Metal–Organic Frameworks Constructed from Branched Oligomers

Metal–organic frameworks (MOFs) prepared from oligomeric or polymeric organic ligands have been studied and are termed oligoMOFs and polyMOFs, respectively. Herein, several oligoMOFs are described that have been prepared from branched oligomers with dendritic or star-like architectures. Branched oligomeric ligands with four (4(H2bdc)-b) or eight (8(H2bdc)-b) 1,4-benzene dicarboxylic acid (H2bdc) groups were prepared and used to synthesize isoreticular-type Zn(II)-based MOFs (IRMOF). A branched tetramer (4(H2bdc)-b) produced an oligoIRMOF-1 with improved ambient stability compared with IRMOF-1 or previously described oligoMOFs. To understand the effect of the ligand architecture, oligoIRMOFs were also prepared from a linear tetramer (4(H2bdc)-l). For a branched octamer (8(H2bdc)-b), it was found that the addition of an organic base was required to produce crystalline oligoIRMOFs. Multivariate MOFs (MTV-MOFs) could also be readily prepared with a combination of an octamer (8(H2bdc)-b) and H2bdc.

To the mixture, 2 M LiBH4 in THF (73 mL, 147 mmol) was slowly added.The reaction mixture was stirred at 75 ºC for 24 h.The volatile solvent was removed under vacuum using a rotary evaporator.To the mixture, 60 mL of water was slowly added.The resulting white precipitate was collected by filtration, washed with water, and dried under vacuum to give a white solid.

Alternative activation method for oligoIRMOF-1-8(bdc)-b
Supercritical CO2 drying.The process was performed by a Tousimis Samdri PVT-3D critical point drier.Liquid CO2 was used to exchange CH2Cl2 three times.The material was heated above 31 °C (P = 1072 psi), the critical point of CO2.The gas state CO2 slowly released at 0.1-1 cm 3 /min.The sample was activated at 25 °C for 18 h before nitrogen adsorption experiment.
Solvent exchange with n-hexane or diethyl ether.As-synthesized oligoMOFs were washed with DMF three times (3´20 mL) and exchanged with CH2Cl2 three times (3´20 mL).The CH2Cl2 exchanged samples were immersed in 20 mL of n-hexane or diethyl ether for solvent exchange and repeated three times.The samples were evacuated under vacuum at 25 °C for 18 h before nitrogen adsorption experiment.Zn(NO3)2•6H2O (89 mg, 0.30 mmol), 8(H2bdc)-b (20 mg, 9.4 µmol), and terephthalic acid (4.7 mg, 28 µmol) were dissolved in 1.2 mL of DEF in 1.5 dram vial.The vial was placed in a preheated oven at 100 °C for 72 h.The vial was removed and allowed to cool to room temperature.The mother liquor was decanted and oligoMOF sample was washed with fresh DMF three times and rinsed with CH2Cl2 three times.Prior to analysis, oligoMOF samples were activated under vacuum at 50 °C for 18 h.Yield: 19 mg (63%, molecular formula: Zn4O(bdc)2.25(8(bdc)-b)0.094).

Nitrogen adsorption isotherm analysis
Around 20-50 mg of dried oligoMOF was transferred to a preweighed sample tube and degassed at 50 °C on a Micromeritics ASAP 2020 Adsorption analyzer for 18 h and the sample tube was

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then reweighed to determine the sample mass.Nitrogen adsorption was then performed at 77 K on the same instrument.BET surface areas were determined from sorption isotherms using the BETSI program.Powder X-Ray Diffraction (PXRD)Dry oligoMOF powder (10 mg) was mounted on a silicon sample holder.PXRD data were collected at ambient temperature on a Bruker D8 Advance diffractometer using LynxEye detector at 40 kV, 40 mA for Cu Kα (λ = 1.5418Å), with a scan speed of 0.5 sec/step, a step size of 0.02° in 2θ, and a 2θ range of 4-40°.