Titanium(IV) Surface Complexes Bearing Chelating Catecholato Ligands for Enhanced Band-Gap Reduction.

Protonolysis reactions between dimethylamido titanium(IV) catecholate [Ti(CAT)(NMe)] and neopentanol or tris(-butoxy)silanol gave catecholato-bridged dimers [(Ti(CAT)(OCHBu))(HNMe)] and [Ti(CAT){OSi(OBu)}(HNMe)], respectively. Analogous reactions using the dimeric dimethylamido titanium(IV) (3,6-di--butyl)catecholate [Ti(CATBu-3,6)(NMe)] yielded the monomeric Ti(CATBu-3,6)(OCHBu)(HNMe) and Ti(CATBu-3,6)[OSi(OBu)](HNMe). The neopentoxide complex Ti(CATBu-3,6)(OCHBu)(HNMe) engaged in further protonolysis reactions with Si-OH groups and was consequentially used for grafting onto mesoporous silica KIT-6. Upon immobilization, the surface complex [Ti(CATBu-3,6)(OCHBu)(HNMe)]@[KIT-6] retained the bidentate chelating geometry of the catecholato ligand. This convergent grafting strategy was compared with a sequential and an aqueous approach, which gave either a mixture of bidentate chelating species with a bipodally anchored Ti(IV) center along with other physisorbed surface species or not clearly identifiable surface species. Extension of the convergent and aqueous approaches to anatase mesoporous titania (m-TiO) enabled optical and electronic investigations of the corresponding surface species, revealing that the band-gap reduction is more pronounced for the bidentate chelating species (convergent approach) than for that obtained via the aqueous approach. The applied methods include X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and solid-state UV/vis spectroscopy. The energy-level alignment for the surface species from the aqueous approach, calculated from experimental data, accounts for the well-known type II excitation mechanism, whereas the findings indicate a distinct excitation mechanism for the bidentate chelating surface species of the material [Ti(CATBu-3,6)(OCHBu)(HNMe)]@[m-TiO].