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

A series of novel bimetallic TiIV 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. Bimetallic titanium‐oxo complexes generate singlet oxygen under visible light irradiation, which is used to photo‐oxygenate α‐terpinene to ascaridole under flow conditions. The activity of five titanium amine bis(phenolate) complexes was investigated and the compounds were found to be selective for the production of ascaridole over p‐cymene.