Catalytic Reduction of Dinitrogen to Ammonia by Molybdenum

Abstract

IntroductionIn the 1960s it was fi rst recognized that dinitrogen is reduced to ammonia in the environment by a metalloenzyme, an FeMo nitrogenase [1 -5] . Although " alternative " nitrogenases have been discovered that do not contain Mo [6 -8] , the FeMo nitrogenase is the most accessible. The FeMo nitrogenase has been purifi ed, crystallized, and studied for several decades. It appears to be the most effi cient at reducing dinitrogen to ammonia, with only approximately one equivalent of dihydrogen being produced 1 -50 atm of dinitrogen. It has also been subjected to X -ray studies that have elicited additional discussion concerning the mechanism of dinitrogen reduction [9 -12] . However, no defi nitive conclusions have yet been reached.After the discovery of the fi rst dinitrogen complex ([Ru(NH 3 ) 5 (N 2 )] 2 + ) by Allen and Senoff [13] , reduction of dinitrogen to ammonia under mild conditions with an abiological catalyst seemed imminent [14 -21] . However, the problem proved to be a great deal more challenging than anticipated. Over a period of several decades, hundreds of man years were invested in attempting to demonstrate that it is possible to reduce dinitrogen abiologically and catalytically under mild conditions (25 ° C and 1 atm of N 2 ). Perhaps it was not fully appreciated how diffi cult reduction of dinitrogen to ammonia in solution with protons and electrons would be; dinitrogen is an extraordinarily stable molecule, and protons are reduced readily to dihydrogen, a reaction that itself may be catalyzed by transition metals. The only reaction in which dinitrogen is reduced catalytically (to an approximately 10 : 1 mixture of hydrazine and ammonia) under mild conditions was reported by Shilov [22] . That reaction also requires molybdenum and is catalytic with respect to it. The solvent is methanol and a relatively strong reducing agent such as sodium amalgam is required. Hydrazine is the primary product with ammonia being formed through a metal -catalyzed disproportionation of hydrazine to dinitrogen and ammonia, a reaction that is accomplished relatively readily by transition metals. Shilov proposed that dinitrogen is bound and reduced between two metal centers, although direct evidence was not provided.

Defect Engineering Strategies for Nitrogen Reduction Reactions under Ambient Conditions

“…Also NH 3 is an important energy carrier and storage intermediate. There is an output exceeding 150 million tons every year to sustain the development of the modern society and the growth of the population . As we know, Fritz Haber and Carl Bosch developed an industrial‐scale method (Haber–Bosch process) to reduce the N 2 to NH 3 over a century ago, using Fe‐based catalysts under both high‐temperature (400–600 °C) and high‐pressure (150–300 atm) conditions .…”

Protonation Studies of Molybdenum(VI) Nitride Complexes That Contain the [2,6-(ArNCH2)2NC5H3]2– Ligand (Ar = 2,6-Diisopropylphenyl)

“…Some form of proton-coupled electron transfer (PCET) has been proposed to assist addition of the first proton and electron to (HIPTN 3 N)­Mo­(N 2 ) to yield (HIPTN 3 N)­Mo­(NNH) (HIPT = [{3,5-(2,4,6- i -Pr 3 C 6 H 2 ) 2 C 6 H 3 NCH 2 CH 2 } 3 N] 3– ) and to be involved at some other stage in a variety of other homogeneous reductions of dinitrogen by molybdenum, iron, or other catalysts. Although it is not obvious why PCET would be required to add a proton and electron to a nitride, an interesting fundamental question is whether 4a would be reduced by cobaltocene to yield “[Ar 2 N 3 ]­Mo­(NH)­(O- t -Bu)”.…”

“…The homogeneous catalytic reduction of molecular nitrogen to ammonia with protons and electrons under mild conditions is most efficiently carried out with catalysts that contain molybdenum, , iron, or osmium . The reducing agent is usually a metallocene or KC 8 , and the proton source is often the [B­(3,5-(CF 3 ) 2 C 6 H 3 ) 4 ] − (BAr F 4 – ) or triflate salt of some protonated nitrogen base such as a lutidinium or diphenylammonium.…”

Molecular Catalysts for N2 Reduction: State of the Art, Mechanism, and Challenges

“…Electronic effects can also affect the stability of the catalyst, which represents a key issue for N 2 reduction catalysts, demetallation being recognized as the most important cause for their deactivation. This was established in the case of Schrock and PNP‐based Nishibayashi Mo 0/II dimers, containing amide/pyridine functions that are easily protonated or hydrogenated thus inducing metal dissociation. Conversely, the strong donor properties of TP i Pr B, PPP and Bim R3 ‐PCP ligands afford robust metal‐ligand linkages, especially in Bim R3 ‐PCP Mo 2 complexes in which the NHC carbene acts simultaneously as very strong σ‐donor and π‐acceptor.…”