Catalytic Silylation of N_2 and Synthesis of NH_3 and N_2H_4 by Net Hydrogen Atom Transfer Reactions Using a Chromium P_4 Macrocycle

摘要

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