Metal-organic frameworks (MOFs) are highly tunable crystalline materials formed through the coordination-driven self-assembly between organic ligands and metal nodes.[1] Through careful ligand design, a vast number of reactive groups or functionalities can be incorporated into MOF structures to provide characteristics that can be tailored to specific applications. Post-synthetic metal ion exchange is an additional, valuable strategy for modifying the secondary building units of MOFs, and thereby enhancing their stability or properties for targeted applications.[2,3] Additionally, through this approach, MOFs that are difficult or impossible to synthesise directly through conventional methods can be made (e.g. Titanium-based MOFs can be made from a Zirconium-based MOF precursor).
As part of our interest in forming chemically and thermally robust flexible MOFs, this work presents the synthesis of a family of isostructural group 13-based MOFs prepared from flexible pyrazole carboxylate linkers and group 13 metal salts (where the metal is Aluminium, Gallium and Indium). These group 13-based MOFs feature hexagonal pores with notably large pore diameters of approximately 15.4 Å and vacant bispyrazole sites poised for post-synthetic metalation. Due to their structural flexibility and the relative stability of the metal-carboxylate bonds, the Indium analogue does not show permanent porosity, while the Al-MOF undergoes a structural change upon activation. Therefore, as a pathway to enhance the chemical and structural stability and to access the expected porosity for these materials, we explored post-synthetic metal ion exchange within this family of group 13 metal-containing MOFs. Additionally, we were also interested in developing a general method to access single crystals of Al-based MOFs suitable for X-ray crystallography. This would allow us to probe the effect of activation on structure, as crystals of Al-based MOFs are generally too small for structure determination, even at the MX beamlines of the Australian Synchrotron.