Dinitrogen Activation and Hydrogenation by CMeSiMe-Ligated Di- and Trinuclear Chromium Hydride Complexes.

Activation of dinitrogen (N) by well-defined metal hydrides is of much interest and importance, but studies in this area have remained limited to date. We report here N activation and hydrogenation by CMeSiMe-ligated di- and trinuclear chromium polyhydride complexes. Hydrogenolysis of [Cp'Cr(μ-Me)CrCp'] (Cp' = CMeSiMe) () with H in a dilute hexane solution under N-free conditions affords the dichromium dihydride complex [Cp'Cr(μ-H)CrCp'] (), while hydrogenolysis of in a concentrated solution or without solvent provides the trinuclear chromium tetrahydride complex [(Cp'Cr)(μ-H)(μ-H)] (). When the reaction is carried out in the presence of N in a dilute hexane solution, the tetranuclear diimide/dihydride complex [(Cp'Cr)(μ-NH)(μ-H)] () is formed via N-N bond cleavage and N-H bond formation. The reaction of with N at room temperature gives the tetranuclear imide/nitride/dihydride complex [(Cp'Cr)(CMe(CH)SiMe)Cr(μ-NH)(μ-N)(μ-H)] () via N cleavage and hydrogenation and C-H bond activation of a Cp methyl group. At -30 °C, the reaction of with N affords the dinitride intermediate [(Cp'Cr)(μ-N)(μ-H)] (), which is quantitatively transformed to at room temperature. Complex reversibly converts to the stereoisomer . The hydrogenation of a mixture of and with H affords . The reaction of with N proceeds at 100 °C to afford [(Cp'Cr)(μ-NH)] (). This transformation has also been investigated by DFT calculations. Both experimental and computational studies suggest that N incorporation into the chromium hydride cluster is involved in the rate-determining step. This work represents the first example of N cleavage and hydrogenation by well-defined chromium hydride complexes.