Abstract
The conversion of metal nitrides to NH(3) 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 NH(3) 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(SiMe(3))(2) ligands, is resistant toward protonation by weak acids, while stronger acids result in ligand loss by protonolysis. Moreover, the basic -N(SiMe(3))(2) ligands promote the N-H heterolytic bond cleavage of NH(3), yielding a "naked" diuranium complex containing three bridging ligands, a nitride (N(3-)) and two NH(2) ligands. Conversely, in the nitride-bridged diuranium(iv) complex supported by -OSi(O (t) Bu)(3) ligands, the nitride group is easily protonated to afford NH(3), which binds the U(iv) ion strongly, resulting in a mononuclear U-NH(3) complex, where NH(3) can be displaced by addition of strong acids. Furthermore, the U-OSi(O (t) Bu)(3) bonds were found to be stable, even in the presence of stronger acids, such as NH(4)BPh(4), therefore indicating that -OSi(O (t) 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 N(2) to NH(3).