Catalytic Dinitrogen Reduction to Ammonia at a Triamidoamine-Titanium Complex

Abstract

Catalytic reduction of N₂ to NH₃ by a Ti complex has been achieved, thus now adding an early d-block metal to the small group of mid- and late-d-block metals (Mo, Fe, Ru, Os, Co) that catalytically produce NH₃ by N₂ reduction and protonolysis under homogeneous, abiological conditions. Reduction of [Ti^IV (Tren^TMS )X] (X=Cl, 1A; I, 1B; Tren^TMS =N(CH₂ CH₂ NSiMe₃ )₃ ) with KC₈ affords [Ti^III (Tren^TMS )] (2). Addition of N₂ affords [{(Tren^TMS )Ti^III }₂ (μ-η¹ :η¹ -N₂ )] (3); further reduction with KC₈ gives [{(Tren^TMS )Ti^IV }₂ (μ-η¹ :η¹ :η² :η² -N₂ K₂ )] (4). Addition of benzo-15-crown-5 ether (B15C5) to 4 affords [{(Tren^TMS )Ti^IV }₂ (μ-η¹ :η¹ -N₂ )][K(B15C5)₂ ]₂ (5). Complexes 3-5 treated under N₂ with KC₈ and [R₃ PH][I], (the weakest H⁺ source yet used in N₂ reduction) produce up to 18 equiv of NH₃ with only trace N₂ H₄ . When only acid is present, N₂ H₄ is the dominant product, suggesting successive protonation produces [{(Tren^TMS )Ti^IV }₂ (μ-η¹ :η¹ -N₂ H₄ )][I]₂ , and that extruded N₂ H₄ reacts further with [R₃ PH][I]/KC₈ to form NH₃ .

Keywords: ammonia; dinitrogen reduction; nitrogen fixation; phosphonium salts; titanium.