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    Photo-redox reactivity of titanium-oxo clusters: Mechanistic insight into a two-electron intramolecular process, and structural characterisation of mixed-valent Ti(iii)/Ti(iv) products


    Krämer, Tobias and Tuna, Floriana and Pike, Sebastian D. (2019) Photo-redox reactivity of titanium-oxo clusters: Mechanistic insight into a two-electron intramolecular process, and structural characterisation of mixed-valent Ti(iii)/Ti(iv) products. Chemical Science, 10 (28). pp. 6886-6898. ISSN 2041-6539

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    Abstract

    Small titanium-oxo-alkoxide clusters, [TiO(OR)(O2PR0 2)]4, synthesised by the stoichiometric reaction of Ti(Oi Pr)4, phosphinic acid and water, undergo a photo-redox transformation under long-wave UV light. The photo-reaction generates blue coloured, mixed-valence Ti(III)/Ti(IV)-oxo clusters alongside acetone and isopropanol by-products. This reactivity indicates the ability for photoactivated charge separation to occur in even the smallest of Ti-oxo clusters. EPR and NMR spectroscopic studies support a photoredox mechanism that occurs via an intramolecular, two-electron pathway, directly relating to current doubling effects observed at TiO2 photoanodes in the presence of alcohols. The rate of photo-reaction is solvent dependent, with donor solvents supporting the formation of low coordinate Ti(III) sites. The nature of the electronic transition is identified by DFT and TDDFT calculations as an oxygen to titanium charge transfer and it is possible to finetune the UV absorption onset observed by changing the phosphinate ligand. A two-electron photo-reduced cluster, [Ti4O4(O2PPh2)6], forms spontaneously from the photo-reaction and its structure is identified by X-ray crystallography with supporting DFT calculations. These indicate that [Ti4O4(O2PPh2)6] is high-spin and contains two ferromagnetically coupled electrons delocalised over the Ti4 core. [Ti4O4(O2PPh2)6] undergoes rapid oxidation in air in the solid-state and performs a remarkable single-crystal to single-crystal transformation, to form a stable cluster-superoxide salt.

    Item Type: Article
    Keywords: Photo-redox; reactivity; titanium-oxo clusters; mechanistic; two-electron; intramolecular; process; structural; characterisation; mixed-valent; Ti(III); Ti(IV); products;
    Academic Unit: Faculty of Science and Engineering > Chemistry
    Item ID: 13983
    Identification Number: https://doi.org/10.1039/c9sc01241a
    Depositing User: Tobias Kraemer
    Date Deposited: 09 Feb 2021 17:17
    Journal or Publication Title: Chemical Science
    Publisher: Royal Society of Chemistry
    Refereed: Yes
    URI:

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