4.1.2 Imine/Acylhydrazone Gels and Metal Ions
The properties of imine gels can be readily controlled and tuneable by metal ions
considering imines are good ligands for metal ions. Bunzen and co-workers synthesized in situ imine metallogelators with cholesterol groups 20 from steroidal amine 19,
2-pyridinecarboxaldehyde and metal ions (Cu
2+ , Ni
2+ , Zn
2+ ) in a 3:3:1 ratio
(Scheme 4.7) [16]. The resulting gelator molecules contain three cholesterol groups
because the selected metal ions are hexacoordinate with octahedral coordination
geometry. The precursor molecules spontaneously self-assemble around template
metal ions via simultaneous imine and coordination bond formation. Compounds 20
induce gels in various alcohols at room temperature. The gels are multi-responsive and
are also sensitive to stoichiometry, thermal and chemical stimuli.
O
O
N
H
H2N
OHOH
N
N
NH
O
NH2
+
_
M +
N
N
N
N
N
H
N
N N
N
H
O
H
H
N
H
N
N
N
N
O H
O
H
N
H
N H
N N
O
H
N
H
H
O
N
H
H2N
HO OH
O
O
NH
H2N
OH
OH
O
O
H
N NH2
OH
HO
O
HN
NH2
HO
HO
O
O
+ R-CHO
- R-CHO
18
17
16
R-CHO =
O
NaO3S
CHO
N
CHO
O P O
OH
OH
M +
N
N
N
N
N
H
N
N N
N
H
O
H
H
N
H
N
N
N
N
O H
O
H
N
H
N H
N N
O
H
N
H
H
O
N
H
N
HO OH
O
O
NH
N
OH
OH
O
O
H
N N
OH
HO
O
HN
N
HO
HO
O
O
R
R
R
R
M +
M +
Scheme 4.6 Reversible decoration of G-quartet assembly 17 of guanosine hydrazide 16 via
hydrazone formation with aldehydes
H 2 N
N
H
O
H
H
H
H
O
19
N
H
O
+
N
N
H
O
H
H
H
H
O
N
M 2+
3
M 2+
20 M = Cu, Ni or Zn
H 2 N
O
O
22
H
O
21
+
N
O
O
OH
OH
23
N
O
O
O
24
M n+
M 2+
HO
N
O
N
H
O
C 18 H 37
HN
C 18 H 37
25
M 2+
O
N
O
N
H
O
C 18 H 37
HN
C 18 H 37
26
M 2+
M = Zn or Cu
Scheme 4.7 Formation of imine metallogelators 19, 24 and 26
128
4 Dynamic Covalent Gels