CHEMPLUSCHEM
FULL PAPERS
DOI: 10.1002/cplu.201300164
Differences in Cucurbit[7]uril: Surfactant Complexation
Promoted by the Cationic Head Group
[a, b]
[b]
[b]
[a]
Mꢀrcia PessÞgo,
Josꢁ P. Da Silva, Josꢁ A. Moreira, and Luis Garcꢂa-Rꢂo*
In memory of Detlef Schrçder
The formation of host/guest complexes between cucurbit[7]uril
ent of the nature of the surfactant head group. Distinct struc-
+
(
CB7) and tetradecyltrialkylammonium bromide surfactants
tures were found for the 1:1 and 2:1 complexes. C TMA and
14
+
+
+
with trimethyl (C TMA ), triethyl (C TEA ), and tripropyl
C14TPA surfactants form 1:1 inclusion complexes that com-
prise the head group close to the CB7 portal and 2:1 external
complexes with both host molecules near the head group. The
14
14
+
(
C TPA ) head groups was studied. Complexation was studied
14
by ESI-MS, isothermal titration calorimetry (ITC), and NMR spec-
troscopy. The structure of the complexes is strongly dependent
on the surfactant head group. Both 1:1 and 2:1 complexes are
formed with all cationic surfactants. The magnitude of the K1:1
binding constants decreases with the alkyl length of the head
group, whereas that of the K2:1 binding constants is independ-
+
1:1 complexes between C TEA and CB7 showed that the
14
head group was centered inside the CB7 cavity, whereas in the
2:1 complex the second molecule of CB7 was located at the
end of the main alkyl chain.
Introduction
[
12]
The architecture and negative electrostatic potential enable cu-
length of alkyl substituents of the cationic guest. They also
+
+
curbit[n]urils (CBn) to form stable complexes with a variety of
showed that peralkylonium cations, including NR4, PR , and
4
[
1–5]
+
guest molecules in aqueous solution.
The encapsulation of
SR (R=Me, Et, nPr, and nBu), formed very stable inclusion
3
guests is principally driven by hydrophobic effects within the
cavity and ion–dipole interactions at the portals, making am-
phiphilic compounds with positive charges, such as ammoni-
complexes with CB7. The cation moiety of these complexes is
partially or fully located within the host cavity and stronger se-
lectivity is observed for smaller guests. Similar inclusion behav-
ior and cation size dependence was observed for host–guest
interactions between CB7 and N-alkyl and N-aryl cationic
[
6–10]
um or pyridinium ions, the preferred guests.
CB7 has at-
tracted particular interest in recent years owing to its superior
water solubility and appropriate cavity size for the inclusion of
a wide variety of guests. Most cationic amphiphilic guests have
their hydrophobic portion inside the cavity of CB7, whereas
the cationic component is located near the carbonyl oxygen
atoms on the portals outside the cavity.
[13–17]
guests.
We recently demonstrated that CB7 strongly bonds alkyltri-
+
[18]
methylammonium (C TMA ) cations to form 1:1 complexes.
n
Moreover, if CB7 was present in excess, 2:1 complexes were
observed, in which both internal and external complexes were
[18]
Early reports by Mock and Shih on primary alkylammonium
cations and dications revealed that these guests formed very
stable complexes with CB6 in aqueous solution as a result of
proposed (Scheme 1b).
A minimum alkyl chain length of
12 carbon atoms is necessary to observe 2:1 complexes. This
12 carbon distance allows displacement of the first CB7, so
that it can share the guest with another host molecule, while
minimizing electrostatic repulsions between the negative elec-
trostatic potentials of the two host portals.
[
11]
ion–dipole and hydrogen-bonding interactions. The binding
constants depend on the size of the alkyl group and the cat-
[11]
ionic moiety is located outside the cavity near the portals.
Macartney and co-workers found that the stability of host–
guest complexes also depended on the nature, number, and
When the cationic group is included inside CB7 no such dis-
placement of the first CB7 host is expected to form the 2:1
complex. In this case, the second CB7 molecule does not share
[
a] M. PessÞgo, Prof. L. Garcꢀa-Rꢀo
Departamento de Quꢀmica Fꢀsica
Centro de Investigaciꢁn en Quꢀmica Biolꢁgica
y Materiales Moleculares (CIQUS)
Universidad de Santiago, 15782 Santiago (Spain)
E-mail: luis.garcia@usc.es
the cation group, but rather binds the end of the alkyl chain
[18]
(
see Scheme 1c).
With this idea in mind, we synthesized
a series of tetradecyltrialkylammonium bromide surfactants
with different head groups (trimethyl, triethyl, and tripropyl) to
further explore the formation of 2:1 host–guest complexes.
The aim of this study was to explore the binding and struc-
ture of 1:1 and 2:1 host/guest complexes between CB7 and
tetradecyltrialkylammonium surfactants as a function of the
alkyl chain length of the head group. We used well-established
methodologies to investigate the formation and structure of
[b] M. PessÞgo, Dr. J. P. Da Silva, Dr. J. A. Moreira
CIQA, Departamento de Quꢀmica e Farmꢂcia
Faculdade de CiÞncias e Tecnologia, Universidade do Algarve
Campus de Gambelas, 8005-139 Faro (Portugal)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300164.
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2013, 78, 1058 – 1064 1058