improved. Consequently, introducing more sulfonate groups to the aromatic skeleton
of benzo-crown ethers is an ideal strategy to this approach, in which stronger
electrostatic attraction is generated. Unlike the o- and p-diaryoxy benzene that
can react with only one sulfonato reagent, 1,3-diaryoxy benzene has better orientation-directed effect for sulfonation, so it can easily react with two sulfonato reagents
on 4,6-position to obtain 1,3-diaryoxy-4,6-disulfonic acid [22]. Thus, Liu et al.
reported the tetrasulfonated bis(m-phenylene)-26-crown-8 which was prepared
by the simple reaction of bis(m-phenylene)-26-crown-8 with chlorosulfonic acid
(Fig. 8) [23]. They investigated the binding behaviors and thermodynamic parameters for the host-guest complexation of G1 & H3 and G8 & H3 by method of
isothermal titration calorimetry (ITC) measurements (Table 3). In the case of
G1 & H3, the binding process is mainly contributed by positive entropy change.
In another case of G8 & H3, the binding process is governed by both negative
enthalpy and positive entropy changes. The distinct thermodynamic nature of the
two complexes reveals they should have different binding modes.
Crystal structures of G1 & H3 and G8 & H3 (Fig. 9) give direct evidence for their
different binding behaviors. Due to the intensive positive charges of G1, in crystal
structure of G1 & H3, the guest G1 is located outside the cavity of H3 and is in close
contact with the sulfonate groups to form more exclusive electrostatic attraction. On
the other hand, the guest G8 interpenetrated through the cavity to form π-stacking,
hydrogen bonds, and electrostatic attraction with H1, which is attributed to the πelectron-poor aspect of G8.
Although the tetrasulfonated crown ether H3 is a successful water-soluble aromatic crown ether, its weak binding ability indicates that it is still possible to improve
its molecular structure to enhance the binding strength toward cationic guests. In this
context, Liu et al. synthesized two tetrasulfonated naphtho-crown ethers, which are
named tetrasulfonated 1,5-dinaphtho-38-crown-10 (H4) and tetrasulfonated 1,5dinaphtho-32-crown-8 (H5) (Fig. 10) [24]. Possessing more extended π-electronH3
O
O
O
O
O
O
O
O
O 3 S
O 3 S
SO 3
SO 3
Fig. 8 Molecular structure of
tetrasulfonated crown ether
H3
Table 3 Association constants (K a , M
À1
), enthalpy change (ÀΔH
, kJ/mol), and entropy change
(TΔS
, kJ/mol) for intermolecular complexation of H3
Hosts
Guests
lgK a
ÀΔH
TΔS
Ref.
H3
G1
2.4 Â 10
2
3.03
11.46
[23]
G8
1.8Â 10
3
10.18
7.80
10
L. Chen and Y. Liu
of benzo-crown ethers is an ideal strategy to this approach, in which stronger
electrostatic attraction is generated. Unlike the o- and p-diaryoxy benzene that
can react with only one sulfonato reagent, 1,3-diaryoxy benzene has better orientation-directed effect for sulfonation, so it can easily react with two sulfonato reagents
on 4,6-position to obtain 1,3-diaryoxy-4,6-disulfonic acid [22]. Thus, Liu et al.
reported the tetrasulfonated bis(m-phenylene)-26-crown-8 which was prepared
by the simple reaction of bis(m-phenylene)-26-crown-8 with chlorosulfonic acid
(Fig. 8) [23]. They investigated the binding behaviors and thermodynamic parameters for the host-guest complexation of G1 & H3 and G8 & H3 by method of
isothermal titration calorimetry (ITC) measurements (Table 3). In the case of
G1 & H3, the binding process is mainly contributed by positive entropy change.
In another case of G8 & H3, the binding process is governed by both negative
enthalpy and positive entropy changes. The distinct thermodynamic nature of the
two complexes reveals they should have different binding modes.
Crystal structures of G1 & H3 and G8 & H3 (Fig. 9) give direct evidence for their
different binding behaviors. Due to the intensive positive charges of G1, in crystal
structure of G1 & H3, the guest G1 is located outside the cavity of H3 and is in close
contact with the sulfonate groups to form more exclusive electrostatic attraction. On
the other hand, the guest G8 interpenetrated through the cavity to form π-stacking,
hydrogen bonds, and electrostatic attraction with H1, which is attributed to the πelectron-poor aspect of G8.
Although the tetrasulfonated crown ether H3 is a successful water-soluble aromatic crown ether, its weak binding ability indicates that it is still possible to improve
its molecular structure to enhance the binding strength toward cationic guests. In this
context, Liu et al. synthesized two tetrasulfonated naphtho-crown ethers, which are
named tetrasulfonated 1,5-dinaphtho-38-crown-10 (H4) and tetrasulfonated 1,5dinaphtho-32-crown-8 (H5) (Fig. 10) [24]. Possessing more extended π-electronH3
O
O
O
O
O
O
O
O
O 3 S
O 3 S
SO 3
SO 3
Fig. 8 Molecular structure of
tetrasulfonated crown ether
H3
Table 3 Association constants (K a , M
À1
), enthalpy change (ÀΔH
, kJ/mol), and entropy change
(TΔS
, kJ/mol) for intermolecular complexation of H3
Hosts
Guests
lgK a
ÀΔH
TΔS
Ref.
H3
G1
2.4 Â 10
2
3.03
11.46
[23]
G8
1.8Â 10
3
10.18
7.80
10
L. Chen and Y. Liu