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Chapter 2 5
On the other hand, linkers provide various connecting sites with tuned binding strength
and directionality. Linkers utilized in the CP construction may be halides (F, Cl, Br and I),
CN - and SCN - ions, [8-9] cyanometallate anions ([M(CN)x] n- ) as well as neutral, anionic and
cationic organic ligands. Among all these ligands, halides as the smallest and simplest one
are broadly employed to form quasi-1D halogen-bridged mixed-valence compounds (MX
chains), which feature extensive physical properties. [10] Furthermore, halides can be well
incorporated along with neutral organic ligands in the coordination frameworks. [11]
Cyanometallate anions could perform various geometries, similar to the metal ions with
coordination number from 2 to 7 mentioned above. With respect to the neutral organic
ligands, pyrazine (pyz) and 4, 4’-bpy (bpy = bipyridine) are the most often used in the
reports. [12] As to the anionic ligands, di-, tri-, tetra- and hexacarboxylate molecules are
typically employed. [13-15] On the contrary, CPs with cationic ligands are not very common
due to their low affinity for cationic metal ions. [16-17]
The bonding interactions between connectors and linkers in the CPs are mainly
coordination bonds. Additional weak interactions such as hydrogen and metal-metal
bonds, π- π and CH- π, electrostatic and van der Waals interactions can also participate to
construct the structure of CPs. Consequently, both robust and flexible networks could be
made. Many CPs studied in the earlier stage (1 st generation) feature open voids, cavities
or channels which are usually occupied by the solvent molecules or counter-anions. Such
guests might be exchanged post-synthetically against other ions or solvents, what is of
highly interest to study (host-guest chemistry, ion-exchange, etc.). [18-19] However,
complete removal of the guests molecules in this type of polymers is accompanied with
an irreversible structure collapse, what considerably restrict their application in the fields
of gas storage and separation as a result of low structural rigidity and absence of
permanent porosity. On the other hand, structural interpenetrations, in which the voids
constructed by one framework are occupied by one or more other independent
frameworks, [20] frequently happens in chemistry of CPs and might facilitate formation of
robust structures. Providing structural rigidity, this way, however, often leads in
considerable reduction of the pore sizes and precludes creation of highly porous
frameworks. Therefore, in spite of the abundant study on this class of CPs (1 st generation)
in the early stage (before 1990s), synthesis of coordination networks exhibiting
permanent porosity was still an open question until the exploration of [Co2(4,4′-
bpy)3(NO3)4]n(H2O)4 in 1997 by S. Kitagawa. [21] This compound featuring channeling