Recent advances in wireless photofixation of dinitrogen to ammonia under the ambient condition: A review

Ammonia is the most necessitate and second largely produces chemical reagent worldwide to address the need of the fertilizer industry, as a precursor for many value-added chemicals and a competing source (17.6 wt% H₂) for the blooming hydrogen economy. Although N₂ constitutes 78.09 % of the earth's atmosphere, however, its conversion to ammonia is strenuous because of its non-polar and triple bond character. To address the burgeoning demand, ammonia is typically synthesized via the conventional energy and capital intensive Haber-Bosch technique utilizing natural gas and releasing tons of (CO₂) to the environment. On this basis, cost-effective photon-driven dinitrogen reduction reaction (NRR) is aroused thriving attention as a sustainable and eco-friendly process for ammonia production under ambient conditions. Yet, the photocatalytic ammonia production is not up to the mark for industrial application due to low conversion rate, less catalytic selectivity, ambiguous mechanism, and limited faradic or solar-to-chemical efficiency. Further, the NRR activity of a catalyst essentially depends upon its electronic and surface texture; hence the fabrication of advanced materials is of paramount interest to enhance the performance. The present review covers the underlying mechanism of N₂ photoreduction, prevailing theories, different catalytic engineering techniques, various detection methods, and critical challenges encountered in the theme of photofixation of dinitrogen to ammonia. Additionally, the overarching goal of this review is to bestow an outline of recent research articles in earmarking high-caliber photocatalytic systems and hence planting a strong foundation to ensure the succeeding improvement in this promising and hastily stretching field of dinitrogen photofixation research.     

Molybdenum-catalyzed reduction of molecular dinitrogen into ammonia under ambient reaction conditions

Synthesis of transition metal–dinitrogen complexes and stoichiometric transformations of their coordinated dinitrogen into ammonia and hydrazine have so far been well investigated in order to achieve a novel nitrogen fixation under ambient conditions. As an extension of our study, the dimolybdenum–dinitrogen complex bearing PNP pincer ligands has been found to work as an effective catalyst for the formation of ammonia from dinitrogen, where 52 equiv of ammonia are produced based on the catalyst (26 equiv of ammonia are produced based on the molybdenum atom of the catalyst). This is the most effective catalytic reaction system for the formation of ammonia from molecular dinitrogen catalyzed by transition metal–dinitrogen complexes as catalysts under ambient reaction conditions. Herein, we describe recent results concerning the catalytic reaction, including the proposed reaction pathway.     

New avenues in dinitrogen fixation research

The present article focuses on a recent finding concerning dinitrogen fixation by using titanium oxide/conducting polymer composite systems and its comparison with an earlier fixation method (Schrauzer process) that makes use of a powdered titanium oxide. Both processes work under the stimulus of light at room temperature and pressure, but dinitrogen is fixed to a solid ammonium salt crystal in the former and to a gaseous ammonia molecule in the latter process. Differences in the physicochemical concepts between the two processes are discussed.     

Intrinsic dinitrogen activation at bare metal atoms

There is ongoing interest in metal complexes which bind dinitrogen and facilitate either its reduction or oxidation under mild conditions. In nature, the enzyme nitrogenase catalyzes this process, and dinitrogen fixation occurs under mild and ambient conditions at a metal-sulfur cluster in the center of the MoFe protein, but the mechanism of this process remains largely unknown. In the last few years, new important discoveries have been made in this field. In this review are discussed recent findings on the interaction of N(2) with metal atoms and metal-atom dimers from all groups of the periodic table as provided by gas-phase as well as matrix-isolation experiments. Intrinsic dinitrogen activation at such bare metal atoms is then related to corresponding processes at complexes, clusters, and surfaces.     

