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18 Chapter 2
(H2BPDC) [75] or other bridging ligands, [76] which are not part of the inorganic building
blocks of a given framework. Moreover, MOFs with open metal sites featuring diverse or
enhance property can be achieved by using other metal ions in place of those metal
precursors used in the known MOFs. By directly employing metal precursors containing
another metal-ions and the same linkers during the synthesis of MOFs, the formation of
the isostructural MOFs is expected. For example, analogs of HKUST-1 with the general
formula [M3(BTC)2]n where M =Zn, [77] Cr, [78] Ni, [79] Fe, [80] Mo, [81] and Ru [82] etc. were
reported by serveal groups after the appearance of [Cu3(BTC)2]n. [35] These analogs of
HKUST-1 behave rather different properties such as in gas sorption/selectivity. [78,
83] Similarly, by varying the metal in the infinite inorganic rod-type SBUs utilized for the
construction of MOF-74,([Zn2(dobdc)]n) [63] a series of isostructural M-MOF-74 with
various divalent metal ions like Mg, Co, Ni, Mn, Cd, Cu and Fe were described. [84-88] Other
families of the homologous structures prepared using the same principle are M-BTT (BTT
= 1,3,5-benzenetristetrazolate, M = Mn, Fe, Co, Cu, Cd), [89-92] for example.
2.3.3.3 Post-synthetic modification and beyond
Post-synthetic method (PSM), where the modification of ligands or metal containing
nodes is implemented after the formation of MOFs (Figure 2.12), offers an effective way
to overcome the potential for functional-group interference during MOF assembly.
Several research groups, in particular the group of Cohen, have investigated the
modification of amino and aldehyde groups in MOFs via the formation of new covalent
bonds (Figure 2.12 top). [93-100] Still, employing PSM, one can face the problem of the pore
volume decrease (caused by the functionalization reaction), which can lead to the
reduction of gas absorption capacity. Therefore, an optimized method called “postsynthetic
deprotection” was developed (Figure 2.12 bottom). [101] In this case, an organic
linker bearing protected or “masked” functional group is incorporated into a MOF under
standard solvothermal conditions, and the protecting group is subsequently removed by
a thermal [102-103] or light [98, 104] induced deprotection reaction to reveal the desired
functionality. One of the earliest example was presented by Kitagawa with the term of
“protection-complexation-deprotection” process. [102] During the protection step,
hydroxyl group of the 2,5-dihydroxyterephthalic acid (H2dhybdc) was protected by acetyl
group via acetylation to form the 2,5-diacetoxyterephthalic acid (H2dacobdc). In the
complexation/deprotection step, a MOF with a layered-pillared structure was obtained by