð5Þ
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r
aq
r
aq,H
a
a
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9
45 24 12
322.
4
.30, N 33.16, C/N 1.34:1; found: C 38.32, H 4.67, N 29.71, C/N 1.29:1;
[
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4
0
47 24 12
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4
1
49 24 12
(1084.01): C 45.43, H 4.56, N 32.30, C/N 1.41:1; found: C 39.63, H 4.67,
N 29.29, C/N 1.35:1; calcd for C H N O (1098.19): C 45.94, H 4.68,
4
2
51 24 12
N 31.89, C/N 1.35:1; found: C 39.11, H 4.75, N 28.35, C/N 1.38:1.
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Received: April 16, 2007
Published online: July 24, 2007
Angew. Chem. Int. Ed. 2007, 46, 6499 –6501
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6501