The chalcogen difluorides SF₂ and SeF₂ are extremely reactive species that can generally only be isolated by matrix techniques. Selenium tetrafluoride (SeF₄) is a very corrosive and toxic liquid, the preparation of which usually involves gaseous SF₄ and SeO₂ in an autoclave.

Marian Olaru, Jens Beckmann, University of Bremen, Germany, Stefan Mebs, Free University Berlin, Germany, and colleagues  have prepared donor–acceptor complexes between SF₂, SeF₂, and SeF₄ and an N-heterocyclic carbene (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr) that were isolated as the crystalline solids IPrSF₂ (pictured), IPrSeF₂, and IPrSeF₄.

The complexes were prepared via the stoichiometry-controlled fluorination of imidazol-2-thione or the corresponding selenone using either one or two equivalents of solid XeF₂. This approach avoids the generation of the highly unstable and/or toxic parent chalcogen fluorides. When the team tried to extend this chemistry to tellurium, they obtained the metastable ionic complex [IPr₂TeF₃][TeF₅], which slowly converted into the abnormal NHC complex aIPrTeF₄ (aIPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-4-ylidene, i.e., the NHC binds via a carbon atom in its backbone). 

The products were fully characterized using X-ray crystallography and NMR spectroscopy. The electronic structures of the donor–acceptor complexes were studied by density-functional theory (DFT) calculations. The solid complexes IPrSF₂, IPrSeF₂, and IPrSeF₄ can be regarded as “tamed” synthons of their highly reactive parent chalcogen fluorides.

• Donor Acceptor Complexes between the Chalcogen Fluorides SF₂, SeF₂, SeF₄ and TeF₄ and an N‐Heterocyclic Carbene,
  Pascal Komorr, Marian Olaru, Emanuel Hupf, Stefan Mebs, Jens Beckmann,
  Chem. Eur. J. 2022.
  https://doi.org/10.1002/chem.202201023