Metal-carboxylate gels may be further functionalized by modifying the ligands
or metal ions. Dastidar and co-workers found that reaction of C 3 -symmetric
tris-amide-tris-carboxylate ligand 48 (Scheme 3.10) with various metal salts such as
Cd(NO 3 ) 2 , Cu(NO 3 ) 2 , Co(NO 3 ) 2 and Zn(NO 3 ) 2 in pure water resulted in metallogels [58]. Moulton and co-workers reported that metal cluster
Mn 12 O 12 (AcO) 16 (H 2 O) 4 coordinates with polycarboxylate ligands to form gels
[59]. Thiacalix [4] arene-based Co
2+ gel was reported by Jung and co-workers [60].
The gel was prepared from thiacalix [4] arene derivative (49, Scheme 3.10) and
Co
2+ in H 2 O-DMF at pH = 7.0. The gel shows selectively colour changing property in the presence of gases that yield hydrogen chloride upon hydrolysis due to the
coordination geometry, which is proved by single-crystal X-ray. The red gel
derived from Co(NO 3 ) 2 , an octahedral Co
2+ complex, changes to blue gel with a
tetrahedral structure by the introducing of volatile gases containing a chlorine atom
(VGCl). More detailly, the addition of VGCl results in the removal of three DMF
molecules by the intermolecular hydrogen-bonding interaction between DMF and
HCl and forming the tetrahedral structure meanwhile. The gel can be used as a
chemoprobe for VGCl such as HCl, SOCl 2 , (COCl) 2 and COCl 2 .
K
+ /Rb
+ gels based on 49 (Scheme 3.10) were prepared in MeOH-water (v:v 1:1)
by Lee, Park and co-workers [61]. The formation of the K
+
/Rb
+ metallogels suggests metal ion recognition based on the size effect is an important factor for the
gelation because other alkali metal ions are too small (e.g. Li
+
, Na
+ ) or too big (e.g.
Cs
+
) to accommodate in gelator 49. The Rb
+ gel is more stable than K
+ gel. The
analysis of single crystals from the corresponding gels through thermal treatment
indicates that each of two K
+ present in the calix moiety is coordinated by two
phenolic oxygen and two monodentate carboxyl oxygen atoms from different
pendants in a cis-arrangement for 49-K
+
. The association of the dipotassium(I) unit
cross-links via intermolecular hydrogen bonding, generating a pseudo 3D framework. Different from 49-K
+
, the connectivity pattern of 49-Rb
+ is strongly associated with triply bridged additional two Rb
+ ions in different calix moieties. In
short, both the K
+ and the Rb
+ gel have an H-bonded framework, while the Rb
+ gel
is more stable due to extra coordination bonds.
The design principles of metal-carboxylate gels may be extended into various
systems. A high proton conducting metallogel is synthesized by immobilizing
protogenic phytic acid (50, Scheme 3.11) [62] using iron(III) nitrate in DMF. The
gel consists of 20–40 nm nanospheres which interweave into a 3D gel network. The
xerogel has a Brunauer–Emmett–Teller (BET) surface area of 124 m
2 g
−1 and
shows high proton conductivity of 2.4 Â 10
−2 S cm
−1 at 120 °C. Such gel materials are used as solid electrolyte in fuel cell with 0.94 mW cm
−2 power density at
0.6 V.
A series of phenolic gels were prepared through direct gelation of 51
(Scheme 3.11) [63] and group IV metal ions. Polyphenolic compound 51 consists
of five digalloyl ester groups covalently attached to a central glucose core. These
metallogels form in different organic solvents and aqueous solutions through simple
mixing at a range of molar ratios. Redox reactions between 51 and Ti
4+
during gelation, the high oxidation state and formal charge of Ti
4+ play a significant
84
3 Metal–Organic Gels
or metal ions. Dastidar and co-workers found that reaction of C 3 -symmetric
tris-amide-tris-carboxylate ligand 48 (Scheme 3.10) with various metal salts such as
Cd(NO 3 ) 2 , Cu(NO 3 ) 2 , Co(NO 3 ) 2 and Zn(NO 3 ) 2 in pure water resulted in metallogels [58]. Moulton and co-workers reported that metal cluster
Mn 12 O 12 (AcO) 16 (H 2 O) 4 coordinates with polycarboxylate ligands to form gels
[59]. Thiacalix [4] arene-based Co
2+ gel was reported by Jung and co-workers [60].
The gel was prepared from thiacalix [4] arene derivative (49, Scheme 3.10) and
Co
2+ in H 2 O-DMF at pH = 7.0. The gel shows selectively colour changing property in the presence of gases that yield hydrogen chloride upon hydrolysis due to the
coordination geometry, which is proved by single-crystal X-ray. The red gel
derived from Co(NO 3 ) 2 , an octahedral Co
2+ complex, changes to blue gel with a
tetrahedral structure by the introducing of volatile gases containing a chlorine atom
(VGCl). More detailly, the addition of VGCl results in the removal of three DMF
molecules by the intermolecular hydrogen-bonding interaction between DMF and
HCl and forming the tetrahedral structure meanwhile. The gel can be used as a
chemoprobe for VGCl such as HCl, SOCl 2 , (COCl) 2 and COCl 2 .
K
+ /Rb
+ gels based on 49 (Scheme 3.10) were prepared in MeOH-water (v:v 1:1)
by Lee, Park and co-workers [61]. The formation of the K
+
/Rb
+ metallogels suggests metal ion recognition based on the size effect is an important factor for the
gelation because other alkali metal ions are too small (e.g. Li
+
, Na
+ ) or too big (e.g.
Cs
+
) to accommodate in gelator 49. The Rb
+ gel is more stable than K
+ gel. The
analysis of single crystals from the corresponding gels through thermal treatment
indicates that each of two K
+ present in the calix moiety is coordinated by two
phenolic oxygen and two monodentate carboxyl oxygen atoms from different
pendants in a cis-arrangement for 49-K
+
. The association of the dipotassium(I) unit
cross-links via intermolecular hydrogen bonding, generating a pseudo 3D framework. Different from 49-K
+
, the connectivity pattern of 49-Rb
+ is strongly associated with triply bridged additional two Rb
+ ions in different calix moieties. In
short, both the K
+ and the Rb
+ gel have an H-bonded framework, while the Rb
+ gel
is more stable due to extra coordination bonds.
The design principles of metal-carboxylate gels may be extended into various
systems. A high proton conducting metallogel is synthesized by immobilizing
protogenic phytic acid (50, Scheme 3.11) [62] using iron(III) nitrate in DMF. The
gel consists of 20–40 nm nanospheres which interweave into a 3D gel network. The
xerogel has a Brunauer–Emmett–Teller (BET) surface area of 124 m
2 g
−1 and
shows high proton conductivity of 2.4 Â 10
−2 S cm
−1 at 120 °C. Such gel materials are used as solid electrolyte in fuel cell with 0.94 mW cm
−2 power density at
0.6 V.
A series of phenolic gels were prepared through direct gelation of 51
(Scheme 3.11) [63] and group IV metal ions. Polyphenolic compound 51 consists
of five digalloyl ester groups covalently attached to a central glucose core. These
metallogels form in different organic solvents and aqueous solutions through simple
mixing at a range of molar ratios. Redox reactions between 51 and Ti
4+
during gelation, the high oxidation state and formal charge of Ti
4+ play a significant
84
3 Metal–Organic Gels