functional groups react to form dynamic covalent polymers in coordination polymer
gels. Such metal–organic gels usually have infinitely extended structures, and thus
cannot be redissolved upon heating and do not show thermoreversible gel–sol
transitions.
3.1 Discrete Gelators
3.1.1 Metal–Organic Gelators with Monodentate Ligands
Pyridyl ligands are widely studied. Monodendate pyridyl ligands form stable
coordination compounds with Pd
2+ /Pt
2+ , which have square-planar coordination
geometry. A series of gelators Pd
2+ /Pt
2+ dipyridyl gelators have been developed
with the aid of long alkyl/oligoether oxide chain or H-bonding motifs. Fernández
and co-workers reported an oligophenyleneethynylene (OPE)-based amphiphilic
trans-Pt
2+ compound 1-Pt [1]. 1-Pt (Scheme 3.1) forms supramolecular polymeric
structures in aqueous and polar media driven by p–p and weak C–HÁÁÁX (X=Cl, O)
hydrogen-bonding interactions involving chlorine atoms attached to the Pt
II centres
as well as oxygen atoms and polarized methylene groups belonging to the
peripheral glycol chains. They also found that 1-Pd self-assemble in polar media
into fibre-like structures [2]. The self-assembly is governed by cooperative intraand interstrand CHÁÁÁO interactions between peripheral triethylene glycol chains
leading to slipped p-stacks. Aromatic and CHÁÁÁCl interactions are also important.
Nolte and co-workers developed trans-Pd
2+ /Pt
2+ compound 2 (Scheme 3.1) of
gluconamide derivatives by incorporating a pyridine functionality [3]. 2-Pd gave a
gel in THF and helical ribbons were observed in the xerogel by TEM. 2-Pt is also
an effective gelator, giving fibres but without the helical twist. Steed and co-workers
developed cis-Pt(II) complex 3 (Scheme 3.1) of urea derivative [4]. 3 is a gelator
for toluene, o-xylene and others. Interfacial crystallization of cisplatin in a gel–sol
biphasic system is employed to address the insolubility of the drug molecule in
organic solvents, and a new N,N-dimethylacetamide (DMA) solvate of cisplatin has
been identified.
Feng, Fan and co-workers reported three Ag
+ gelators 4 (Scheme 3.1) of pyridine-functionalized poly(aryl ether) dendritic ligands [5]. These gelators gelate
various organic solvents. During the gelation, strong solvophobic interactions
enhance aromatic p–p stacking interactions of the polar pyridine moieties distinctly,
which synergistically enable the formation of gels. These metallogels respond to
different external stimuli, such as temperature, chemicals and shear stress, resulting
in reversible gel–sol phase transitions. Moreover, these dendritic metallogel systems (Fig. 3.1) can be used as templates to in situ form and stabilize silver
nanoparticles.
Rotaxanes-based a-CD and coordination complexes have been employed for
gelation [6]. Addition of [Pd(en)(NO 3 ) 2 ] (en = 1,2-ethylenediamine) to the aqueous
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3 Metal–Organic Gels
gels. Such metal–organic gels usually have infinitely extended structures, and thus
cannot be redissolved upon heating and do not show thermoreversible gel–sol
transitions.
3.1 Discrete Gelators
3.1.1 Metal–Organic Gelators with Monodentate Ligands
Pyridyl ligands are widely studied. Monodendate pyridyl ligands form stable
coordination compounds with Pd
2+ /Pt
2+ , which have square-planar coordination
geometry. A series of gelators Pd
2+ /Pt
2+ dipyridyl gelators have been developed
with the aid of long alkyl/oligoether oxide chain or H-bonding motifs. Fernández
and co-workers reported an oligophenyleneethynylene (OPE)-based amphiphilic
trans-Pt
2+ compound 1-Pt [1]. 1-Pt (Scheme 3.1) forms supramolecular polymeric
structures in aqueous and polar media driven by p–p and weak C–HÁÁÁX (X=Cl, O)
hydrogen-bonding interactions involving chlorine atoms attached to the Pt
II centres
as well as oxygen atoms and polarized methylene groups belonging to the
peripheral glycol chains. They also found that 1-Pd self-assemble in polar media
into fibre-like structures [2]. The self-assembly is governed by cooperative intraand interstrand CHÁÁÁO interactions between peripheral triethylene glycol chains
leading to slipped p-stacks. Aromatic and CHÁÁÁCl interactions are also important.
Nolte and co-workers developed trans-Pd
2+ /Pt
2+ compound 2 (Scheme 3.1) of
gluconamide derivatives by incorporating a pyridine functionality [3]. 2-Pd gave a
gel in THF and helical ribbons were observed in the xerogel by TEM. 2-Pt is also
an effective gelator, giving fibres but without the helical twist. Steed and co-workers
developed cis-Pt(II) complex 3 (Scheme 3.1) of urea derivative [4]. 3 is a gelator
for toluene, o-xylene and others. Interfacial crystallization of cisplatin in a gel–sol
biphasic system is employed to address the insolubility of the drug molecule in
organic solvents, and a new N,N-dimethylacetamide (DMA) solvate of cisplatin has
been identified.
Feng, Fan and co-workers reported three Ag
+ gelators 4 (Scheme 3.1) of pyridine-functionalized poly(aryl ether) dendritic ligands [5]. These gelators gelate
various organic solvents. During the gelation, strong solvophobic interactions
enhance aromatic p–p stacking interactions of the polar pyridine moieties distinctly,
which synergistically enable the formation of gels. These metallogels respond to
different external stimuli, such as temperature, chemicals and shear stress, resulting
in reversible gel–sol phase transitions. Moreover, these dendritic metallogel systems (Fig. 3.1) can be used as templates to in situ form and stabilize silver
nanoparticles.
Rotaxanes-based a-CD and coordination complexes have been employed for
gelation [6]. Addition of [Pd(en)(NO 3 ) 2 ] (en = 1,2-ethylenediamine) to the aqueous
62
3 Metal–Organic Gels