Dynamic covalent exchange on metal-organic framework nanoparticles

Abstract

This Thesis focuses on the development of surface modified metal-organic framework nanoparticles (MOF NPs) with hydrazones, which can be used for potential dynamic covalent exchanges to reversibly modify their surface.

Two hydrazones of complementary reactivity were synthesised, designated as either electrophilic or nucleophilic. Following development of a post-synthetic modification procedure the hydrazone ligands were directly attached to the surface of a zirconium MOF NP through coordinative attachment. New quantitative nuclear magnetic resonance spectroscopic methods were developed to quantify the surface modification, and characterisation confirmed attachment and localisation to the surface of the MOF NPs. On-MOF exchanges were demonstrated using complementary nucleophilic and electrophilic modifiers. Exchanges were reversible, and in one example were cycled across three different exchange series without significant loss in surface-bound material.

Four further electrophilic and nucleophilic dynamic covalent MOF NPs were then prepared with Aluminium-, Hafnium- and mixed Hafnium-/Zirconium- core@shell metal centres. On-MOF exchanges were successful, proving the general applicability of this method. Heterostructural nanoparticle assemblies (two different MOF NPs) were studied using MOFs of complementary reactivity. Zr-MOF-Al-MOF-Zr-MOF interactions were confirmed using transmission electron microscopy and elemental mapping. The interaction of particles also maintained core porosities, morphologies, and structural integrity.

Application of hydrazone exchange was extended to create heteromaterial nanoparticle assemblies (two NPs of different cores) using monolayer-stabilised gold NPs (AuNPs) in combination with MOF NPs. Nanoassemblies of MOF centres saturated with AuNPs were achieved with aluminium, zirconium, and core@shell MOF NPs. Characterisation of the AuNP-Zr-MOF system revealed maintained structure and morphology, with an expected reduction in porosity.

Surface-Anchored MOF NPs were reported to increase catalytic activity by reducing aggregation of MOF NPs. Preliminary results indicated dynamic covalent functionalised MOF NPs were surface-bound to silicon substrates. This is proposed as a future method to modify substrates after initial preparation for tandem catalysis, and potentially improve activity.