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