CHAPTER 1. Cu-MOF-74, Cu 2 (OBA) 2 BPY, MOF-235 AND CARBON HETEROATOM BOND FORMING REACTIONS
1.2 MOF-235, Cu-MOF-74 and Cu 2 (OBA) 2 (BPY)
1.2.1 Synthesis, structure and physicochemical properties of MOF-235
[Fe3O(1,4-BDC)3(DMF)3][FeCl4-](DMF)3, also known as MOF-235, was first synthesized in 2005 by solvothermal method. It was constructed from ferric chloride hexahydrate and terephthalic acid in the presence of ethanol and DMF. The particles of MOF-235 have octahedron morphology in which each iron atom is trivalent yielding an overall cationic (+1 per formula unit) framework. This charge is balanced by FeCl4-
counterion which is located in the hexagonal pore of the structure. The diameter of the primary hexagonal channel in MOF-235 is 6.7 Å [45, 46].
(a) (b)
Figure 1.5. (a) Inorganic building unit of MOF-235; (b) Single-crystal X-ray structure of MOF-235 (Fe, blue; O, red; Cl, teal; C, gray) [47].
MOF-235 is built-up from corner-sharing octahedral iron trimers that are connected through linear terephthalic acid links, the Fe3O plane of each trimer has Fe-(à3-O)-Fe angles of 120o and Fe-Fe distance of 3.33 Å.
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MOF-235 was first used as adsorbent in gas separation and environmental treatment. It was found as a potential adsorbent for the separation of CH4 from a mixture of CH4, H2, CO2. The absolute adsorption capacity was observed highest toward CH4.These results were attributed to the high pore volume and large number of open metal sites in MOF- 235 [48]. Because of the special structure in which the framework had the positive charge balanced by FeCl4¯ counterions, MOF-235 could adsorb very large amount of both anionic dye methyl orange and cationic dye methylene blue via an electrostatic interaction between the dyes and the adsorbent [49].
1.2.2 Synthesis, structure and physicochemical properties of Cu2(OBA)2(BPY) Cu2(OBA)2(BPY) was synthesized by adding aqueous solution of Cu(NO3)2.3H2O into the mixture of 4,4’-oxybis(benzoic) acid (H2OBA) and 4,4’-bipyridine (BPY). In the structure of Cu2(OBA)2(BPY), Cu(II) ions have a trigonal bipyramid geometry formed by four carboxylate oxygen atoms and a nitrogen atom of the 4,4’-bipyridine. The Cu–
O bond length is assumed in the range 1.952–2.172 Å while the Cu–N bond length is 1.999 Å [47]. Hence, the coordination geometry around the copper(II) atoms can be regarded as a Jahn–Teller-distorted trigonal bipyramid.
Figure 1.6. Coordination environment of copper in Cu2(OBA)2(BPY) [47].
In the structure of Cu2(OBA)2(BPY), OBA ligands take responsibility of producing 2D layers in which the carboxylate groups of OBA ligands are connected with the Cu(II) cations forming an eight-membered ring chains. These 2D sheets lying in the ac plane are connected together in the third dimension by axially coordinating BPY ligands to
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give a 3D framework featuring with channel along b-axis. With this structure, Cu2(OBA)2(BPY) has monoclinic crystal system. The channels were believed too small to residue any guests. The potential free volume was 281 Å per unit cell volume (3443 Å) equally to 8.1% of void per unit volume.
(a) (b)
Figure 1.7.(a) 2D helical layers produced by Cu(II) and OBA ligands; (b) The 3D pillared-layer structure of Cu2(OBA)2(BPY) [47].
Because Cu2(OBA)2(BPY) possessed a quite small range of pore diameters, this MOF was rarely utilized in gas adsorption. In term of catalysis, Cu2(OBA)2(BPY) was employed to form C–C bond between aryl iodide and benzothiazole [50]. Those reactions indicated that this catalyst could be tolerant to harsh reaction conditions such as a strong base environment of t-BuOLi and high temperature. The catalyst could be reused at least 7 times under these reaction conditions.
I +
N
S Cu2(OBA)2(BPY)
t-BuOLi, 1,4-doxane N
S
120oC
Scheme 1.1. Direct arylation of benzothiazole with aryl halide [50].
1.2.3 Synthesis, structure and physicochemical properties of Cu-MOF-74
Cu-MOF-74, having the molar composition corresponds to Cu2(DHTP)(CH3OH)2.73(DMF)0.25(H2O)1.33, was first synthesized by Sanz et al [51].
Cu-MOF-74 is structurally homologous to the honeycomb-like M-MOF-74 (M=Ni, Co,
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Mg...) series with large, one dimensional hexagonal channel of 11.7 Å in diameter. It was prepared from the mixture of 2,5-dihydroxyterephthalic acid and copper nitrate trihydrate in the presence of N,N-dimethylformamide. Solvents used in MOF synthesis were found to affect the physical properties of the final material. The presence of isopropanol as co-solvent in the synthesis medium led to higher crystallinity, higher specific surface area and larger pore volume of Cu-MOF-74. Cu-MOF-74 was one of Cu-based MOF materials having the highest densities of Cu(II) sites per unit volume (4.7 nm−3). In Cu-MOF-74 structure, Cu(II) cations are coordinated to six oxygen atoms, resulting in octahedral Cu centers. Three of them are from the oxygen atoms of carboxylate groups, two are from the oxygen atoms of the hydroxyl groups on the ligand and the sixth one is from the oxygen atom in a solvent (such as DMF, methanol or H2O).
Due to the Jahn–Teller effect on the distortion of the coordination environment of the Cu2+ ions, the Cu2+ ions in Cu-MOF-74 exhibit a low partial positive charge [51, 52] leading to the weaker interaction between the Cu2+ sites and adsorbate molecules. As a consequence, the solvent molecules can be easily and completely removed in vacuum or at high temperatures, providing a five-coordinate Cu(II) species and an unsaturated metal site [52, 53].
Figure 1.8. a) Coordination environment of Cu(II) centers in Cu-MOF-74 after thermal solvent removal. (b) Inorganic SBUs crystalline framework (c) 3D honeycomb
structure of Cu-MOF-74. (Cu, blue; O, red; C, gray) [52-54].
The high density of exposed and unsaturated metal ions in Cu-MOF-74 has sparked
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much interest in the field of adsorption and the adsorptive separation of compounds such as NH3, CO2, CH4 , H2, ethylene [53, 55-58].
In the catalytic application, Cu-MOF-74 has been used as a catalyst in many organic transformation reactions. Owing to the structure containing open metal sites, Cu-MOF- 74 has been used as a heterogeneous Lewis acid catalyst in the Friedel-Crafts acylation of anisole [59] or transformation of trans-ferulic acid into vanillin [60]. Cu-MOF-74 was also used as a redox catalyst in oxidation of cyclohexene [61]. It should be noted that the reaction conditions of using Cu-MOF-74 as catalyst were rather mild in most cases [62-64]. The crystalline Cu-MOF-74 was believed to partially turn into amorphous state under the action of alkali (NaOH 1M) and high temperature (120oC) in the reaction of 4-methoxy benzene iodide with imidazole [59].
Scheme 1.2. Reaction of aryl iodides with N-H nucleophiles catalyzed by aCu-MOF- 74 [59].
In general, the structure and characteristics of MOF-235, Cu2(OBA)2(BPY) and Cu- MOF-74 could be summarized in table 1.1.
Table 1.1. Structure and characteristics of MOF-235, Cu2(OBA)2(BPY), Cu-MOF-74.
Parameters
Analysis results Cu-MOF-74
[51, 65]