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Designed Syntheses and Post-Synthetic Modifications of Metal–Organic Frameworks

Designed Syntheses and Post-Synthetic Modifications of Metal–Organic Frameworks
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Abstract Two synthetic strategies, direct synthesis of metal–organic frameworks (MOFs) based on well-defined secondary building units (SBUs) and post-synthetic modifications (PSMs) of known MOFs, were utilized for the construction of various MOFs of different net topologies with various functionalities. The direct syntheses of metal–organic macrocycle (MOM) and MOFs with different network topologies by utilizing well-defined SBUs were described in part I and the constructions of core-shell bimetallic MOFs via post-synthetic and selective metal doping and of hybrid bimetallic MOFs via direct and non-selective metal doping were described in part II. Part I In Part I-1, a double-walled triangular MOM, [Cu6L16(DMF)2(H2O)4], and a microporous MOF, [Zn2L12(dabco)], have been synthesized by solvothermal reactions using 1,4-bis(3-carboxylphenylethynyl)benzene (H2L1) as a long and rigid dicarboxylate ligand in dimethylformamide (DMF). The ditopic ligand L1 adopts syn-conformation for the discrete MOM structure, but anti-conformation for the extended MOF structure based on the isostructural dinuclear square paddle-wheel SUB, M2(COO)4 (M = Cu(II) and Zn(II)). In Part I-2, a twofold interpenetrated MOF, [(Zn4O)2L24(DMF)2(H2O)3], was prepared by using 4,4,4-[1,3,5-benzenetriyltris-(carbonylimino)]trisbenzoic acid (H3L2) as a nanometer-sized trigonal tritopic node and a tetranuclear [Zn4O(COO)6] cluster as a six-connected octahedral hexatopic node, where the MOF based on the hexatopic tetranuclear [Zn4O(COO)6] secondary building unit has a 3,6-connected network with an rtl net topology showing a large solvent cavity. The 3D network is doubly interpenetrated to generate curved 3D solvent channels. Keywords: Microporous metal–organic framework, Interpenetration, Metal–organic macrocycle, Secondary building units, Net topology Part II In Part II-1, selective post-synthetic exchange of framework metal ions in MOFs such as Zn-HKUST-1 [Zn3(BTC)2(H2O)3], Zn-PMOF-2 [Zn24L38(H2O)12 , L3 = 1,3,5-tris(3,5-dicarboxylphenylethynyl)benzene) (H6L3)], and M-ITHD [M6(BTB)4(BP)3, M = Zn(II) (1), Co(II) (2), Cu(II) (3), and Ni(II) (4)] was investigated for the constructions of core-shell bimetallic MOF structures. The N2 sorption studies suggested the significant improvement of the framework stabilities of the core-shell bimetallic MOFs compared to those of the parent MOFs. In Part II-2, a series of Ni(II) doped isostructural hybrid bimetallic MOFs, NixM1-x-ITHD (M = Zn(II), Co(II)), with an ith-d net topology have been prepared. While a ~20% amount of Ni(II) doping to Zn-ITHD is not enough to improve the framework stability of the hybrid bimetallic NixZn1-x-ITHD, only a small amount of Ni(II) doping (~10%) to Co-ITHD leads to a full enhancement of the framework stability of hybrid bimetallic NixCo1-x-ITHD. The highly porous and robust NixCo1-x-ITHD MOFs activated via conventional vacuum drying process show an average BET specific area of ~5370 m2 g-1, which is comparable to that of pure Ni-ITHD. Their CO2 uptake capacities at 196 K and 1 bar are the highest among the reported MOFs at similar conditions. Keywords: Post-synthetic modification, Post-synthetic exchange, Core-shell bimetallic structure, Hybrid bimetallic structure, Framework stability
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