Synthesis and characterization of metal-organic frameworks supported copper catalysts and their applications in photocatalytic organic transformations

Abstract

Metal−organic frameworks (MOFs) are a class of highly crystalline functional materials assembled by metal ions and organic linkers. Owing to their porous properties, diverse structures, high surface area and functional adjustability, MOFs are desirable candidates as efficient heterogeneous catalysts. Except for the catalytic sites from their open metal nodes and functional organic linkers, guest species can also be introduced into the MOFs via the physical encapsulation or the chemical modification of MOF components. Although visible-light copper photocatalysis has recently become a feasible technology for constructing sustainable synthetic processes, homogeneous copper photocatalytic systems have encountered several fundamental challenges such as catalyst deactivation and high usage. This thesis describes the immobilization of copper complexes into diverse MOF structures to dramatically improve photostability and photocatalytic activity in heterogeneous systems. A dimeric copper(I)–binap (binap = 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) photosensitizer (di-Cu) has been embedded on a zinc-based pillar-layered MOF (PL-MOF) to generate the PL-MOF-di-Cu catalyst via surface ligand exchange strategy. The PL-MOF-di-Cu was well-characterized by a series of experiments. This catalyst was active to multiple intramolecular C(sp3)–H aminations triggered by a photoinduced N–O bond cleavage. In addition, the heterogenized binap-ligated monomeric copper iodide species (PL-MOF-mono-Cu) has also been prepared through post-synthetic metalation of a PL-MOF-binap (PL-MOF installed by benzoic acid-containing binap). The mechanistic studies in copper photoredox catalysis were explored. A stable phenanthroline-functionalized UiO-69-phen MOF has been prepared and chosen as a desirable platform for the post-synthetic metalation with binap-ligated heteroleptic copper(I) complex [Cu(binap)(MeCN)2](PF6)] to produce UiO-69-phen(binap)Cu. The extended X-ray absorption fine structure revealed each copper(I) center of UiO-69-phen(binap)Cu adopts a distorted tetrahedral geometry via the coordination with two nitrogen atoms and two phosphine atoms. The UiO-69-phen(binap)Cu was used as a triplet photosensitizer in intermolecular crossed [2+2] cycloadditions. This catalyst is superior to its homogeneous counterparts, with a turnover number of up to 4750 for the crossed [2+2] cycloadditions between different styrenes. The catalyst can be recycled six times without any loss of photocatalytic activity and crystallinity. A stable Zr-based MOF (MOF-808), [Zr6(μ3-O)4(μ3-OH)4(BTC)2(HCOO)5(OH)(H2O)] (BTC = 1,3,5-benzenetricarboxylate), was chosen as a support to accommodate external binap-ligated copper(I) complexes due to the exchangeable formates and suitable pore size. The MOF-808 supported photocatalysts have been characterized. The reactivity for dearomatic [2+2] cycloadditions was tested. Unlike the homogeneous counterpart (binap)2Cu2I2 which catalyzed the generation of 2H-biphenyleno [8b,1-b]furan derivatives, the MOF-808 supported binap-ligated copper(I) catalyst exhibited higher activity toward the synthesis of naphtho[1',2':1,4]cyclobuta[1,2-b]furan derivatives. A MOF-808 supported iridium/copper dual metal photocatalyst (MOF-808-Ir/Cu) was synthesized based on a sequential metalation strategy by incorporating the copper(II) triflate to the free −OH/−OH2 sites and then exchanging the formates with iridium complex. This catalyst was found to promote radical amino-functionalizations in 40−73% yields. The extended X-ray absorption fine structure revealed each copper(II) center has a Cu–O4 coordination environment, originating from two oxygen atoms of −OH/−OH2 site and two water molecules. This catalyst can be recycled six times without the loss of activity and crystallinity.