Experimental and Computational Studies of Dinitrogen Activation and Hydrogenation at a Tetranuclear Titanium...

The activation of N₂ by a tetranuclear titanium(III) diimide/tetrahydride complex, [(Cp'Ti)₄(μ₃-NH)₂(μ-H)₄] (1) (Cp' = C₅Me₄SiMe₃), which was obtained by the reaction of the Cp'-ligated titanium trialkyl complex Cp'Ti(CH₂SiMe₃)₃ with H₂ and N₂, was investigated in detail by experimental and density functional theory studies. The reaction of 1 in the solid state with N₂ (1 atm) at 180 °C gave the dinitride/diimide complex [(Cp'Ti)₄(μ₃-N)₂(μ₃-NH)₂] (2) through the incorporation, cleavage, and partial hydrogenation of one molecule of N₂ and release of two molecules of H₂. At 130 °C, the formation of 2 was not observed, but instead, dehydrogenation of 1 took place through cleavage of the N-H bond in an imide ligand, followed by deprotonation of the other imide ligand with a hydride ligand, affording the dinitride/tetrahydride complex [(Cp'Ti)₄(μ₃-N)₂(μ-H)₄] (3). Upon heating under N₂ (1 atm) at 180 °C, 3 was quantitatively converted to the dinitride/diimide complex 2. This transformation was initiated by migration of a hydride ligand to a nitride ligand to give one imide unit, followed by N₂ coordination to a Ti atom and H₂ release through the reductive elimination of two hydride ligands. The other imide ligand in 2 was formed by hydride migration to one of the two nitride ligands generated through the cleavage of the newly incorporated N₂ unit. The hydrogenation of 2 with H₂ (100 atm) at 180 °C afforded the tetraimide complex [(Cp'Ti)₄(μ₃-NH)₄] (4). This reaction was initiated by σ-bond metathesis between H₂ and a titanium-nitride bond, followed by migration of the resulting hydride ligand to the remaining nitride ligand. In all of these transformations, the interplay among the hydride, imide, and nitride ligands, including the reversible dehydrogenation/hydrogenation of imide and nitride species, at the multimetallic titanium framework has a critically important role.