absorbance data, the corresponding K a values was determined to be 2.0 Â 10
4 M
À1
for G9 & H10 and 1.7 Â 10
3 M
À1 for G9 & H9. H9 and H10 have similar crown
ether moiety and the same negative charge, but the K a value of G9 & H10 is about 11
times higher than that of G9 & H9, indicating that the third arm contributed about
one order of magnitude to its binding to G9. Huang et al. also performed the
assembly and disassembly of G9 & H10 controlled by acid/base treatment. When
complex of G9 & H10 is treated by adding HCl solution, the carboxylate groups on
the third arm are protonated, and H10 loses its negative charge and water solubility.
The protonated H10 can also be turned back to negatively charged H10 by adding
NaOH, making the cryptand H10 be controlled in aqueous solution.
1.5
Molecular Assembly Based on Water-Soluble Aromatic
Crown Ethers
In the preceding sections, we have gained a deep insight into structural features and
molecular recognition process of water-soluble aromatic crown ethers, which endowed them significant applications in several fields, including supramolecular metalorganic frameworks (MOFs), amphiphilic aggregations, controlled release, nanoassembly, and so on.
Loeb and co-workers prepared and characterized a large collection of supramolecular MOFs with rotaxane linkers. These MOF systems usually employed a
pseudorotaxane templating motif involving 1,2-bis(4-pyridyl-pyridinium)ethane
axles and dibenzo[24]crown-8 wheels [32]. To eliminate independent counter
anions and increase the stability of pseudorotaxane templating containing G4, the
disulfonated crown ether H1 is more beneficial to be used as a wheel to form a
neutrally charged [2]pseudorotaxane of G4 & H1. To this end, G4Á(BF4) 2 and
[Me 4 N] 2 ÁH1 are used as reagents to synthesize MOFs containing [2]rotaxane linkers
(Fig. 17). This ligand forms reaction mixture one-periodic coordination polymer
consisting of [Cu 2 (BnO)] with formula [Cu 2 (BnO) 4 (G4 & H1)] (MeOH) 2 (DMF)
[33]. The MOF is a one-periodic coordination polymer consisting of Cu(II)
paddlewheel nodes and [G4 & H1] inkers. The negative charges of the sulfonate
groups on H1 counteract the positive charge of G4 to form an independent
bipyridine ligand. Each [Cu 2 (BnO) 4 ] cluster acts as a binuclear complex to coordinate to pyridine groups of G4 & H1 to form one-periodic polymer chain in crystals.
Besides the one-periodic polymer chain of copper MOFs based on [2]pseudorotaxane ligand of G4 & H1, Loeb et al. also reported one-, two- and three-periodic
MOFs using G4 & H1 ligand and zinc ions (Fig. 18). By mixing G4 & H1 with
one equivalent of Zn(NO 3 ) 2 (H 2 O) 6 in methanol, MOF crystals with formula [Zn
(G4 & H1) 2 (H 2 O) 2 (MeOH)][NO 3 ]Á2MeOH were obtained [34]. Each Zn(II) adopts
an octahedral geometry to coordinate with two pyridine groups on G2 & H1 and two
water molecules. A methanol molecule and a sulfonate group from H1 wheel occupy
the remaining coordinating site. H1 is stopped on the G2 axles during the coordination process. The neighboring linear chains are joined together through a head-totail fashion by coordination of a sulfonate group to neighboring Zn(II) center on each
18
L. Chen and Y. Liu
4 M
À1
for G9 & H10 and 1.7 Â 10
3 M
À1 for G9 & H9. H9 and H10 have similar crown
ether moiety and the same negative charge, but the K a value of G9 & H10 is about 11
times higher than that of G9 & H9, indicating that the third arm contributed about
one order of magnitude to its binding to G9. Huang et al. also performed the
assembly and disassembly of G9 & H10 controlled by acid/base treatment. When
complex of G9 & H10 is treated by adding HCl solution, the carboxylate groups on
the third arm are protonated, and H10 loses its negative charge and water solubility.
The protonated H10 can also be turned back to negatively charged H10 by adding
NaOH, making the cryptand H10 be controlled in aqueous solution.
1.5
Molecular Assembly Based on Water-Soluble Aromatic
Crown Ethers
In the preceding sections, we have gained a deep insight into structural features and
molecular recognition process of water-soluble aromatic crown ethers, which endowed them significant applications in several fields, including supramolecular metalorganic frameworks (MOFs), amphiphilic aggregations, controlled release, nanoassembly, and so on.
Loeb and co-workers prepared and characterized a large collection of supramolecular MOFs with rotaxane linkers. These MOF systems usually employed a
pseudorotaxane templating motif involving 1,2-bis(4-pyridyl-pyridinium)ethane
axles and dibenzo[24]crown-8 wheels [32]. To eliminate independent counter
anions and increase the stability of pseudorotaxane templating containing G4, the
disulfonated crown ether H1 is more beneficial to be used as a wheel to form a
neutrally charged [2]pseudorotaxane of G4 & H1. To this end, G4Á(BF4) 2 and
[Me 4 N] 2 ÁH1 are used as reagents to synthesize MOFs containing [2]rotaxane linkers
(Fig. 17). This ligand forms reaction mixture one-periodic coordination polymer
consisting of [Cu 2 (BnO)] with formula [Cu 2 (BnO) 4 (G4 & H1)] (MeOH) 2 (DMF)
[33]. The MOF is a one-periodic coordination polymer consisting of Cu(II)
paddlewheel nodes and [G4 & H1] inkers. The negative charges of the sulfonate
groups on H1 counteract the positive charge of G4 to form an independent
bipyridine ligand. Each [Cu 2 (BnO) 4 ] cluster acts as a binuclear complex to coordinate to pyridine groups of G4 & H1 to form one-periodic polymer chain in crystals.
Besides the one-periodic polymer chain of copper MOFs based on [2]pseudorotaxane ligand of G4 & H1, Loeb et al. also reported one-, two- and three-periodic
MOFs using G4 & H1 ligand and zinc ions (Fig. 18). By mixing G4 & H1 with
one equivalent of Zn(NO 3 ) 2 (H 2 O) 6 in methanol, MOF crystals with formula [Zn
(G4 & H1) 2 (H 2 O) 2 (MeOH)][NO 3 ]Á2MeOH were obtained [34]. Each Zn(II) adopts
an octahedral geometry to coordinate with two pyridine groups on G2 & H1 and two
water molecules. A methanol molecule and a sulfonate group from H1 wheel occupy
the remaining coordinating site. H1 is stopped on the G2 axles during the coordination process. The neighboring linear chains are joined together through a head-totail fashion by coordination of a sulfonate group to neighboring Zn(II) center on each
18
L. Chen and Y. Liu