Titanium-oxo-clusters (TOCs), also known as polyoxotitanates, are structurally defined titanium-oxygen aggregates stabilised by organic surface ligands. Such materials are often considered intermediates between molecules and nanoparticles.1 In the case of TOCs, many can inherit the photoactive and catalytic properties reminiscent of bulk TiO2, however the inclusion of surface ligands, including amines, carboxylates and alcohols, provides diverse opportunities for modifying physical, chemical, mechanical and electronic properties such as solubility, electronic structure and conductivity.2 Consequently, various TOCs have been developed for advanced applications including photocatalysis3 and solar cells4. Additionally, numerous TOCs have been used as synthetic precursors to titanium-containing metal-organic-frameworks (Ti-MOFs), where the preformed cluster prevents rapid hydrolysis to TiO2 during the MOF synthesis5. Despite these promising applications, a fundamental understanding of structure-property relationships in this class of materials is undeveloped, particularly with respect variable temperature and pressure measurements. To-date there are no high-pressure structures of any TOC reported on the Cambridge Structural Database.
The Ti6O6(abz)6(OiPr)6 (Habz = 4-aminobenzoic acid, OiPr = isopropanol) cluster is one of the most common TOCs used in MOF synthesis, as well as having desirable CO2 adsorption properties in the crystalline state6. Here we report the first high-pressure structural characterisation of this compound during compression to 5.04 GPa in the isopropanol mother liquor. The structural compression is highly anisotropic, with up to a 20% difference in compression observed between orthogonal crystallographic axes. Consequently, we find that the solvent accessible voids within the crystal structure change dramatically in volume and shape during compression, which may have implications for its gas sorption properties. Despite being mechanically soft, with an isothermal bulk modulus of 7(1) GPa, the structure retains excellent crystallinity up to 5 GPa. Further investigations of electronic and bulk properties of this compound as a function of pressure will also be discussed.