12 Kinetic Assembly of Porous Coordination Networks …
223
MIL-101, and UiO-66 [24] are obtained using this technique. These types of synthesis
are classified as thermodynamic assembly because the final product, including the
solvent and adducts, represents a global energy minimum on the reaction coordinate
landscape. In contrast, kinetic assembly traps metastable states during crystallization
before equilibrium is reached, which requires much faster formation rates. The kinetic
products tend to form polycrystalline powders rather than single crystals suitable for
single-crystal structure analysis. As a result, the kinetic network formation has not
been widely explored because of the difficulty of ab initio structure determination of
crystalline powders by X-ray diffraction.
There are several examples of the kinetic assembly of coordination networks
[4, 25–31]. The Cd-based network, [Cd(bpy)(bdc)] (bpy = 4,4
-bipyridine, bdc =
1,4-benzenedicarboxylate) can be generated as a single non-interpenetrating net by
kinetic control at low temperature or high concentration. Whereas an interpenetrating network is obtained under thermodynamic control at high temperature or low
concentration [25]. The interpenetration creates more stable structures relative to
their non-interpenetrating analogues due to increase of crystal density and reduction of void space. Therefore, an increase in the degree of interpenetration tends
to stabilize network structures. The consequence of this behavior is the increase of
the accessible pore space inside the kinetically assembled networks, which could
be beneficial for several applications, such as gas storage and separation. However,
the fine balance between stability and porosity needs to be carefully controlled to
achieve the desired material. Recently, this concept was applied to the synthesis of
Fe-bpdc (bipy = biphenyl-4,4
-dicarboxylate) MOFs [26].
Other methods for the control of reaction pathways during coordination polymer
synthesis have been reported. The most common approach is simply changing the
reaction temperature to produce different network polymorphs. Cheetham et al.
reported that they could obtain five structures from an identical starting mixture
of Co(OH) 3 and succinic acid by running the reaction at five different temperatures
[32]. In another example, a one-dimensional (1D) coordination polymer was formed
by kinetical trapping of the reaction mixture of ZnI 2 and pyrimidine [33]. These
results highlight the tendency of kinetically assembled networks to contain larger
internal voids and exhibit lower density [34–39].
Another important feature of kinetic assembly is the creation of interactive sites
within the structure [40, 41]. Since this process halts the network formation before it
can reach a thermodynamic equilibrium by trapping metastable intermediate states,
the resultant structures could contain “under-reacted” sites. In contrast, the thermodynamic structures typically have minimal latent reactivity and therefore are
comparatively inert.
Figure 12.1 shows the schematic difference between thermodynamic and kinetic
assemblies. In the former situation, the coordinating groups on the linkers and
open sites around the coordination sphere of metal centers are fully paired up.
Whereas in the latter case, some connecting sites remain unoccupied, resulting in
pore environments that can interact with guest molecules.
In the following section, we will describe how interactive pores can be obtained by
kinetic assembly and discuss their applications. In particular, we will demonstrate the
223
MIL-101, and UiO-66 [24] are obtained using this technique. These types of synthesis
are classified as thermodynamic assembly because the final product, including the
solvent and adducts, represents a global energy minimum on the reaction coordinate
landscape. In contrast, kinetic assembly traps metastable states during crystallization
before equilibrium is reached, which requires much faster formation rates. The kinetic
products tend to form polycrystalline powders rather than single crystals suitable for
single-crystal structure analysis. As a result, the kinetic network formation has not
been widely explored because of the difficulty of ab initio structure determination of
crystalline powders by X-ray diffraction.
There are several examples of the kinetic assembly of coordination networks
[4, 25–31]. The Cd-based network, [Cd(bpy)(bdc)] (bpy = 4,4
-bipyridine, bdc =
1,4-benzenedicarboxylate) can be generated as a single non-interpenetrating net by
kinetic control at low temperature or high concentration. Whereas an interpenetrating network is obtained under thermodynamic control at high temperature or low
concentration [25]. The interpenetration creates more stable structures relative to
their non-interpenetrating analogues due to increase of crystal density and reduction of void space. Therefore, an increase in the degree of interpenetration tends
to stabilize network structures. The consequence of this behavior is the increase of
the accessible pore space inside the kinetically assembled networks, which could
be beneficial for several applications, such as gas storage and separation. However,
the fine balance between stability and porosity needs to be carefully controlled to
achieve the desired material. Recently, this concept was applied to the synthesis of
Fe-bpdc (bipy = biphenyl-4,4
-dicarboxylate) MOFs [26].
Other methods for the control of reaction pathways during coordination polymer
synthesis have been reported. The most common approach is simply changing the
reaction temperature to produce different network polymorphs. Cheetham et al.
reported that they could obtain five structures from an identical starting mixture
of Co(OH) 3 and succinic acid by running the reaction at five different temperatures
[32]. In another example, a one-dimensional (1D) coordination polymer was formed
by kinetical trapping of the reaction mixture of ZnI 2 and pyrimidine [33]. These
results highlight the tendency of kinetically assembled networks to contain larger
internal voids and exhibit lower density [34–39].
Another important feature of kinetic assembly is the creation of interactive sites
within the structure [40, 41]. Since this process halts the network formation before it
can reach a thermodynamic equilibrium by trapping metastable intermediate states,
the resultant structures could contain “under-reacted” sites. In contrast, the thermodynamic structures typically have minimal latent reactivity and therefore are
comparatively inert.
Figure 12.1 shows the schematic difference between thermodynamic and kinetic
assemblies. In the former situation, the coordinating groups on the linkers and
open sites around the coordination sphere of metal centers are fully paired up.
Whereas in the latter case, some connecting sites remain unoccupied, resulting in
pore environments that can interact with guest molecules.
In the following section, we will describe how interactive pores can be obtained by
kinetic assembly and discuss their applications. In particular, we will demonstrate the