Thermolabile protecting groups in metal-organic frameworks : a thesis presented in partial fulfilment of the requirements of the degree of Doctor of Philosophy in Chemistry at Massey University, Manawatu, New Zealand
Prior to the work carried out for this thesis, there were no publications in which bpy was used as a ligand backbone, or in which a carboxylate was incorporated into a MOF using a TPG. Also, to the best of our knowledge there are no examples in the literature of an ethyl carbamate TPG in MOFs.
In this thesis the range of TPG protected ligands has been expanded to include 1,4-bdc
(Chapter 2) and bpy (Chapters 4 and 5). The bpy-NHBoc and bpy-TBE materials are the first examples of N-donor type ligands protected by TPGs. Furthermore, the bpy-TBE ligand is the first example of a TPG protected carboxylate in a MOF.
In Chapter 2, 1,4-bdc-NH2 was protected as both the ethyl carbamate and the tert-butylcarbamate, giving 1,4-bdc-NHCOOEt and 1,4-bdc-NHBoc, which there then incorporated into a MOF-5-type framework. It was envisaged that thermolysis of the carbamate esters could generate an isocyanate group, though this was not expected for 1,4-bdc-NHBoc due to the tendency of tert-butylcarbamates to decompose to the amine. Despite thermolysis on the TGA apparatus only generating the amine, it was found that thermolysis under vacuum enabled not only enabled ~ 60 % conversion of the ethylcarbamate to 1,4-bdc-NCO, but also a ~20 % conversion of the tert-butylcarbamate to 1,4-bdc-NCO. The MOF-5 analogues in this work also proved sufficiently stable to survive the thermolysis conditions with little discernible effect on the porosity of the material.
In Chapter 3, 1,3-bdc-NH2 was protected as both the ethyl carbamate and the tert-butylcarbamate, giving 1,3-bdc-NHCOOEt and 1,3-bdc-NHBoc, which there then incorporated into a lon-e-type framework. It became apparent the lon-e was a poor choice in MOF for use with TPGs as the framework was prone to collapse from desolvation, and it was not possible to thermolyse the materials without complete collapse of the MOFs.
In Chapter 4, bpy-NH2 and bpy-CO2H were protected with TPGs to give bpy-NHBoc and bpy-TBE respectively. The ligands were combined with bpdc and zinc to obtain the BMOF-1-bpdc analogues MUF20-Aβ and MUF20-Aγ. Whilst the thermolysed materials MUF20-Aβt and MUF20-Aγt demonstrated significant gas uptakes compared to their protected counterparts, comparison of MUF20-Aβt with the directly synthesised material MUF20-Aβ’ revealed significantly higher uptakes than the thermolysed materials. This discrepancy indicates that the BMOF-1-bpdc/MUF20 framework is partially degraded under thermolysis conditions. These results strongly imply that this framework is not compatible with TPGs. However, TPGs did allow for the installation of a carboxylate group into the BMOF-1-bpdc/MUF20 framework which was not obtainable through direct synthesis methods.
In Chapter 5, bpy-TBE was combined with btb and Zn/Cu to obtain Zn-DUT-23-TBE and Cu-DUT-23-TBE. These materials were then thermolysed to produce Zn/Cu-DUT-23-CO2H, materials that were not able to be directly synthesised using bpy-CO2H. Unfortunately, the thermolysed materials demonstrated significant decreases in uptakes compared to their protected counterparts. However, the TPG containing materials also had markedly lower uptakes than the parent Zn-DUT-23 and Cu-DUT-23 materials, which has been attributed to pore collapse. This partial pore collapse may have sufficiently weakened the MOF framework to increase its sensitivity to the thermolysis conditions, resulting in a much larger decrease in uptake than would have been the case with a defect free material.
The results of this thesis revealed that MOF stability is a key factor in the compatibility of a material. Specifically, the MOF must be resistant to solvent removal and subsequent heating at elevated temperatures for extended periods. This is most clearly observed in Chapter 3, where the lon-e materials were very susceptible to solvent removal, and later were completely collapsed by thermolysis. These findings have led to the recommendations outlined in section 6.2 for the screening of MOFs for their compatibility with TPGs.