Studying uranium nitride complexes, actinide chemists have gained important insights into 5f covalency and discovered novel modes of reactivity. However, isolable molecular thorium nitride complexes have remained unknown.

Guang Wu and Trevor Hayton, University of California, Santa Barbara, USA, Jochen Autschbach, University of Buffalo, State University of New York, USA, and colleagues have synthesized the first isolable molecular thorium nitride complex, [Na(18-crown-6)(Et₂O)][(R₂N)₃Th(m-N)Th(NR₂)₃] (R = SiMe₃) or in short [Na][1]. The team synthesized this bridged thorium nitride complex by reacting the thorium metallacycle [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] (R = SiMe₃) with NaNH₂ in THF, in the presence of 18-crown-6. In a first reaction step, [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] is deprotonated by NaNH₂. The thorium bis(metallacycle) complex, [Na(THF)_x][Th{N(R)(SiMe₂CH₂)}₂(NR₂)], and NH₃ is formed. NH₃ reacts with unreacted [Th{N(R)(SiMe₂)CH₂}(NR₂)₂], forming [Th(NR₂)₃(NH₂)]. This protonates [Na(THF)_x][Th{N(R)(SiMe₂CH₂)}₂(NR₂)] to give [Na][1].

[Na][1] was isolated in 66 % yield after work-up. The complex is thermally stable. The team characterized the thorium nitride complex by ¹⁵N NMR spectroscopy and DFT calculations. This has allowed them to evaluate the degree of 5f covalency within the Th=N=Th unit. The degree of covalency in this compound is greater than in a related oxo compound.

According to the researchers, this work further strengthens the use of NMR spectroscopy as an important tool for probing the electronic structure of the actinides.

• Use of ¹⁵N NMR spectroscopy to probe covalency in a thorium nitride,
  Selena L. Staun, Dumitru-Claudiu Sergentu, Guang Wu, Jochen Autschbach, Trevor W. Hayton,
  Chem. Sci. 2019.
  https://doi.org/10.1039/c9sc01960j