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 [TiIV(TrenTMS)X] (X=Cl, 1A; I, 1B; TrenTMS=N(CH2CH2NSiMe3)3) with KC8 affords [TiIII(TrenTMS)] (2). Addition of N2 affords [{(TrenTMS)TiIII}2(μ‐η1:η1‐N2)] (3); further reduction with KC8 gives [{(TrenTMS)TiIV}2(μ‐η1:η1:η2:η2‐N2K2)] (4). Addition of benzo‐15‐crown‐5 ether (B15C5) to 4 affords [{(TrenTMS)TiIV}2(μ‐η1:η1‐N2)][K(B15C5)2]2 (5). Complexes 3–5 treated under N2 with KC8 and [R3PH][I], (the weakest H+ source yet used in N2 reduction) produce up to 18 equiv of NH3 with only trace N2H4. When only acid is present, N2H4 is the dominant product, suggesting successive protonation produces [{(TrenTMS)TiIV}2(μ‐η1:η1‐N2H4)][I]2, and that extruded N2H4 reacts further with [R3PH][I]/KC8 to form NH3.

Bimetallic Cooperative Cleavage of Dinitrogen to Nitride and Tandem Frustrated Lewis Pair Hydrogenation to Ammonia

“…Previously, we reported that [Ti(Tren TMS )Cl] [ 1 ; Tren TMS ={N(CH 2 CH 2 NSiMe 3 ) 3 } 3− ] can be reduced by K (K mirror, KC 8 , or KC 10 H 8 ) to give [Ti(Tren TMS )]; this species can bind to N 2 to give the end‐on:end‐on N 2 complex [{Ti(Tren TMS )} 2 (N 2 )], and further reduction with K gives [{Ti(Tren TMS )} 2 (N 2 K 2 )] ( 2 ), which formally contains N 2 4− where the N−N bond is retained (Scheme ). Nevertheless, 2 produces up to 18 equivalents of NH 3 when treated with excess H + /e − . As it was clear that K ( E °=−2.92 V) plays a cooperative, directing role in that reduction chemistry, we investigated what effect replacing it with the milder reducing agent Mg ( E °=−2.37 V) would have .…”

“… The Ti complex 1 gives different N 2 reduction products depending on the nature of the reducing agent, producing the hydrazido complex 2 when reduced with KC 8 , or the dinitride complex 3 when reduced with elemental Mg (this work).…”

“…Complex 3 is unreactive towards H 2 (1–4 bar) in various hydrocarbon, aromatic, or ethereal solvents. However, as the strongly basic nitride ligands in 3 were readily protonated by the relatively weak phosphonium acid [Cy 3 PH][I], we surmised that 3 might be reacted with H 2 in tandem via the H 2 ‐activated compound [ t Bu 3 PH][HBCF] (BCF=B(C 6 F 5 ) 3 ), generated from the known P t Bu 3 /BCF FLP system using H 2 in toluene . Gratifyingly, treatment of 3 with [ t Bu 3 PH][HBCF] (6 equiv) in THF yielded NH 3 (0.97 equiv, 49 %), which was isolated from the reaction by distillation onto frozen DCl (200 equiv; 2 m in Et 2 O) to form NH x D y Cl (NH 3 DCl expected; Scheme ).…”

Single Electron Transfer to Diazomethane–Borane Adducts Prompts C−H Bond Activations

“…Organometallic chemists have studied metal-N 2 systems since the seminal report of A. D. Allen [4] who described the first transition metal-dinitrogen complex. Over the past 50 years numerous advances have emerged from the luminaries of organometallic chemistry including Schrock, [5] Cummins, [6] Peters, [7] Fryzuk, [8] Evans, [9] Gambarotta, [10] Nishibiashi, [11] Holland, [12] Chatt, [13] and Liddle [14] among others. [15] Avenues to metal-mediated N 2 chemistry have typically involved stoichiometric reductants.…”

Titanium(IV) Cations with Trigonal Monopyramidal Geometry: Unusual Lewis Acids Supported by a Triaryl Triamidoamine Ligand

“…Due to the high Lewis acidity of titanium(IV) centers, the isolation of coordinatively unsaturated titanium(IV) complexes of triamidoamine ligands with TMP geometry is more difficult. No structurally characterized titanium triamidoamine complexes of TMP geometry in the solid state have been reported so far . By using the 3,5‐dimethylphenyl‐substituted triamidoamine ligand [{(3,5‐Me 2 C 6 H 3 )NCH 2 CH 2 } 3 N] 3− , we report herein on thermally stable titanium(IV) cations with TMP geometry around the titanium center.…”