Catalytic Silylation of N 2 and Synthesis of NH 3 and N 2 H 4 by Net Hydrogen Atom Transfer Reactions Using a Chromium P 4 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 ) (P N )] (P Cr(N ) ), bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-P N ligand is critical to preserve the structural integrity of the catalyst. P Cr(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 Me SiCl, 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 N H ); 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 ) (P N )] 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.