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

Abstract

Although reductive cleavage of dinitrogen (N 2 )t o nitride (N 3À )and hydrogenation with dihydrogen (H 2 )toyield ammonia (NH 3 )i sa ccomplished in heterogeneous Haber-Boschi ndustrial processes on av ast scale,s equentially coupling these elementary reactions together with as ingle metal complex remains am ajor challenge for homogeneous molecular complexes.H erein, we report that the reaction of ac hloro titanium triamidoamine complex with magnesium effects complete reductive cleavage of N 2 to give ad initride dititanium dimagnesium ditriamidoamine complex. Tandem H 2 splitting by ap hosphine-borane frustrated Lewis pair (FLP) shuttles Hatoms to the N 3À ,e volving NH 3 .I sotope labelling experiments confirmed N 2 and H 2 fixation. Though not yet catalytic,t hese results give unprecedented insight into coupling N 2 and H 2 cleavage and N À Hb ond formation steps together,h ighlight the importance of heterobimetallic cooperativity in N 2 activation, and establish FLPs in NH 3 synthesis. Ammonia, NH 3 ,i sakey chemical feedstock for the production of av ast array of organonitrogen compounds. [1] Nitrogenase enzymes produce NH 3 from dinitrogen, N 2 ,a t ambient temperature and pressure,b ut this process requires eight protons,H + ,e ight electrons,e À ,a nd hydrolysis of sixteen ATPm olecules per N 2 molecule. [2] In contrast, chemical industry produces NH 3 using the Haber-Bosch (HB) process where N 2 and H 2 combine to give NH 3 over heterogeneous metal catalysts. [2] However,high temperatures and pressures are required in HB NH 3 synthesis,reflecting the challenge of cleaving the N 2 triple bond.Following the inception of HB NH 3 synthesis,c onsiderable efforts have been made to gain insight into the chemical and physical processes involved. [3] Therate-limiting step is N 2 chemisorption on the metal catalyst and N À Nbond cleavage to give surface-bound nitrides,N 3À ,which then combine with adsorbed Hatoms,p roduced by essentially barrierless H 2 dissociation on the metal surface,t og ive NH 3 .A st he reaction is heterogeneous,itisdifficult to be studied in atomic detail;however, there is evidence that multi-metallic sites are integral to N 2 activation, and that the presence of various Group 1a nd 2m etal oxides enhances catalyst reactivity by direct interaction with N 2 or as "electronic" promoters. [3] Thus there is great interest in studying homogeneous complexes as surrogates for heterogeneous HB NH 3 synthesis as they can provide detailed information on reaction intermediates and mechanisms. [1][2][3] Ti,V ,Mo, Fe,Ru, Os,and Co complexes have been shown to catalytically reduce N 2 to NH 3 with H + /e À . [4,5] These catalytic cycles are based on sequential biomimetic protonation/reduction of M-N 2 linkages,c ontrasting the direct cleavage of N 2 /H 2 in HB chemistry.Indeed, molecular species that spontaneously cleave N 2 to N 3À are rare, [6] and none of the resulting metal nitrides react with H 2 ;other metal nitrides can react with H 2 ,t om ake am etal amide,M -NH 2 ,o rN H 3 , but they derive fr...

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

“…Using a triamidoamine ligand scaffold, Liddle and co‐workers further illustrated the influence of alkali or alkali‐earth reducing agents mediating the cleavage of dinitrogen by a Ti IV complex 48 [56, 57] . When potassium graphite was used as the reductant, four‐electron reduction of N 2 was observed (Scheme 10, left arrow).…”

Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen

“…Targeting the production of NH 3 , Liddle and co-workers generated a Ti 2 Mg 2 -nitride species formulated as [N­(CH 2 CH 2 N­(SiMe 3 )) 3 ­MgNTi] 2 . This species reacted with H 2 in the presence of the FLP t- Bu 3 P/B­(C 6 F 5 ) 3 , reducing the N 2 fragment and liberating NH 3 and a product formulated as [N­(CH 2 CH 2 N­(SiMe 3 )) 3 ­MgTi] 2 ­[HB­(C 6 F 5 ) 3 ] 6 (Scheme ).…”

Boosting Efficient Ammonia Synthesis over Atomically Dispersed Co-Based Catalyst via the Modulation of Geometric and Electronic Structures

“…In contrast to the N 2 dissociative mechanism, most of the enzyme-catalyzed N 2 fixation processes followed the associative mechanism, wherein the N≡N triple bond breaks stepwise after partial hydrogenation of the N 2 molecule. 10,11 Notably, the dissociation energy of the N-N bond (297 kJ/mol) from *N 2 H 4 intermediates is less than a third of that from the N≡N triple bond. Consequently, the exploitation of advanced catalysts that obey the N 2 hydrogenation process instead of direct N 2 dissociation is promising for realizing the desired NH 3 synthesis performance at mild conditions.…”