Molecular and Surface Catecholato Complexes of Titanium(IV)

Abstract

Surface functionalized titanium dioxides (TiO2) are among the most studied hybrid materials for photocatalytic reactions and their potential application in the field of biomedicine, e.g. for light-induced targeted drug delivery, was pointed out. Catechols are prominent organic dyes, which generate different surface species on TiO2, dependent on the latter´s surface curvature and the surface loading. In order to investigate the surface species, which has previously been detected, only on very small nanoparticles, heteroleptic Ti(IV) catecholate complexes were synthesized, aiming at a monomeric Ti(IV) catecholate complex, whose basic actor ligands would readily react with the surface hydroxyls of the substrate, and whose catecholato binding mode (chelating) is predefined and retained upon the grafting reaction. In a first approach Ti(NMe2)4 was reacted with five differently substituted catechols, giving oxo-bridged dimers (except for [Ti(CATtBu-3,6)(NMe2)2]2, where one nitrogen atom was involved in the bridging, too). A ligand scrambling test with the largest and the smallest catecholate complexes indicated the presence of monomeric species in solution. Therefore, the sterically most demanding dimer [Ti(DHN)(NMe2)2]2 was further investigated according to two grafting routes – the sequential approach gave the same dominant surface species as the convergent approach, but in both cases more than one species was present at the surface. To prevent dimerization, the complexes with none and with the highest degree of catecholato- oxygen shielding by tert-butyl groups at the benzene ring – [Ti(CAT)(NMe2)2]2 and [Ti(CATtBu-3,6)(NMe2)2]2 – were reacted with neopentanol to exchange the small amido groups with bulkier actor ligands. In both cases HNMe2 stayed coordinated at the Ti(IV) centers to ensure an octahedral geometry. But only for the bulkier catecholato ligand a monomeric complex, [Ti(CATtBu-3,6)(OCH2tBu)2(HNMe2)2], was obtained. This was further used for the grafting onto KIT-6 for characterization of the surface species and onto m-TiO2 for optical and electronic investigations. Only the convergent approach gave exclusively the desired bidentate chelating surface species [Ti(CATtBu-3,6)(OCH2tBu)2(HNMe2)2]@[KIT-6], which was compared to an aqueous route species H2CATtBu-3,6@Ti(OH)x@[KIT-6] on the same support. The energy levels of the hybrid m-TiO2 from the aqueous route mirrored the well-known type-II excitation mechanism, which is accompanied by a slight increase of energy levels and a reduced bandgap, compared to virgin m-TiO2. The novel hybrid-m-TiO2 from the convergent approach displayed decreased energy levels compared to virgin m-TiO2, and an even smaller bandgap than the aqueous approach material.