crown ethers showed moderate binding strengths toward alkali earth metal ions and
lanthanide ions. This is acceptable because alkali earth metal ions and lanthanide
ions have two or three positive charges, which are favorable to form electrostatic
attraction to increase the binding strength. Shigeo et al. even synthesized
disulfonated dibenzo-18-crown-6 and dibenzo-24-crown-8, but they never investigated the host-guest interactions between sulfonated crown ethers and π-electronpoor organic cations, which is achieved by Loeb et al. 10 years later.
N
N
N
N
N
N
O
O
O
O
N
N
O
O
O
O
N
N
N
N
N
N
N
N
N
EtOOC
COOEt
N
N
EtOOC
COOEt
N
N
N
N
N
N
N
N
N
G1
G2
G3
G4
G5
G13
G15
G16
G12
n
n
N
S
N
n
N
N
N
N
N
N
n = 1, G23
n = 2, G24
G19
G20
G22
N
S
N
G21
n = 0, G8
n = 1, G9
n = 3, G10
N
O
OHOH
O
OHOH
O P O P O
O
O
OH O
N
O
NH 2
N
N
N
NH 2
N
G18
G17
N
N
G11
N
N
G14
N
N
NH
O
O
HN
OH
O
G25
N
N
N
N
G6
OOC
COO
N
N
N
N
G7
HOOC
COOH
Fig. 3 The structure of most common guests that associate with water-soluble aromatic crown
ethers
O
O
O
O
O 3 S
O
O
O
O
O
O
O 3 S
SO 3
m
n
n
m = 1, 2, 3
n = 1, 2
Fig. 4 Molecular structures of sulfonated benzo-crown ethers synthesized by Shigeo et al.
1 Water-Soluble Aromatic Crown Ethers
7