Discrete Ti-O-Ti Complexes: Visible-Light-Activated, Homogeneous Alternative to TiO₂ Photosensitisers

A series of novel bimetallic Ti^IV amine bis(phenolate) complexes was synthesised and fully characterised. X-ray crystallography studies revealed distorted octahedral geometries around the Ti centres with single or double oxo-bridges connecting the two metals. These robust, air- and moisture-stable complexes were employed as photosensitisers generating singlet oxygen following irradiation with visible light (420 nm) LED module in a commercial flow reactor. All five complexes showed high activity in the photo-oxygenation of α-terpinene and achieved complete conversion to ascaridole in four hours at ambient temperature. The excellent selectivity of these photosensitisers towards ascaridole (vs. transformation to p-cymene) was demonstrated with control experiments using a traditional TiO₂ catalyst. Further comparative studies employing the free pro-ligands as well as a monometallic analogue highlighted the importance of the 'TiO₂ -like' moiety in the polymetallic catalysts. Computational studies were used to determine the nature of the ligand to metal charge transfer (LMCT) states and singlet-triplet gaps for each complex, the calculated trends in the UV-vis absorption spectra across the series agreed well with the experimental results.