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

<p>The conversion of metal nitrides to NH3&nbsp;is an essential step in dinitrogen&nbsp;fixation, but there is limited knowledge of the reactivity of nitrides with protons (H+). Herein, we report comparative studies for the reactions of H+&nbsp;and NH3&nbsp;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&nbsp;&ndash;N(SiMe3)2&nbsp;ligands, is resistant toward protonation by weak acids, while stronger acids result in ligand loss by protonolysis. Moreover, the basic&nbsp;&ndash;N(SiMe3)2&nbsp;ligands promote the N&ndash;H heterolytic bond cleavage of NH3, yielding a&nbsp;&ldquo;naked&rdquo;&nbsp;diuranium complex containing three bridging ligands, a nitride (N3) and two NH2&nbsp;ligands. Conversely, in the nitride-bridged diuranium(IV) complex supported by&nbsp;&ndash;OSi(OtBu)3&nbsp;ligands, the nitride group is easily protonated to afford NH3, which binds the U(IV) ion strongly, resulting in a mononuclear U&ndash;NH3&nbsp;complex, where NH3&nbsp;can be displaced by addition of strong acids. Furthermore, the U&ndash;&nbsp;OSi(OtBu)3&nbsp;bonds were found to be stable, even in the presence of stronger acids, such as NH4BPh4, therefore indicating that&nbsp;&ndash;OSi(OtBu)3&nbsp;supporting ligands are well suited to be used when acidic conditions are required, such as in the H+/e&nbsp;mediated catalytic conversion of N2&nbsp;to NH3.</p>