Indenyl zirconium dinitrogen chemistry: N2 coordination to an isolated zirconium sandwich and synthesis of side-on, end-on dinitrogen compounds

Exposure of the isolable zirconocene sandwich compounds, (eta(5)-C5Me5)(eta(5)-C9H5-1-R(1)-3-R(2))Zr (R(1) = Me, (i)Pr, (t)Bu; R(2) = Me) to one atmosphere of dinitrogen resulted in N2 coordination. X-ray diffraction and NMR spectroscopy establish that the resulting dimeric dinitrogen compounds contain an unusual mu2,eta(2)-bridging indenyl ring and a weakly activated N2 ligand. N2 coordination from the isolable zirconium sandwich compounds is extremely sensitive to the number and size of the indenyl subsituents. Compounds bearing two [(i)Pr] or three methyl substituents are stable as eta(9) sandwich compounds for weeks under dinitrogen likely due to the inability to dimerize through a two-atom N2 bridge. Performing the reduction of (eta(5)-C5Me5)(eta(5)-C9H5-1-R(1)-3-R(2))ZrCl2 (R(1) = (i)Pr, (t)Bu; R(2) = Me; R(1) = R(2) = SiMe3) under an N2 atmosphere produced a different outcome; rare examples of side-on, end-on zirconium dinitrogen compounds were isolated and in one case, crystallographically characterized. Protonolysis studies with weak Br?nsted acids were used to evaluate the relative activation of the bridging dinitrogen ligands.     

Visible light decomposition of ammonia to dinitrogen by a new visible light photocatalytic system composed of sensitizer (Ru(bpy)(3)2+), electron mediator (methylviologen) and electron acceptor (dioxygen)

Visible light decomposition of aqueous ammonia to dinitrogen was successfully achieved by using a new photocatalytic system based on a molecular photoelectron relay composed of a sensitizer (Ru(bpy)(3)2+), an electron mediator (methylviologen) and an electron acceptor (dioxygen), which can be used as a visible light-driven photocatalyst instead of UV-driven semiconductors.     

Understanding reaction mechanisms of metal-free dinitrogen activation by methyleneboranes

Dinitrogen activation under mild conditions is important but extremely challenging due to the inert nature of the N≡N triple bond evidenced by high bond dissociation energy(945 k J/mol) and large HOMOLUMO gap(10.8 e V). In comparison with largely developed transition metal systems, the reported main group species on dinitrogen activation are rare. Here, we carry out density functional theory calculations on methyleneboranes to understand the reaction mechanisms of their dinitrogen activation. It...     

Molybdenum-catalyzed reduction of molecular dinitrogen under mild reaction conditions

Quite recently we have found two nitrogen fixation systems catalyzed by molybdenum-dinitrogen complexes under mild reaction conditions; one is the transformation of molecular dinitrogen into its synthetic equivalent of ammonia and the other is that into ammonia. A molybdenum-dinitrogen complex bearing two ferrocenyl diphosphines works as a good catalyst in the transformation of molecular dinitrogen into silylamine, where up to 226 equiv are produced based on the catalyst. A dinitrogen-bridged dimolybdenum complex bearing a PNP-type pincer ligand works as a good catalyst in the direct transformation of molecular dinitrogen into ammonia, where up to 23 equiv are produced based on the catalyst. We believe that both systems provide a new aspect in the development of novel nitrogen fixation.     

Conversion of ammonia to dinitrogen in wastewater by Nitrosomonas europaea

Because Nitrosomonas europaea contains ammonia-oxidizing enzyme, nitrite reductase, and nitrous oxide reductase, the conversion of ammonia to dinitrogen was tried with different reaction conditions. In aerobic reaction conditions, ammonium was converted to nitrite (NO ₂ ⁻ ), while under oxygen-limiting or oxygen-free conditions, NO ₂ ⁻ -N formed from ammonia oxidation by N. europaea was reduced to N₂O and dinitrogen with 22% conversion. During denitrification, optimal pH for the production of N₂O and dinitrogen was found to be 7.0–8.0. Dinitrogen was not produced in acidic pH<7.0. A low partial oxygen pressure as well as oxygen-free conditions are favorable for high production of dinitrogen.     

