Kinetic phase-tuning in the synthesis of iron and chromium metal-organic frameworks

Bara, Dominic James (2020) Kinetic phase-tuning in the synthesis of iron and chromium metal-organic frameworks. PhD thesis, University of Glasgow.

Abstract

Metal-Organic Frameworks (MOFs) are solid-state materials which display exceptional permanent porosity and unprecedented structural diversity. These unique properties have garnered significant attention for potential applications, including gas storage, catalysis, and drug delivery.
This thesis focuses on kinetic phase-tuning in the synthesis of Fe and Cr-MOFs and aims to explore novel strategies for phase-control, as well as assessing their biocompatibility for therapeutic applications such as drug delivery.
The introduction chapter discusses reticular chemistry and other key concepts relating to the synthesis of MOFs, as well as exploring some of the synthetic methods employed for high valent MOFs and their subsequent development for biomedical applications. In Chapter 2, a comprehensive investigation of self-assembly in the Fe–biphenyl-4,4′-dicarboxylate system is conducted by exploring the use of coordination modulation - the addition of monotopic ligands which can compete with the bridging linkers during the self-assembly process - as a tool for phase control. It was found that coordination modulation can reliably tune between the kinetic product, non-interpenetrated MIL-88D(Fe), and the thermodynamic product, two-fold interpenetrated MIL-126(Fe). Coordination modulation slows self-assembly and therefore selects the thermodynamic product MIL-126(Fe), while also offering fine control over defectivity and inducing mesoporosity. Interpenetration control is also demonstrated using the 2,2′-bipyridine-5,5′-dicarboxylate linker; it is energetically prohibitive for it to adopt the twisted conformation required to form the interpenetrated phase, although multiple alternative phases are identified due to additional coordination of Fe cations to its N donors. In addition, oxidation modulation - the deliberate use of metal precursors in different oxidation states to that found in the resulting MOF - is employed as a novel method to kinetically control self-assembly. Combining coordination and oxidation modulation enables the synthesis of pristine MIL-126(Fe) with surface areas close to the predicted maximum for the first time, suggesting that combining the two may be a powerful methodology for the controlled self-assembly of high-valent MOFs.
Chapter 3 details work conducted on the synthesis of Fe-terephthalate MOFs, utilising the two methodologies that were developed in the previous chapter - coordination and oxidation modulation - to control the phase that crystallises. An extensive crystallisation study was performed by variation of key parameters such as synthesis time, temperature, quantity of modulator, and Fe precursor. The results have established synthetic routes to various MOFs containing the common metal hydroxide chain and oxo-centred trigonal secondary building units (SBUs), as well as gaining insight into the kinetic and thermodynamic relationships between the different phases. The thermodynamic relationship between two topologically identical frameworks - MOF-235(Fe) and MIL-88B(Fe) – could subsequently be ascertained, with MOF-235(Fe) being the thermodynamically preferred product when using chloride salts as the metal precursor. MOF-235(Fe) differs from MIL-88B(Fe) in that it contains non-coordinating [FeCl4]- counterions, and as such a novel analogue of MOF-235(Fe) could be obtained when using Fe(BF4)2 which provides suitable [BF4]- anions. The role of the counterion was found to be crucial for directing the structure and has broad implications for other MOF syntheses. In addition, these synthetic efforts yielded suitably large crystals of a prototypical flexible MOF - MIL-53(Fe) - which enabled study of its flexibility by single-crystal X-ray diffraction.
In Chapter 4 the synthesis of Cr-MOFs is explored using coordination modulation as a strategy for phase control. The ligand exchange rate of Cr3+ is much slower than for Fe3+, making it more challenging to obtain crystalline materials as there is less ‘error-checking’, hence, modulation was considered a viable strategy for enhancing crystallisation. Two synthetic routes were established in this study and were found to reliably control the crystallinity and SBU in the resulting MOF: synthesis in water and pyridine with acetic acid as modulator yields frameworks with the trigonal [Cr3O(RCO2)6(H2O)2X] (X = monoanion) SBU, while synthesis in water with HCl yields those with chain [Cr(OH)(RCO2)2] SBU. The ability to control the SBU in this manner is extremely useful and has enabled the synthesis of many novel Cr-MOFs in this study. A series of flexible dicarboxylate MOFs with MIL-88 topology were synthesised successfully using the acetic acid route, and their flexible behaviour was investigated by powder X-ray diffraction. Interestingly, synthesis with the longest linker - 4,4′-stilbenedicarboxylic - yields a two-fold interpenetrated framework (Cr-SDC) which, despite interpenetration, exhibits comparable flexibility to the MIL-88 frameworks. The use of higher connectivity linkers has also been explored, generating novel frameworks with greater rigidity and permanent porosities surpassing the dicarboxylates. The most notable of these frameworks is MIL-100(Cr)_BTB, containing benzene-1,3,5-tribenzoate (BTB) as linker, which demonstrates exceptional sorption capacity (4000 m2g-1). The HCl route enabled the synthesis of novel isoreticular analogues of MIL-53(Cr), two of which could be grown as large single crystals, which has never previously been reported for directly synthesised Cr-MOFs. The remarkable stability of Cr-MOFs suggests that many of these frameworks could find use in demanding applications where their high porosities and flexible properties can be utilised.
MOFs have been widely studied for their potential use in biomedical applications, particularly as drug delivery systems, however these have typically been limited to frameworks consisting of either Zr4+ or Fe3+, with those consisting of Cr3+ largely neglected due to the associated stigma relating to the toxicity of Cr6+. In Chapter 5 the biocompatibility of several of the Cr-MOFs developed in Chapter 4 were tested using cell culture experiments. The results showed that all the MOFs were biocompatible within the doses expected to be used in drug delivery applications.
In summary, novel synthetic strategies for phase-tuning in Fe3+ and Cr3+ carboxylate systems have been developed, providing better control over the obtained phase, and enhancing crystallinity. These enhanced synthetic protocols have enabled discovery of novel MOFs, investigation of the flexibility, gas sorption, and biocompatibility of these and existing benchmark materials, and has revealed that Cr-MOFs are promising candidates for biomedical applications.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QD Chemistry
• Colleges/Schools: College of Science and Engineering > School of Chemistry
• Supervisor's Name: Forgan, Prof. Ross
• Date of Award: 2020
• Embargo Date: 19 June 2023
• Depositing User: Mr Dominic Bara
• Unique ID: glathesis:2020-81470
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 22 Jun 2020 14:58
• Last Modified: 22 Jun 2020 14:58
• URI: https://theses.gla.ac.uk/id/eprint/81470