coordinated Ag links three TCB units with dimensions of ca 1 nm to form cavities, in the centers of which are the triflate anions. The layers are stacked in an
ABCABC manner.
A similar methodology was developed simultaneously by Yaghi. In this instance, he used 4,4
0 -bipyridine as the organic component, using the two nitrogens
of the molecule for ligation. This lead to three-dimensional compounds with interpenetrating subnetworks; the cations used were Cu 2 Cl 2 dimers, Cu and Ag
[125]. It is noteworthy that the cavities generated in this family of compounds accommodate either solvent molecules or exchangeable anions, at variance to the
situation in zeolites. However, the vast majority of cyano- or amino-based coordination polymers do not remain crystalline after solvent removal.
In terms of stability, carboxylates and phosphonates have proven to be the most
useful coordinating groups for hybrid systems. Indeed, many examples remain
crystalline following solvent removal, often yielding high surface areas. Yaghi and
coworkers have prepared several structures based on zinc and aromatic carboxylic
acids [126], including a zinc 1,4-benzenedicarboxylate compound, MOF-5 (MOF ¼
metal organic framework) [127]. This structure, illustrated in Figure 18.8, consists
of Zn 4 O(OOCaR) 6 clusters where six carboxylate groups coordinate to four tetrahedral zinc atoms surrounding a central oxygen atom. This arrangement, known
in molecular form from coordination chemistry [128], may be pictured as an octahedron of carboxylate groups. The bifunctional 1,4-benzenedicarboxylate serves
to bridge these clusters into a three-dimensional fcc framework with comparable
pore volume and adsorption–desorption characteristics to the zeolite faujasite. The
structural versatility possible in coordination polymer systems has been dramatically illustrated by the synthesis of 16 variations [129] of the MOF-5 structure type,
with cavities ranging from 0.38 to 2.88 nm, simply by changing the dicarboxylate
used in the synthesis. These structures show promising methane adsorption characteristics, and have recently been investigated for hydrogen storage [130]. Similarly, Williams and coworkers prepared a related porous structure [131] based on
Cu(II) dimers linked by 1,3,5-benzenetricarboxylate. The channels contain water
bound to Cu sites that can be reversibly exchanged with amines and thus might
find applications in catalysis, separations or chemical sensing. Kim et al. produced a homochiral compound by using only one enantiomer of a chiral coordination complex to build a porous framework [132]. This material shows selectivity
in both adsorption and catalysis based on chirality and may find applications in
enantiomer-specific separations or catalysis. Finally, the squarate ion, has recently
been used (with Co(II) and other metals) to create architectures that are remarkably similar to that of zeolite sodalite [133].
One remarkable aspect of many of these materials, as well as the hybrid metal
oxide materials discussed in the next section, is that templates are rarely used in
their synthesis. Instead, the open architectures are a consequence of interactions
between the organic and inorganic groups. One notable exception is an openframework zinc oxalate where the organic template, guanadinium, occupies a system of channels in the structure [134].
Many intriguing coordination polymers prepared in the last decade utilized flex18 Nanoporous Materials
610
ABCABC manner.
A similar methodology was developed simultaneously by Yaghi. In this instance, he used 4,4
0 -bipyridine as the organic component, using the two nitrogens
of the molecule for ligation. This lead to three-dimensional compounds with interpenetrating subnetworks; the cations used were Cu 2 Cl 2 dimers, Cu and Ag
[125]. It is noteworthy that the cavities generated in this family of compounds accommodate either solvent molecules or exchangeable anions, at variance to the
situation in zeolites. However, the vast majority of cyano- or amino-based coordination polymers do not remain crystalline after solvent removal.
In terms of stability, carboxylates and phosphonates have proven to be the most
useful coordinating groups for hybrid systems. Indeed, many examples remain
crystalline following solvent removal, often yielding high surface areas. Yaghi and
coworkers have prepared several structures based on zinc and aromatic carboxylic
acids [126], including a zinc 1,4-benzenedicarboxylate compound, MOF-5 (MOF ¼
metal organic framework) [127]. This structure, illustrated in Figure 18.8, consists
of Zn 4 O(OOCaR) 6 clusters where six carboxylate groups coordinate to four tetrahedral zinc atoms surrounding a central oxygen atom. This arrangement, known
in molecular form from coordination chemistry [128], may be pictured as an octahedron of carboxylate groups. The bifunctional 1,4-benzenedicarboxylate serves
to bridge these clusters into a three-dimensional fcc framework with comparable
pore volume and adsorption–desorption characteristics to the zeolite faujasite. The
structural versatility possible in coordination polymer systems has been dramatically illustrated by the synthesis of 16 variations [129] of the MOF-5 structure type,
with cavities ranging from 0.38 to 2.88 nm, simply by changing the dicarboxylate
used in the synthesis. These structures show promising methane adsorption characteristics, and have recently been investigated for hydrogen storage [130]. Similarly, Williams and coworkers prepared a related porous structure [131] based on
Cu(II) dimers linked by 1,3,5-benzenetricarboxylate. The channels contain water
bound to Cu sites that can be reversibly exchanged with amines and thus might
find applications in catalysis, separations or chemical sensing. Kim et al. produced a homochiral compound by using only one enantiomer of a chiral coordination complex to build a porous framework [132]. This material shows selectivity
in both adsorption and catalysis based on chirality and may find applications in
enantiomer-specific separations or catalysis. Finally, the squarate ion, has recently
been used (with Co(II) and other metals) to create architectures that are remarkably similar to that of zeolite sodalite [133].
One remarkable aspect of many of these materials, as well as the hybrid metal
oxide materials discussed in the next section, is that templates are rarely used in
their synthesis. Instead, the open architectures are a consequence of interactions
between the organic and inorganic groups. One notable exception is an openframework zinc oxalate where the organic template, guanadinium, occupies a system of channels in the structure [134].
Many intriguing coordination polymers prepared in the last decade utilized flex18 Nanoporous Materials
610