Activation and protonation of dinitrogen at the FeMo cofactor of nitrogenase

The protonation of N2 bound to the active center of nitrogenase has been investigated using state-of-the-art density-functional theory calculations. Dinitrogen in the bridging mode is activated by forming two bonds to Fe sites, which results in a reduction of the energy for the first hydrogen transfer by 123 kJ/mol. The axial binding mode with open sulfur bridge is less reactive by 30 kJ/mol and the energetic ordering of the axial and bridged binding modes is reversed in favor of the bridging dinitrogen during the first protonation. Protonation of the central ligand is thermodynamically favorable but kinetically hindered. If the central ligand is protonated, the proton is transferred to dinitrogen following the second protonation. Protonation of dinitrogen at the Mo site does not lead to low-energy intermediates.     

Reaction of thebaine with dinitrogen tetroxide

Treatment of thebaine with dinitrogen tetroxide gave a mixture of 14β-nitrocodeinone (23%) and 8-nitrothebaine (7%).     

Thermal decomposition of dinitrogen complexes of Cr(III)

The thermal behaviour of two dinitrogen complexes of chromium with EDTA and CDTA have been studied using DTA and TG. IR spectroscopy is used for the characterization of intermediate products. The loss of dinitrogen occurs at 240°C in both cases.     

EPR study of the dinitrogen tetroxide dissociation in organic solvents

The dinitrogen tetroxide homolytical dissociation constants K _DIS are determined in organic solvents. The values are the highest in non-donor and the lowest in n-donor solvents. The variations of K _DIS with solvent are interpreted in terms of electron donor-acceptor interactions between dinitrogen tetroxide and electron donor molecules.     

N2 hydrogenation from activated end-on bis(indenyl) zirconium dinitrogen complexes

Sodium amalgam reduction of the bis(indenyl)zirconium dihalide complexes, (eta5-C9H5-1-iPr-3-Me)2ZrX2 (X = Cl, Br, I), yielded the corresponding end-on dinitrogen complexes, [(eta5-C9H5-1-iPr-3-Me)2Zr(NaX)]2(mu2, eta1, eta1-N2), with inclusion of 1 equiv of salt per zirconocene. The solid state structures of the chloro and iodo congeners establish short Zr N and elongated N N bonds, consistent with modest to strong activation of the coordinated dinitrogen molecule. Exposure of the N2 compounds to 1 atm of dihydrogen resulted in rapid N H bond formation to yield a hydrido zirconocene hydrazido compound concomitant with salt elimination. These studies establish a new structural type of zirconocene dinitrogen complex and demonstrate that side-on coordination of the N2 ligand in the ground state is not a prerequisite for dinitrogen hydrogenation.     

Carbon-nitrogen bond formation via the reaction of terminal alkynes with a dinuclear side-on dinitrogen complex

The dinuclear dinitrogen complex ([P2N2]Zr)2(mu-eta2:eta2-N2) reacts with terminal aryl alkynes to generate a new species in which the dinitrogen unit has been functionalized. The products formed have the general formula ([P2N2]Zr)2(mu-eta2:eta2-N2CCAr)(mu-CCAr) and display a styryl-hydrazido unit bridging the two Zr centers along with a bridging arylalkynide. The crystal structures of three of these products are reported. A mechanism is proposed for this process that involves cycloaddition of the alkyne to the side-on dinitrogen unit followed by protonation of the Zr-C bond by a second equivalent of terminal alkyne. A fluxional process is operative in solution that equilibrates the phosphorus nuclei at high temperature; in the slow exchange limit, the two [P2N2]Zr ends of complex are inequivalent as evidenced by four resonances in the 31P NMR spectrum for the inequivalent phosphorus donors. This C-N bond-forming reaction is unique in that an activated dinitrogen fragment undergoes a reaction with an alkyne.     

Terminal end-on coordination of dinitrogen versus isoelectronic CO: A comparison using the charge displacement analysis

The metal dinitrogen bonding in a wide series of terminal end-on dinitrogen complexes is investigated with the charge displacement analysis based on natural orbitals of chemical valence (CD-NOCV). The effect of the σ donation and π backdonation on the N N bond are discussed and compared with the observations for a series of carbonyl complexes, published in 2016 by Tarantelli et al. The σ donation is relative invariant over the series of dinitrogen complexes and has no significant effect on the N N bond strength, whereas the π backdonation causes a considerable elongation of the N N bond. Some uncommon examples of weakly bound dinitrogen with blue-shifted stretching frequency compared to free N₂ (ν = 2330 cm⁻¹) are known. The dinitrogen bonding in these complexes is simulated with a point charge. Apparently, electrostatics account for the shortened N─N bond in these systems.     

