Catalytic Silylation of N2 and Synthesis of NH3 and N2H4 by Net Hydrogen Atom Transfer Reactions Using a Chromium P4 Macrocycle

We report the first discrete molecular Cr-based catalysts for the reduction of N2. This study is focused on the reactivity of the Cr-N2 complex, trans-[Cr­(N2)2­(PPh 4NBn 4)] (P 4 Cr­(N 2 ) 2 ), bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-PPh 4NBn 4 ligand is critical to preserve the structural integrity of the catalyst. P 4 Cr­(N 2 ) 2 was found to mediate the reduction of N2 at room temperature and 1 atm pressure by three complementary reaction pathways: (1) Cr-catalyzed reduction of N2 to N­(SiMe3)3 by Na and Me3SiCl, affording up to 34 equiv N­(SiMe3)3; (2) stoichiometric reduction of N2 by protons and electrons (for example, the reaction of cobaltocene and collidinium triflate at room temperature afforded 1.9 equiv of NH3, or at −78 °C afforded a mixture of NH3 and N2H4); and (3) the first example of NH3 formation from the reaction of a terminally bound N2 ligand with a traditional H atom source, TEMPOH (2,2,6,6-tetramethylpiperidine-1-ol). We found that trans-[Cr­(15N2)2­(PPh 4NBn 4)] reacts with excess TEMPOH to afford 1.4 equiv of 15NH3. Isotopic labeling studies using TEMPOD afforded ND3 as the product of N2 reduction, confirming that the H atoms are provided by TEMPOH.