Switching on photoreactivity in Ti ₄ -oxo clusters by increasing the size of 1, <i>n</i> -alkane diolate bridging ligands

A series of new isostructural Ti-oxo clusters containing bridging bidentate 1,<i>n</i>-alkane diolate ligands with the formula [Ti₄O₄(O₂PR₂)₄{O(CH₂) _<i>n</i> O}₂] (R = Ph, <i>n</i> = 2-5; R = Cy, <i>n</i> = 2-4) were prepared by an alkoxide exchange strategy. The cluster with the 1,5-pentane diolate ligand undergoes productive photoredox chemistry in solution under UV light, resulting in the oxidation of one end of the alkane diolate, and subsequent cyclisation into the lactone tetrahydro-2<i>H</i>-pyran-2-ol, along with formation of a two-electron reduced Ti-oxo cluster stabilised by pyridine. Clusters with smaller bridging alkane diolates show no productive photoredox reactivity, except for R = Cy, <i>n</i> = 3, in which the photoredox products are unstable to further redox processes. Ultrafast electronic absorption spectroscopy studies reveal that all clusters undergo a similar initial photoexcitation step, therefore, productive redox pathways are controlled by the availability of a suitable transition state for rapid proton-coupled electron transfer from the initially generated pendant alkane diolate radicals {Ti-O(CH₂) _<i>n</i> O˙}. This is dependent on the flexibility (<i>i.e.</i> size) of the 1,<i>n</i>-alkane diolate ligand backbone. Interestingly, the productive photoredox pathway of the 1,5-pentane diolate cluster is turned off when the flexibility of the cluster is restricted in the single crystal phase.