A personal account of some dinitrogen and organometallic chemistry research at the University of Sussex

This article reviews the development of dinitrogen chemistry and some associated organometallic chemistry at the University of Sussex with which the author was directly involved. The establishment of the basic heavy-element halide phosphine chemistry laid the ground for the discovery of dinitrogen complexes of rhenium, osmium, molybdenum and tungsten. From there, some of the first well-defined reactions of coordinated dinitrogen (especially protonation and alkylation) were discovered and the essential mechanisms of such reactions were established. This allowed the development of models for the action of nitrogenases that are still probably the best available. Later work has produced similar models in iron chemistry and a range of organometallic chemistry has been uncovered in the effort to discover parallels between the basic organometallic chemistry of substances such as metal carbonyls, dinitrogen complexes and hydrides in their interactions with acetylenes and cyclpropene.     

Nitride formation by thermolysis of a kinetically stable niobium dinitrogen complex

The reduction of [P(2)N(2)]NbCl (where [P(2)N(2)] = PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh) with KC(8) under a dinitrogen atmosphere generates the paramagnetic dinuclear dinitrogen complex ([P(2)N(2)]Nb)(2)(mu-N(2)) (2). Complex 2 has been characterized crystallographically and by EPR spectroscopy. Variable-temperature magnetic susceptibility measurements indicate that 2 displays antiferromagnetic coupling between two Nb(IV) (d(1)) centers. A density functional theory calculation on the model complex [(PH(3))(2)(NH(2))(2)Nb](2)(mu-N(2)) was performed. Thermolysis of ([P(2)N(2)]Nb)(2)(mu-N(2)) in toluene generates the paramagnetic bridging nitride species where one N atom of the dinitrogen ligand inserts into the macrocycle backbone to form [P(2)N(2)]Nb(mu-N)Nb[PN(3)] (3) (where [PN(3)] = PhPMe(CHSiMe(2)NSiMe(2)CH(2)P(Ph)CH(2)SiMe(2)NSiMe(2)N)). Complex 3 has been characterized in the solid state as well as by variable-temperature magnetic susceptibility measurements. The reaction of ([P(2)N(2)]Nb)(2)(mu-N(2)) with phenylacetylene displaces the dinitrogen fragment to generate a paramagnetic eta(2)-alkyne complex, [P(2)N(2)]Nb(eta(2)-HCCPh) (4).     

Oxygen extrusion from amidate ligands to generate terminal Ta=O units under reducing conditions. How to successfully use amidate ligands in dinitrogen coordination chemistry

A series of mixed Cp* amidate tantalum complexes Cp*Ta(RNC(O)R')X(3) (where R = Me(2)C(6)H(3), (i)Pr, R' = (t)Bu, Ph, X = Cl, Me) have been prepared via salt metathesis and their fundamental reactivities under reducing conditions have been explored. Reaction of the tantalum chloro precursors with potassium graphite under N(2) or Ar leads to the stereoselective formation of the terminal tantalum oxo species, Cp*Ta=O(η(2)-RN=CR')Cl. This represents the formal extrusion of oxygen from the amidate ligand to the reduced tantalum center and is accompanied by the formation of the iminoacyl fragment bound to Ta(v). Amidate dinitrogen complexes, [Cp*TaCl(RNC(O)(t)Bu)](2)(μ-N(2)) (where R = Me(2)C(6)H(3), (i)Pr) were synthesized via salt metathesis from the known [Cp*TaCl(2)](2)(μ-N(2)) precursor, establishing that amidate ligands can support dinitrogen complexes, but not the reduction process often necessary for their synthesis.     

Photocatalytic decomposition of dinitrogen monoxide on zinc-supporting porous Vycor glass

Zinc-supporting (0.05–0.61 wt.%) porous Vycor glass was found to act as a photocatalyst for the decomposition of dinitrogen monoxide at 278 K. The active species of zinc ion coordinated by oxygen ion is suggested.