Urea-containing metal-organic frameworks as heterogeneous organocatalysts

Abstract

Two novel pillared metal-organic frameworks (MOFs) including urea-functional groups are introduced. Herein, urea functional groups were incorporated into the MOF backbone by preparing urea-ditopic ligand. These frameworks (TMU-18 and TMU-19) were fabricated using the synthesized urea-containing ligand, 4,4'-Bipyridine (bpy) and 1,2-Bis(4-pyridyl)ethane (bpe), respectively using zinc nitrate as metal source. Subsequently, TMU-18 and TMU-19 were characterized by X-ray diffraction, IR spectroscopy, elemental analysis, scanning electron microscopy (SEM) and thermogravimetric analysis. Furthermore, their potential talent as organocatalysts was evaluated in the regioselective methanolysis of epoxides. 4 (urea-based ligand) is 4,4'-(carbonylbis(azanediyl))dibenzoic acid, bipy and bpe are 4,4'bipyridine and 1,2-bis(4-pyridyl)ethane, respectively, Figure 1. Experimental Section Apparatus and ReagentsAll starting materials, including 1,1'-Carbonyldiimidazole, 4-Aminobenzoic acid were purchased from commercial suppliers (Sigma-Aldrich, Merck) and used as received. The infrared spectra were recorded on a Nicolet Fourier Transform IR, Nicolet 100 spectrometer in the range 500-4000 cm -1 using the KBr disk technique. Elemental analyses (carbon, hydrogen, and nitrogen) were performed using an ECS 4010 CHN made in Costech, Italy. Melting points were obtained by a Bamstead Electrothermal type 9200 melting point apparatus and corrected.Thermogravimetric analyses (TGA) of the compounds were performed on a computer-controlled PL-STA 1500 apparatus. The 1 H-NMR spectrum was recorded on a Bruker AC-250 MHz spectrometer at ambient temperature in d 6 -DMSO and CDCl 3 . X-ray powder diffraction (XRPD) measurements were performed using a Philips Xpert diffractometer with monochromated Cu-Kα radiation (λ = 1.54056 Å). The samples were also characterized by field emission scanning electron microscope (FE-SEM) SIGMA ZEISS and TESCAN MIRA (Czech) with gold coating. Single-Crystal Diffraction.X-ray crystal structure determinations: Crystals in viscous paraffin oil were mounted on cryoloops and intensity data were collected on the Australian Synchrotron MX1 beamline at 100 K with wavelength (λ = 0.71073 Å). The data were collected using the BlueIce 24 GUI and processed with the XDS 25 software package. The structures were solved by conventional methods and refined by full-matrix least-squares on all F 2 data using SHELX97 26 or SHELX2014

Interplay between hydrophobicity and basicity toward the catalytic activity of isoreticular MOF organocatalysts

“…10,11 Among common Lewis acid based MOF catalysts, those bearing basic functional groups provide deserving heterogeneous catalysts for organic transformations. 23 Application of MOFs as heterogeneous organocatalyst with pure Brønsted-basic moieties has only been scarcely explored. Actually, overall catalysis reactions proceeded via synergic effect of the Lewis acid metal nodes placed at neighbouring basic sites on the pore surface of MOF structure eventually make bifunctional acid-base heterogeneous catalysts.…”

Influence of Hydrogen Bond Donating Sites in UiO‐66 Metal‐Organic Framework for Highly Regioselective Methanolysis of Epoxides

“…[26][27][28] Thus, methanol is one of the frequently used nucleophiles for the ring-opening of epoxides due to its high reactivity and importance of the resulting product. [29] Alcoholysis of epoxides is a well-established chemical transformation reaction that can be achieved by various homogeneous and heterogeneous catalysts. [24,[30][31][32][33] Alcohols having relatively poor nucleophilicity are not preferred candidates due to the production of lower yields in the ring-opening of epoxides reaction.…”

“…[38,39] In recent times, numerous attempts have been dedicated to develop MOF based heterogeneous catalysts possessing LA metal center or BA functional groups in the linker like À SO 3 H and À OH for the methanolysis of epoxides. [24,29,34,[40][41][42][43][44] However, some of these catalytic reactions still have drawbacks like unsatisfactory selectivity of product, robustness and recyclability of catalyst and harsh reaction conditions. [24,25] Hence, developing new heterogeneous catalysts to promote methanolysis of epoxide with very high regioselectivity at very high conversion of epoxide is of great interest.…”

Crystallographic investigation into the self-assembly, guest binding, and flexibility of urea functionalised metal-organic frameworks

“…Urea-functionalised MOFs in particular have been found to catalyse FriedelCrafts reactions between pyrroles and nitroalkenes (22,24,27,29,31), Henry reactions between aldehydes and nitromethane (23), and the methanolysis of epoxides (25).…”

“…For example, grafting hydrogen bond donor functionality, such as amides (11)(12)(13)(14)(15)(16)(17)(18)(19)(20), ureas (21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32), thioureas (33), and squaramides (34)(35)(36), (Figure 1(a)) onto the organic scaffold of the MOF can enhance properties including carbon dioxide capture (13,15,17,18), anion binding (20,21), sensing (28), and organocatalytic activity (6). Urea-functionalised MOFs in particular have been found to catalyse FriedelCrafts reactions between pyrroles and nitroalkenes (22,24,27,29,31), Henry reactions between aldehydes and nitromethane (23), and the methanolysis of epoxides (25).…”