Synthesis and characterization of ruthenium, iron and gold porphyrin-based metal-organic frameworks (MOFs) and their applications in catalysis

Abstract

Metal-organic frameworks (MOFs), an emerging class of highly porous and tunable crystalline materials composed of inorganic metal ions/clusters and organic ligands/linkers, have found applications in various areas such as gas storage/separation, heterogeneous catalysis, drug delivery and sensing. This thesis describes the synthesis, characterization and structural properties of new porphyrin-based MOFs, including their X-ray crystal structures, and explores their applications in heterogeneous catalysis. A series of ruthenium(II) porphyrin-based MOFs have been synthesized. Single-crystal X-ray crystallography revealed their three-dimensional (3D) porous structures, and two new topological nets were identified in MOFs [M2(Ru(CO)-TCPP)(HCOO)]n (M = Mn, Co; TCPP = meso-tetrakis(4-carboxyphenyl)porphyrin) and [Co3O(Co(H2O)6)(Ru(CO)-cis-DCPP)3(bpy)3]n (cis-DCPP = 5,10-bis(4-carboxyphenyl)-15,20-diphenylporphyrin; bpy = 4,4′-bipyridine). These MOFs showed catalytic activity towards the intermolecular cyclopropanation of styrene with ethyl diazoacetate. The use of [Zr8O6(Ru(CO)-TCPP)3]n as a catalyst led to excellent product yields (>99%), with a diastereoselectivity (trans/cis) ratio of >95:5; this heterogeneous catalyst could be recycled 10 times without an appreciable decline in product yield or selectivity. This catalyst exhibited higher size selectivity than its homogeneous counterpart towards alkene substrates of different sizes. In addition, [Zr8O6(Ru(CO)-TCPP)3]n showed catalytic abilities in the intramolecular cyclopropanation of alkenes, tandem nitrone formation/1,3-dipolar cycloaddition for isoxazolidines and epoxidation of alkenes. A new class of isoreticular zirconium MOFs based on meso-tetrakis(4-(4-carboxyphenyl)-2,3,5,6-tetrafluorophenyl)porphyrin (TCPTFPP) ligand and the corresponding palladium(II), ruthenium(II), cobalt(II) and iron(III) porphyrins have been synthesized. Single-crystal X-ray crystallography revealed that [Zr6(OH)4O4(H2-TCPTFPP)3]n, which displayed good thermal and chemical stabilities, features a ftw-a topology with 2.5 nm large cubic cages. The permanent porosities of these MOFs were verified by N2 sorption measurements. The iron(III) porphyrin-based MOF, [Zr6(OH)4O4(FeCl-TCPTFPP)3]n, was used as a biomimetic catalyst in nitrene C-H insertion reactions. This MOF catalysed the formation of saturated N-heterocycles by the challenging direct C(sp3)-H amination of alkyl azides (including amination of benzylic, allylic, secondary, tertiary, and primary C-H bonds) in 61−91% yields. The MOF catalyst can be reused 5 times in natural product synthesis. In addition, a “[Zr6(OH)4O4(FeCl-TCPTFPP)3]n + aryl azide + visible-light” protocol was developed for the intermolecular C-H bond amination of indane and ethylbenzene, as well as the aziridination of alkenes. [Zr6(OH)4O4(FeCl-TCPTFPP)3]n also represents the first example of an iron porphyrin-based MOF catalyst for the photo-reduction of CO2 with a TON (CO) of 288 under blue LED irradiation. The catalytic activity of [Zr6(OH)4O4(FeCl-TCPTFPP)3]n towards the oxidation of benzylic and tertiary C-H bonds of hydrocarbons was examined. A gold(III) porphyrin-based MOF, [(Zr6(µ3-O)8(H2O)12)2(Au-TCPP)3Cl3]n, has been prepared and characterized by single-crystal X-ray crystallography. The 3D porous structure of this MOF features a she-a topology with 1.9 × 1.9 nm2 channels. This structure exhibited higher CO2 adsorption capacity than an isostructural metal-free porphyrin-based MOF and selective adsorption of CO2 over N2, CO and H2. [(Zr6(µ3-O)8(H2O)12)2(Au-TCPP)3Cl3]n is capable of catalysing photo-induced aerobic cross-dehydrogenative-coupling reactions and hydroxylation of arylboronic acid. This catalyst was also used as a Lewis acid catalyst for the cycloisomerization of alkynyl ketones. Additionally, a one-dimensional (1D) gold(III) corrole-based chain [Co(Au-DCPC)2(bpy)2(NO3)2]n (DCPC = 5,15-bis(4-carboxyphenyl)-10-phenyl corrole), which represents a unique example of a corrole-based coordination polymer, has also been synthesized.