Synthesis, characterization, and surface tethering of sulfide- functionalized Ti-oxo-alkoxy cages

The parent cage [TiO(OEt)] readily undergoes ligand exchange with a range of primary alcohols to yield species of the type [TiO(OEt)(OR′) ], with R′ = Me, n-Pr, and n-Bu with x of up to 8. Attempted ligand exchange using the thiol [HO(CH)SH] in an attempt to produce functionalized cages suitable for tethering to Au surfaces failed, resulting only in the polymerization of the Ti oxo-cores. However, the use of the thioether [HO(CH)SCH] resulted in successful thio-functionalization and preservation of the Ti cage core, likely due to methyl protection on the sulfide which precludes further intermolecular reaction with other cage molecules. ESI-MS and NMR showed that the resulting substituted cage [TiO(OEt) {O(CH)SCH}] contained eight methylthio-n-butoxy ligands in two groups of four pseudoequivalent positions. High resolution XPS and STM demonstrated that this sulfide-functionalized cage underwent covalent tethering to Au surfaces involving five sulfur linkages per cage, forming a monolayer of adsorbed species in which the molecular integrity had been preserved. In contrast, the parent oxo-alkoxy cage underwent extensive decomposition, rendering it useless as a building block for specific surface architectures.