Dinitrogen Activation and Hydrogenation by C5Me4SiMe3‑Ligated Di- and Trinuclear Chromium Hydride Complexes

Activation of dinitrogen (N2) by well-defined metal hydrides is of much interest and importance, but studies in this area have remained limited to date. We report here N2 activation and hydrogenation by C5Me4SiMe3-ligated di- and trinuclear chromium polyhydride complexes. Hydrogenolysis of [Cp′Cr­(μ-Me)2CrCp′] (Cp′ = C5Me4SiMe3) (1) with H2 in a dilute hexane solution under N2-free conditions affords the dichromium dihydride complex [Cp′Cr­(μ-H)2CrCp′] (2), while hydrogenolysis of 1 in a concentrated solution or without solvent provides the trinuclear chromium tetrahydride complex [(Cp′Cr)3(μ3-H)­(μ-H)3] (3). When the reaction is carried out in the presence of N2 in a dilute hexane solution, the tetranuclear diimide/dihydride complex [(Cp′Cr)4(μ3-NH)2(μ3-H)2] (4) is formed via N–N bond cleavage and N–H bond formation. The reaction of 2 with N2 at room temperature gives the tetranuclear imide/nitride/dihydride complex [(Cp′Cr)3(C5Me3(CH2)­SiMe3)­Cr­(μ3-NH)­(μ3-N)­(μ-H)2] (5) via N2 cleavage and hydrogenation and C–H bond activation of a Cp methyl group. At –30 °C, the reaction of 2 with N2 affords the dinitride intermediate [(Cp′Cr)4(μ3-N)2(μ3-H)2] (6), which is quantitatively transformed to 5 at room temperature. Complex 5 reversibly converts to the stereoisomer 5′. The hydrogenation of a mixture of 5 and 5′ with H2 affords 4. The reaction of 3 with N2 proceeds at 100 °C to afford [(Cp′Cr)3(μ3-NH)2] (7). This transformation has also been investigated by DFT calculations. Both experimental and computational studies suggest that N2 incorporation into the chromium hydride cluster is involved in the rate-determining step. This work represents the first example of N2 cleavage and hydrogenation by well-defined chromium hydride complexes.