Bis(acyl)phosphides (BAPs) such as bis(mesitoyl)phosphide (^mesBAP) are phosphorus analogs of acetylacetonate (acac) derivatives. These compounds can be used in f-element coordination chemistry as chelating ligands. So far, bis(acyl)phosphide complexes of uranium had been limited to tetravalent uranium, as in [U^IV(^mesBAP)₄].

Karsten Meyer, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany, and colleagues have isolated the complex salts K[U^III(^mesBAP)₄] (pictured) and K(2.2.2-cryptand)][U^III(^mesBAP)₄]. The compounds feature a genuine trivalent uranium complex with four bidentate chelating ^mesBAP ligands. The complex was obtained by the reduction of its tetravalent analogue [U^IV(^mesBAP)₄] using KC₈ in tetrahydrofuran (THF) at room temperature. The products were characterized spectroscopically, electrochemically, and magnetochemically to confirm the uranium ion’s +III oxidation state.

Overall, the work introduces a new ligand to low-valent uranium coordination chemistry. It allows for the synthesis of an isostructural U^III/U^IV redox pair and could be promising for small-molecule activation chemistry.

• A Genuine Trivalent Bis‐Acylphosphide (BAP) Complex of Uranium,
  Jakob Hochholzer, Pablo Waldschmidt, Frank W. Heinemann, Hansjörg Grützmacher, Karsten Meyer,
  Eur. J. Inorg.Chem. 2023.
  https://doi.org/10.1002/ejic.202300592