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K. Sada and K. Kokado
25.1 Introduction
Metal-organic Frameworks (MOF) are a new class of crystalline materials composed
of polytropic ridged organic ligands and metal ions as shown schematically in
Fig. 25.1a. They are self-organized to form an infinite periodic framework structure as the crystalline lattice due to the rigidity and large space of the organic ligands
and multivalence of the metal ions. The most important feature of the MOF is the
robust framework structures with large nanopores. The infinite combinations of the
metal ions and the organic ligands provide large numbers of nanoporous materials
with different porosities [1–3]. Moreover, as compared to other inorganic porous
materials, such as zeolite and mesoporous silica, they can be easily synthesized
under relatively mild conditions. From the library of the MOF, we can now design
the frameworks of the MOF as well as the size and shape of the nanopores. The
controlled nanoporous structures are useful for various applications such as storage
and separation of gases [4, 5], heterogeneous catalysis [6] or supports for the immobilization of homogeneous catalysts [7, 8], drug delivery and bioimaging [9, 10], and
transport materials for electronic and photophysical devices [11, 12].
For these many demands on preparing advanced functional materials based on the
nanoporous structures of the MOF [13, 14], the methodology for the modification
of the MOF has attracted significant attention for controlling the environments of
the nanopores. The most successful approach is post-synthetic modification (PSM)
[15–17], as shown in Fig. 25.1b. PSM is defined as the modification of the organic
ligands in the frameworks of the MOF by chemical reactions, especially organic
reactions, after the formation of the MOF as crystalline materials. This approach has
the advantage for the preparation of the MOF with the functional groups that are
sensitive or degradable under the solvothermal conditions used in the preparation
of the MOF. PSM has become a powerful tool for the rational functionalization
starting from the readily available MOF with the reactive functional groups on the
organic ligands. The nanopores large enough for the organic reactions and the high
stability against some organic reagents enable us to use many organic reactions for the
PSM. This should be the most prominent difference between the MOF and the other
nanoporous materials, especially the inclusion crystals of organic host molecules.
The strong electrostatic interaction and coordination bonds between the metal ions
and the organic ligands as the connectivity should contribute to the high stability,
compared to the weak non-covalent bonds such as hydrogen bonds and π-π stacking
among the host molecules in the inclusion crystals.
Although there are some early examples for the PSM of the MOF, in 2007,
Cohen and coworker pointed out the significance of the PSM of the MOF [18].
They demonstrated the chemical modification of the amino-tagged MOF derived
from IRMOF-3 prepared from 2-aminoterephthalic acid and Zn(II) ion. Isocyanates
and acid anhydrides directly reacted with the amino group in the organic ligand of
the MOF to convert the amide and urethane groups without loss of the crystallinity
and the nanoporous structures of the MOF. This report suggested that the MOF
has a relatively high stability against the organic reagents, and the organic reagents
K. Sada and K. Kokado
25.1 Introduction
Metal-organic Frameworks (MOF) are a new class of crystalline materials composed
of polytropic ridged organic ligands and metal ions as shown schematically in
Fig. 25.1a. They are self-organized to form an infinite periodic framework structure as the crystalline lattice due to the rigidity and large space of the organic ligands
and multivalence of the metal ions. The most important feature of the MOF is the
robust framework structures with large nanopores. The infinite combinations of the
metal ions and the organic ligands provide large numbers of nanoporous materials
with different porosities [1–3]. Moreover, as compared to other inorganic porous
materials, such as zeolite and mesoporous silica, they can be easily synthesized
under relatively mild conditions. From the library of the MOF, we can now design
the frameworks of the MOF as well as the size and shape of the nanopores. The
controlled nanoporous structures are useful for various applications such as storage
and separation of gases [4, 5], heterogeneous catalysis [6] or supports for the immobilization of homogeneous catalysts [7, 8], drug delivery and bioimaging [9, 10], and
transport materials for electronic and photophysical devices [11, 12].
For these many demands on preparing advanced functional materials based on the
nanoporous structures of the MOF [13, 14], the methodology for the modification
of the MOF has attracted significant attention for controlling the environments of
the nanopores. The most successful approach is post-synthetic modification (PSM)
[15–17], as shown in Fig. 25.1b. PSM is defined as the modification of the organic
ligands in the frameworks of the MOF by chemical reactions, especially organic
reactions, after the formation of the MOF as crystalline materials. This approach has
the advantage for the preparation of the MOF with the functional groups that are
sensitive or degradable under the solvothermal conditions used in the preparation
of the MOF. PSM has become a powerful tool for the rational functionalization
starting from the readily available MOF with the reactive functional groups on the
organic ligands. The nanopores large enough for the organic reactions and the high
stability against some organic reagents enable us to use many organic reactions for the
PSM. This should be the most prominent difference between the MOF and the other
nanoporous materials, especially the inclusion crystals of organic host molecules.
The strong electrostatic interaction and coordination bonds between the metal ions
and the organic ligands as the connectivity should contribute to the high stability,
compared to the weak non-covalent bonds such as hydrogen bonds and π-π stacking
among the host molecules in the inclusion crystals.
Although there are some early examples for the PSM of the MOF, in 2007,
Cohen and coworker pointed out the significance of the PSM of the MOF [18].
They demonstrated the chemical modification of the amino-tagged MOF derived
from IRMOF-3 prepared from 2-aminoterephthalic acid and Zn(II) ion. Isocyanates
and acid anhydrides directly reacted with the amino group in the organic ligand of
the MOF to convert the amide and urethane groups without loss of the crystallinity
and the nanoporous structures of the MOF. This report suggested that the MOF
has a relatively high stability against the organic reagents, and the organic reagents