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

An article from the Pike Group has been published in Chemical Science. In this study, the photooxidation of diolate ligands coordinated to titanium-oxo clusters is investigated. It matters how big the diolate is, as a crucial proton-coupled electron transfer step in the mechanism is only possible when the diolate is large and flexible. For example, 1,5-pentane diolate is selectively oxidised to the corresponding 1,5-hydroxyaldehyde (which undergoes further cyclisation to the lactol).

https://pubs.rsc.org/en/content/articlelanding/2025/sc/d5sc08522e

The study probes the photoredox reactivity of atomically precise titanium-oxo clusters in detail, revealing that at ultrafast timescales all clusters behave similarly (e.g. oxygen to titanium charge transfer occurs), however, only large flexible diolates can adjust to find a suitable transition state for onward proton coupled electron transfer, allowing oxidation at one end of the ligand to form a hydroxyaldehyde molecule (which cyclises to a lactol). In the case of smaller diolates, radical recombination allows for relaxation back to the starting compound. The study provides design parameters for the selective oxidation of diols/triols using Earth abundant photocatalysts.

This was started as the first Warwick MChem project in the Pike Group in 2020, well done to Alex Veale for the initial results, to Ashwani Chikara for completing the work and to Jack Woolley and WCUS for some very cool ultrafast spectroscopy!