Nitride protonation and NH₃ binding versus N-H bond cleavage in uranium nitrides

The conversion of metal nitrides to NH₃ is an essential step in dinitrogen fixation, but there is limited knowledge of the reactivity of nitrides with protons (H⁺). Herein, we report comparative studies for the reactions of H⁺ and NH₃ with uranium nitrides, containing different types of ancillary ligands. We show that the differences in ancillary ligands, leads to dramatically different reactivity. The nitride group, in nitride-bridged cationic and anionic diuranium(iv) complexes supported by -N(SiMe₃)₂ ligands, is resistant toward protonation by weak acids, while stronger acids result in ligand loss by protonolysis. Moreover, the basic -N(SiMe₃)₂ ligands promote the N-H heterolytic bond cleavage of NH₃, yielding a "naked" diuranium complex containing three bridging ligands, a nitride (N^3-) and two NH₂ ligands. Conversely, in the nitride-bridged diuranium(iv) complex supported by -OSi(O Bu)₃ ligands, the nitride group is easily protonated to afford NH₃, which binds the U(iv) ion strongly, resulting in a mononuclear U-NH₃ complex, where NH₃ can be displaced by addition of strong acids. Furthermore, the U-OSi(O Bu)₃ bonds were found to be stable, even in the presence of stronger acids, such as NH₄BPh₄, therefore indicating that -OSi(O Bu)₃ supporting ligands are well suited to be used when acidic conditions are required, such as in the H⁺/e^- mediated catalytic conversion of N₂ to NH₃.