Understanding actinide–ligand bonding is important from a fundamental perspective, but also for the rational development of ligands for lanthanide-actinide separation in nuclear-fuel reprocessing. Uranium–sulfur interactions are typically more covalent than uranium–oxygen interactions. Thus, uranium–selenium interactions may be anticipated to be even more covalent. However, there are only a few examples of uranium–thioether (SR₂) complexes, and uranium –selenoether (SeR₂) complexes had not definitively been identified so far.

David J. H. Emslie, McMaster University, Hamilton, Ontario, Canada, and colleagues have synthesized the uranium complexes [(κ³-AS₂^Ph2)₂UI₂] and [(κ³-ASe₂^Ph2)₂UI₂], with H[AS₂^Ph2] = 4,5-bis(phenylsulfido)-2,7,9,9-tetramethylacridan and H[ASe₂^Ph2] = 4,5-bis(phenylselenido)-2,7,9,9-tetramethylacridan. The Se-containing complex (pictured) is the first example of a uranium–selenoether complex.

The team first prepared the pincer proligands H[AS₂^Ph2] and H[ASe₂^Ph2], which were deprotonated to obtain the potassium salts [K(AS₂^Ph2)(dme)] and [K(ASe₂^Ph2)(dme)₂] (dme = 1,2-dimethoxyethane). Reactions of two equivalents of one of these salts with [UI₄(dioxane)₂] then gave the desired products. Computational analyses of the resulting complexes, as well as a hypothetical ether analogue, indicate an increase of covalent character in the U–ER₂ bonds from O to S to Se.

• Uranium(IV) Thio‐ and Selenoether Complexes: Syntheses, Structures, and Computational Investigation of U−ER₂ Interactions,
  Novan A. G. Gray, Jeffrey S. Price, David J. H. Emslie,
  Chem. Eur. J. 2021.
  https://doi.org/10.1002/chem.202103580