Engineering new Zr-MOFs for the structure determination of site-isolated metal complexes

Abstract

Transient, coordinatively unsaturated metal sites found in biological systems and important industrial catalysts have intrigued scientists for their role in facilitating selective chemical reactions. Researchers aim to synthetically mimic and study these sites for new chemical transformations and to facilitate selective small molecule binding. Metal-organic frameworks (MOFs), due to their robust and crystalline structures, offer a platform to trap and study these entities by single crystal X-ray diffraction (SCXRD). However, many MOFs lack the properties, along with the chemical and thermal stability, needed for reliable X-ray diffraction based characterization of metal complexes located at binding sites within their structures. This thesis aims to synthesize chemically robust Zirconium-based MOFs with free bis-pyrazole sites for coupled post-synthetic metalation and SCXRD. These Zr-MOFs can act as crystalline sponge materials, with the bis-pyrazole groups being used to site-isolate and facilitate SCXRD characterisation of mononuclear and dinuclear transition metal complexes. Leveraging significant prior art in the formation of MOFs from pyrazole carboxylate linkers, the work described in the thesis investigated the synthesis of chemically and thermally stable Zr-MOFs from bis-pyrazole and tetrakis-pyrazole carboxylate linkers. The initial linker that was studied for developing new crystalline sponges with Zr-based nodes, 1,1'-methylenebis(1H-pyrazole-4-carboxylic acid), gave MOFs that did not fully meet the targeted aims of the thesis, although, the study provided important insights into the synthesis of Zr-MOFs for this type of linker. In chapter 2, the templated synthesis of zirconium(IV)-based 2D-metal-organic frameworks (2D-MOFs) using the aforementioned di-topic linker is reported. This gave two different but structurally related 2D-MOFs depending on whether a templating metal ion was used in the synthesis or not. Without the template, a close packed 3D solid comprising 2D MOF was formed, whereas using a synthetic approach with a Cu(I) template provided a more open 3D solid comprising related 2D layers. Chemical delamination of this latter material produced stable and polydisperse 2D-MOF nanosheets that were capable of post-synthetic metalation (PSMet). However, the delamination process unavoidably caused a loss of long-range order, preventing the study of resulting metal complexes by SCXRD. To address these limitations and pursue the goal of generating robust crystalline sponges, a tetratopic linker, 1,1,2,2-tetrakis[4-(4-carboxyphenyl)-1H-pyrazol-1-yl]ethane (TCPE), was employed for the study presented in Chapter 3. This linker led to the synthesis of a new Zr-MOF, UAM-1001, which enable structure determination of unusually distorted dimeric complexes trapped within the pre-organised metalation site. The chapter reports the synthesis and characterisation of the crystalline sponge UAM-1001, by first preparing a more flexible MOF (UAM-1000) and then post-synthetically bracing the structure to ensure PSMet could occur without loss of crystallinity. The study explored PSMet of UAM-1001 with different metal salts, revealing the ability to accommodate of both monomeric and dimeric metal complexes, depending on the choice of metal salt and its coordination geometry. The semi-rigid and preorganised arrangement of the donor groups enforced notably shorter metal-metal distances for some of the dinuclear complexes. Chapter 4 presents an in-depth study into the formation of Zr-MOFs from tetratopic linkers containing free bis-pyrazole units, combining experimental and computational analyses, to understand how these MOFs are formed and the effect of linker length and synthetic conditions on the resulting topology. MOFs with three different topologies (scu, flu, and sqc) are encountered as the overall length and width (insertion of a spacer in the arm) of the linker are modified. Only one of the original three MOFs (UAM-1001, sqc topology, chapter 3) could be post-synthetically metalated. Using the same TPCE linker that was studied previously to form UAM-1000/1001, a fourth MOF could be synthesised by changing the modulator. This material, UAM-1002, has a different topology to UAM-1001, and is only able to form mononuclear complexes due to spatial isolation of the bis-pyrazole sites. Finally, a short study, which covers a small body of work, is presented in chapter 5, to highlight how changing the positioning of the donor groups on the arms of the organic linker can also affect the topology of the MOF. The resulting material, UAM-1006, has the same scu topology of UAM-1002, but is formed as a “closed” phase. In summary, this thesis has focused on the synthesis of chemically and thermally robust and highly porous crystalline sponges for site-isolating and investigating the structures and chemistry occurring at transition metal complexes anchored to the MOF. It has reported several new Zr-based MOFs, including two new MOFs that serve the targeted purpose. One of these MOFs is capable of trapping either mononuclear or dinuclear complexes (the former with a vacant non-coordinating site near the metal), while a second MOF is set-up to provide spatially isolated bidentate binding sites. These new materials significantly expand the library of MOFs capable of acting as crystalline sponges for metal complexes but combined the work highlights the challenges of forming such materials. The crystalline sponges identified are being used to investigate chemistry occurring at metal sites within the confined, chemically controlled environment of the MOF pores by SCXRD.