Solvothermal synthesis and structural characterization of three polyoxotitanium-organic acid clusters

Three new titanium oxo-clusters Ti₄O₂(OⁱPr)₁₀(OOCPhMe)₂ (I), Ti₆O₄(OEt)₈(OOCPhMe)₈ (II) and Ti₆O₆(OEt)₆(OOCCHPh₂)₆ (III) were obtained by easy one-step solvothermal reactions of titanium(iv) isopropoxide, alcohols and carboxylic acids. The three compounds were characterized by single-crystal and powder X-ray diffraction, TGA/DSC, optical and electron microscopy, and FTIR and NMR spectroscopy. X-ray powder diffraction and spectroscopy confirmed the purity of the compounds. Structural analysis indicates that in all compounds the titanium(iv) ions are six-coordinated (distorted octahedra). (I) is a tetranuclear complex containing a Ti₄(μ₄-O)(μ₂-O) core, which is linked by two (μ₂-OOCPhMe), four (μ₂-OⁱPr) and six OⁱPr ligands. (II) and (III) are hexanuclear complexes with different cores, respectively Ti₆(μ₃-O)₂(μ₂-O)₂ and Ti₆(μ₃-O)₆. The coordination sphere of the Ti atoms is filled by eight (μ₂-OOCPhMe), two (μ₂-OEt) and six OEt in (II) and six (μ₂-OOCHPh₂) and six OEt in (III). Different steric hindrance of substituents attached to the carboxyl group or different concentrations lead to three main different cluster geometries with two ligands. The tetranuclear and the hexanuclear clusters were obtained with the OOCPhMe ligand, while the hexagonal prism cluster was obtained with the OOCCHPh₂ ligand. Hirshfeld surface calculations indicated that the packing is driven by C-O⋯H-C weak hydrogen bonds. The clusters can be used as molecular models of organic molecules bonded to titania surface, used in organic photovoltaic (dye sensitized solar cells) or other optoelectronic applications.