Closing the Loop on Transition-Metal-Mediated Nitrogen Fixation: Chemoselective Production of HN(SiMe3)2 from N2, Me3SiCI, and X-OH (X = R, R3Si, or Silica Gel)

Treatment of the Mo(IV) terminal imido complex, (eta(5)-CsMes) [N (Et) C (Ph)N (Et)] Mo (NSiMe3) (3), with a 1:2 mixture of iPrOH and Me3SiCI resulted in the rapid formation of the Mo(IV) dichloride, (eta(5)-CsMe5)[N(Et)C(Ph)N(Et)]MoCl2 (1), and the generation of 1 equiv each of HN(SiMe3)(2) and iPrOSiMe(3). Similarly, a 1:2 mixture of Me3SiOH and Me3SiCI 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 HCI across the Mo=N imido bond through initial hydrogen-bonding between X OH and the N-atom of 3 to form the adduct Illb, followed by chloride delivery from Me3SiCI to the metal center via a six-membered transition state (IV) that leads to the intermediate, (reCsMes)[N(Et)C(Ph)N(Et)]Mo(C1)(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.