Modular Synthesis of Templated Multicomponent Active Sites in Metal–Organic Frameworks via Cross-linking Strategies

Abstract

Binuclear metal active sites are found throughout all subfields of catalysis, from homogeneous and heterogeneous systems to enzymes. The installation of bimetallic active sites within metal–organic framework (MOF) pores is an enticing strategy to leverage the intrinsic benefits of MOFs for catalysis – their porosity derived site isolation, rigid periodic secondary structure, and high degree of tunability. However, the actual construction of bimetallic sites is nontrivial. While a small number of bimetallic sites in MOFs have been reported, progress in this space is limited by synthetic challenges in controlling both the local coordination environments and relative metal positioning within the framework. The work herein describes progress towards the installation of precisely templated bimetallic active sites within MOF pores, and the exploration of these bimetallic sites as catalysts for oxidative catalysis. Chapter 1 provides an overview and perspective of the current landscape for the installation of bimetallic sites within MOFs for catalysis. Particular emphasis is placed on the synthetic strategies employed, as well as the spectroscopic shortcomings in designing active sites that are explicitly bimetallic in nature. Chapter 2 describes the development of an initial templating strategy. The strategy leverages simple protecting group chemistry (i.e. tertiary esters) to install cross-linked ligand dimers into the framework, Mg2dotpdc (dotpdc4- = 4,4″-dioxido-[1,1′:4′,1″-terphenyl]-3,3″ dicarboxylate), wherein the length of the cross-linking tether restricts the tethered struts to a single conformation (~ 7Å down the pore channel). Subsequent thermal removal of the cross linker under microwave conditions exposes templated carboxylate pairs. The generalizability of this templating method is a key advantage over other synthetic approaches. Chapter 3 details the expansion of the strategy first explored in Chapter 2 to other functional group pairs, specifically templated aryl and alkyl amines via tertiary carbamate cross linkers. The ability to install templated amine pairs is particularly exciting because they are amenable to a variety of post-synthetic covalent modifications to generate diverse chelating sites for metal cations. As initial examples, the quantitative conversion of the aryl amine and alkyl amine pairs to iminopyridine (IP) and dipicolylamine (DPA) sites, respectively, is described. The iminopyridine and pyridyl amine sites can then be metalated with a variety of M(I/II) cations (M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Cu(I)). Detailed characterization of the metalated materials, including electron paramagnetic resonance (EPR) spectroscopy and extended X-ray absorption fine structure spectroscopy (EXAFS) are provided. Chapter 4 details a departure from the installation of bimetallic sites to explore the structural implications of cross-linker incorporation in flexible MOFs. Incorporation of a primary ester cross-linker into a previously unreported terphenyl expanded analogue of MIL53(Al) revealed that the otherwise highly flexible framework was stabilized in an open-pore configuration. The degree of flexibility could be further tuned by changing the concentration of ligand dimer used. While the parent framework is inactive, the cross-linked MOF is a competent Prins condensation catalyst. This work highlights the potential versatility of the templating strategy to dictate other parameters beyond installing functional groups. Appendix A marks a return to the installation of bimetallic sites, specifically the expansion of the previously discussed post-synthetic chemistry to other N-donor ligands. Here, the modularity of the post-synthetic covalent modification is highlighted. Beyond the aforementioned IP and DPA scaffolds, di[2-(2-pyridyl)ethyl]amine, di[2 (diethylamino)ethyl]amine, alkylamine-based iminopyridine, and thiazole-2-carboxaldehyde based ligand scaffolds and their ability to bind transition metals are discussed. An initial investigation of our Cu(II) metalated DPA and iminothiazole materials as catalysts for the oxidation of catechols and is explored in Appendix B, including a foray into statistical modeling and the use of design of experiments (DoE) to elucidate key parameters. Our results suggest that while our materials are catalytically competent, there is not a major templating effect, highlighting the importance of a metal–metal distance match between the framework active sites and the target reaction. Appendix C provides additional context for how the strategies presented in this body of work compare to other common heterogeneous platforms: mesoporous silicas and zeolites. While both mesoporous silicas and zeolites can be post-synthetically modified, the molecular level precision and modularity of our MOF chemistry detailed in Chapters 2 and 3 cannot be replicated in those materials. As highlighted in Chapter 4, the rich landscape of MOF architectures also provides an exciting and unique opportunity to explore structural implications such as rigidification in a way that cannot be done with amorphous materials like silica nor rigidly crystalline zeolitic materials.