Closing the Loop on Transition-Metal-Mediated Nitrogen Fixation: Chemoselective Production of HN(SiMe3)2 from N2, Me3SiCl, and X-OH (X = R, R3Si, or Silica Gel)

Abstract

Treatment of the Mo(IV) terminal imido complex, (η⁵-C₅Me₅)[N(Et)C(Ph)N(Et)]Mo(NSiMe₃) (3), with a 1:2 mixture of iPrOH and Me₃SiCl resulted in the rapid formation of the Mo(IV) dichloride, (η⁵-C₅Me₅)[N(Et)C(Ph)N(Et)]MoCl₂ (1), and the generation of 1 equiv each of HN(SiMe₃)₂ and iPrOSiMe₃. Similarly, a 1:2 mixture of Me₃SiOH and Me₃SiCl provided 1, HN(SiMe₃)₂, and O(SiMe₃)₂. Finally, silica gel, when coupled with excess equivalents of Me₃SiCl, was also effectively used as the X-OH reagent for the generation of 1 and HN(SiMe₃)₂. A proposed mechanism for the 3 → 1 transformation involves formal addition of HCl across the Mo═N imido bond through initial hydrogen-bonding between X-OH and the N-atom of 3 to form the adduct IIIb, followed by chloride delivery from Me₃SiCl to the metal center via a six-membered transition state (IV) that leads to the intermediate, (η⁵-C₅Me₅)[N(Et)C(Ph)N(Et)]Mo(Cl)(NHSiMe₃) (V), and XOSiMe₃ as a co-product. Metathetical exchange of the new Mo-N amido bond of V by a second equivalent of Me₃SiCl then generates 1 and HN(SiMe₃). These results serve to complete a highly efficient chemical cycle for nitrogen fixation that is mediated by a set of well-characterized transition-metal complexes.