Switching on photoreactivity in Ti4-oxo clusters by increasing the size of 1,n-alkane diolate bridging ligands.

A series of new isostructural Ti-oxo clusters containing bridging bidentate 1,n-alkane diolate ligands with the formula [Ti4O4(O2PR2)4{O(CH2) n O}2] (R = Ph, n = 2-5; R = Cy, n = 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-2H-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, n = 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) n O˙}. This is dependent on the flexibility (i.e. size) of the 1,n-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.