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