Can [M(H)₂(H₂)(PXP)] Pincer Complexes (M=Fe, Ru, Os; X=N, O, S) Serve as Catalyst Lead Structures for NH₃ Synthesis from N₂ and H₂?

Abstract

Eine Doppelbegabung: Der Russe Alexander Borodin war ein wegweisender Komponist und wichtiger Chemiker seiner Zeit. Dass er als der Entdecker der Aldol‐Reaktion angesehen werden muss, ist heute weitgehend unbekannt.

Neutral Tridentate PNP Ligands and Their Hybrid Analogues: Versatile Non‐Innocent Scaffolds for Homogeneous Catalysis

“…[63] Reversible exchange of the H 2 ligand for N 2 was observed and was subsequently used as a starting point for a theoretical investigation into the applicability of 25 as a catalyst for N 2 reduction. [64] More notably, rapid H-D exchange was noted when complex 25 was treated with [D 8 ]toluene, with a high preference for exchange at the sp 2 carbon atoms. Initial deuteration occurred at the para-CH as well as the CH 2 spacer groups, with subsequent additional scrambling into the hydride and tert-butyl fragments.…”

Catalytic NH3 Synthesis using N2/H2 at Molecular Transition Metal Complexes: Concepts for Lead Structure Determination using Computational Chemistry

“…The thermodynamically much more stable end‐on complex 1 will form; however, according to calculations by our group it seems impossible to transfer a metal‐bound hydride centre to the terminal N atom of the N 2 molecule. Instead, we have repeatedly shown that the transfer of the hydride onto the N atom of a side‐on coordinating N 2 molecule like 2 is possible, even with low barriers …”

“…The fact that the associated local minima and transition states for the cycle shown in Scheme were located in a DFT study by our group for ruthenium pincer model complex [Ru(POP)(H) 2 (N 2 )] (POP=2,5‐bis(dimethylphosphanylmethyl)furan) show the reaction to be possible in principal, and we have also subjected other though related ruthenium pincer complexes to such calculations, which have confirmed the general applicability of this approach and also its limitations. A multitude of single‐hydrogen transfer steps in such a mechanism suffer from unreasonable high activation barriers, which define the challenge of this approach.…”

“…Instead, we have repeatedly shown that the transfer of theh ydride onto the Na tom of as ide-on coordinating N 2 molecule like 2 is possible, even with lowb arriers. [19] After initial hydridet ransfer to side-on bonded N 2 resulting in the diazene species 3 the vacantc oordination site at the metal is occupied by H 2 forming an on-classical H 2 complex 4. Dependingo nt he typeo ft he metal an oxidative additiono f H 2 could also occur creating two new hydride centres( 4a).…”

“…[1] On the technical scale NH 3 is produced by treatingg aseous dihydrogen (H 2 )a nd dinitrogen (N 2 )a th eterogeneous catalysts under harsh conditions (T = 400-500 8C, P = 250-350bar) by using the process, which after its inventors Fritz Haber and Carl Bosch, is known worldwide as the Haber-Bosch process. The importance of ammonia synthesis for societya nd the scientific achievements that needed to be made to develop catalytic NH 3 synthesis, to understand it and to implement as an industrialp rocess have been stressed repeatedly by the awards of the Nobel Prizes of 1918, 1931and 2007 [2] Bosch [3] and Ertl, [4] respectively,w ho contributed substantially to our mechanistic understanding of this reaction. The total NH 3 production in 2012 amounted to about 198 million tons and used approximately 1-2 %o ft he global energy supply,w ith the largestf raction of energy usage being related to steam-reforming processes for the generation of H 2 .A ccordingt om arket studies the projection for 2019 predicts an increase of ammonia production of around3 5m illion tons with at otal business volumeo fa bout 102 billion US dollars.…”