Functionalization of Titanium Oxide Cluster Ti O (O C H ) with Catechols: Structures and Ligand-Exchange Reactivities.

The functionalization of Ti O (O C H ) (Ti ) 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 Ti (denoted as Ti site) adopting the mono-protonated, chelate-bidentate binding mode, whereas it binds to a six-coordinated surface-Ti (denoted as Ti site) adopting the mono-dentate mode. With low numbers of equivalents of added catechols the Ti sites show higher reactivity than the Ti sites toward functionalization. Two binding modes may co-exist and equilibrate in solution. Our results also imply that at most eight of the twenty O C H ligands of Ti 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 Ti 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.