Functionalization of Titanium Oxide Cluster Ti₁₇ O₂₄ (Oⁱ C₃ H₇ )₂₀ with Catechols: Structures and Ligand-Exchange...

The functionalization of Ti₁₇ O₂₄ (Oⁱ C₃ H₇ )₂₀ (Ti₁₇ ) with substituted catechols is studied by using crystallography, Raman spectroscopy, and stopped-flow kinetics. The knowledges on the number of accessible functionalities, their exact location correlated with their Raman assignment, and the kinetic parameters are acquired. A catecholate ligand binds to a five-coordinated surface Ti of Ti₁₇ (denoted as Ti_a site) adopting the mono-protonated, chelate-bidentate binding mode, whereas it binds to a six-coordinated surface-Ti (denoted as Ti_b site) adopting the mono-dentate mode. With low numbers of equivalents of added catechols the Ti_a sites show higher reactivity than the Ti_b sites toward functionalization. Two binding modes may co-exist and equilibrate in solution. Our results also imply that at most eight of the twenty Oⁱ C₃ H₇ ligands of Ti₁₇ are exchangeable without damage of the core structure. The kinetic studies point out that the ligand-exchange reaction is second order and occurs very fast. The current findings are helpful for the controlled functionalization of Ti₁₇ and other Ti oxide clusters, and the further application of them as building blocks in supramolecular chemistry for the assembly of well-defined organic-inorganic hybrid materials.