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Chapter 2 17
concentrations (e.g. 84 mg/L). The pre-functionalization method is, however, somehow
limited because certain kinds of groups favor to coordinate to metal ions, which result
into frameworks with completely blocked organic sides.
2.3.3.2 Introduction of open metal sites
Open metal sites are CUS where an actual coordination number of the metal center is
lower than typically expected for its oxidation state, charge or actual ligand field. Such
CUSs can serve as binding sites for certain guest molecules (hydrogen, methane, etc.) as
well as catalytic active sites for particular reactions under Lewis acidic conditions. The
most common method to obtain CUSs is to activate (i.e. heat under vacuum) MOFs to
remove the metal-bound volatile species (e.g. water, DMF, methanol, etc.). Indeed, this is
often a case for several well-known MOFs such as HKUST-1 ([Cu3(BTC)2]n) and other
members of this family, as well as MOF-74 ([(M2(dobdc)H2O]n) (Figure 2.11). [70-71]
Figure 2.11. Representation of the crystal structure of HKUST-1 as well as the generation of Cu-
CUS at the PW units upon activation. Green, Cu atoms; red, O atoms; gray, C atoms. Hydrogen
atoms are omitted for clarity.
Apart from this conventional method, the unsaturated metal centers can be introduced by
employing similar metal chelating dicarboxylates, [72-74] 2,2’-bipyridine-5,5’-dicarboxylate