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)

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.