Studying Transition Metal Chemistry inside a Metal-Organic Framework

Abstract

Metal-organic Frameworks (MOFs) are porous, crystalline materials built from metal ‘nodes’ and interconnecting organic ligands. The combination of crystallinity, porosity and building block design allows MOFs to be tailored at the nanoscale and functionalised to suit specific applications. Reactive metal complexes can be installed inside MOFs, producing well-defined reactive sites with long range order, thereby allowing metal-centred chemical processes to be studied in-situ via X-ray crystallography. Furthermore, the physical isolation of active metal complexes prevents unwanted side-reactions such as cluster formation from occurring, allowing reactive species to be trapped within the crystalline matrix while the surrounding microenvironment can be tuned, via judicious ligand design, to augment their reactivity. The Mn-based MOF [Mn3(L)2(L')] (where L = bis-(4-carboxyphenyl-3,5-dimethylpyrazolyl)methane; 1) is well suited to this application because its pores are decorated with well-defined N,N-chelation sites that bind metal complexes, which allows 1 to be post-synthetically metalated and the resulting metal complexes can be studied using X-ray crystallography. In chapter 2, a series of transition metal nitrate complexes were incorporated within 1 and their structures determined. The structural features of the complexes were compared to those of solution and solid-state analogues to elucidate the effect of the MOF pore-environment on their coordination chemistry. The distribution of metal sites within a MOF framework is important to consider for heterogenous catalysis applications; quantitative metalation can result in the MOF pores becoming ‘burdened’ with metal sites that impede mass transport through the crystal. Work in chapter 3 demonstrates that by using a mixed ligand synthesis approach, the ligand bearing the free N,N-chelating site in 1 could be partially replaced with a modified ligand that is incapable of metalation. The resulting structure possesses the same topology as 1 but is doped, specifically in the non-coordinated donor sites, with a ligand that will not readily bind to metal complexes, allowing the overall degree of metalation to be tuned. Work in chapter 4 utilised the observation that within 1 the metal complexes are site-isolated and separated by 13 Å. By incorporating a Mn(I) azide complex within 1, the site-isolation was harnessed to perform site-selective ‘click’ chemistry on small dialkynes that are shorter than the azide separation. Within 1, this ‘click’ chemistry cycle, using both simple mono- and di-alkynes, was monitored using X-ray crystallography. This work demonstrates that the nanoscale spatial control of reactive sites within MOFs, supported with X-ray crystallographic insights, can affect highly selective chemical transformations. Finally in chapter 5, advancing the concept of site-isolation within MOFs, 1 was functionalised with a Mn(I) carbonyl complex. When exposed to visible light the complex releases a portion of its carbonyl ligands which escape the porous crystalline lattice. It was envisaged that photolysis could generate site-isolated, reactive metal complexes that can activate small molecules. Preliminary experiments demonstrated that CO is successfully liberated under photolysis, while in-situ X-ray crystallography suggests that the CO ligands are replaced by weakly coordinating solvent molecules. In this way, 1 acts as a matrix for isolating and studying the reactive metal complexes formed using photolysis.