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.