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 (N3-) and two NH2 ligands. Conversely, in the nitride-bridged diuranium(iv) complex supported by -OSi((OBu)-Bu-t)(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((OBu)-Bu-t)(3) bonds were found to be stable, even in the presence of stronger acids, such as NH4BPh4, therefore indicating that -OSi((OBu)-Bu-t)(3) supporting ligands are well suited to be used when acidic conditions are required, such as in the H+/e(-) mediated catalytic conversion of N-2 to NH3.