Catalytic Silylation of N and Synthesis of NH and NH by Net Hydrogen Atom Transfer Reactions Using a Chromium P Macrocycle.

We report the first discrete molecular Cr-based catalysts for the reduction of N. This study is focused on the reactivity of the Cr-N complex, trans-[Cr(N)(PN)] (PCr(N)), bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-PN ligand is critical to preserve the structural integrity of the catalyst. PCr(N) was found to mediate the reduction of N at room temperature and 1 atm pressure by three complementary reaction pathways: (1) Cr-catalyzed reduction of N to N(SiMe) by Na and MeSiCl, affording up to 34 equiv N(SiMe); (2) stoichiometric reduction of N by protons and electrons (for example, the reaction of cobaltocene and collidinium triflate at room temperature afforded 1.9 equiv of NH, or at -78 °C afforded a mixture of NH and NH); and (3) the first example of NH formation from the reaction of a terminally bound N ligand with a traditional H atom source, TEMPOH (2,2,6,6-tetramethylpiperidine-1-ol). We found that trans-[Cr(N)(PN)] reacts with excess TEMPOH to afford 1.4 equiv of NH. Isotopic labeling studies using TEMPOD afforded ND as the product of N reduction, confirming that the H atoms are provided by TEMPOH.