The reduction of N₂ to NH₃ (N₂RR) is a globally significant reaction that is challenging due to the inertness of N₂. Transition metals can activate N₂ and mediate catalytic N₂RR, but challenges remain with respect to catalytic efficiency in terms of overpotential and selectivity. We discuss the role of the N–H bond dissociation free energy (BDFE) of metal diazenidos (M-NNH), the first intermediates of N₂RR, in determining N₂RR efficiency.

Chromium has long been one of the most reluctant metals to coordinate dinitrogen. Historical accounts and recent developments of molecular Cr-N₂ complexes are highlighted herein. Exciting new discoveries have firmly shown Cr complexes are cabable of activating N₂, and catalyzing the reduction of N₂ to NH₃.

A semiconducting iron titanate thin film functions as dual catalyst for visible light nitrogen fixation. Photocatalysis generates ammonia via intermediate hydrazine whereas aerial oxidation of ammonia to nitrate is a thermal catalysis process. The results point to the possibility of non-enzymatic natural nitrogen fixation at oxidic mineral surfaces.

Functionalization of dinitrogen with molecular hydrogen has been a core area of interest for several decades. The competitive binding of molecular H₂ over N₂ makes the process difficult. This Minireview explains the role of the transition metal and the ancillary ligands for the activation and functionalization of N₂ using molecular H₂ in the presence/absence of some organic co-catalysts.

Two-electrode ammonia photosynthesis systems were constructed to enhance the efficiency both by increasing the electron supply using a novel photoanode composed of Au film, TiO₂, and Au-NPs with hybrid energy states due to modal strong coupling and by increasing the surface area of the Zr cathode.

N₂ splitting into terminal nitrides by chemical and electrochemical reduction of [ReCl₂{N(CHCHPtBu₂)₂}] is presented. Comparison of electrochemical data with that of the previously reported, related pincer complex [ReCl₂{N(CH₂CH₂PtBu₂)₂}] allowed for identifying key parameters that control the efficiency of the reaction sequence, which defines reductive N₂ splitting.

Catalytic conversion from N₂ to (Me₃Si)₃N proceeds with the mononuclear enamidoimonophorane Co^I complex, Co^I(NpNPiPr), but not with Fe^I(NpNPiPr). The inevitable repulsive interaction of metal centers with N₂ due to the occupation of d_πu orbitals in the Co^I complex avoids the formation of inactive N₂-bridged dinuclear complex.

Reaction of the titanium alkyl complex with the dinuclear ruthenium hydride complex under H₂ afforded a Ti/Ru heteromultimetallic hydride complex, which can react with N₂ to give a nitrido/imido complex through the cleavage and partial hydrogenation of N₂.

What way would the alkyllithium take? Intrigued by earlier results of nucleophilic attack by RLi reagents at N₂ coordinated to Mn, we wanted to check whether the same applies for group 6 phosphine-N₂ complexes. Treatment of the complexes [ML₄(N₂)₂] (M = Mo or W) with methyllithium affords contrastingly labile anionic methyl-dinitrogen complexes as a result of substitution of one N₂ ligands by the Me anion.

A new, simple yet record-breaking efficient electrosynthesis process for NH₃ production is reported with optimization of catalyst and configuration under various temperatures, including ambient temperatures from H₂O and N₂. The system combines a cheap calcinated iron-graphite particle catalyst, a hydroxide electrolyte, a monel cathode, and a nickel anode.

The tridentate phosphine ligand prPP(Ph)P-pln with phospholane end groups is synthesized and used for the synthesis of MoX₃ (X = Cl, Br, I) precursors. Reduction in the presence of N₂ and either mono- or bidentate coligands led to molybdenum(0) mono- and bis(dinitrogen) complexes and subsequent protonation to a hydrazido complex.

Reduction of a novel heterobimetallic VFe complex derived from 1,1'-bis(arylamido)vanadocene was found to result in the formation of a three-coordinate iron dinitrogen complex. In X-ray crystallographic study of this complex, the dinitrogen moiety shows one of the longest N–N bond lengths for reported iron dinitrogen complexes.

Two novel µ-dinitrogen-divanadium complexes with triamidoamine derivatives have been characterized spectroscopically and structurally. The protonation reactions of the µ-N₂ ligands for them have given ammonia. Furthermore, the mixed valence Na⁺ adduct of the dinitrogen complex has been prepared and characterized. It is also evaluated by DFT calculations.

A new quinolone-based pincer ligand was developed, and its coordination chemistry with Co^II was examined. Upon reduction, a square-planar dinitrogen cobalt(I) derivative is formed, which is in equilibrium with a T-shaped complex. A preliminary study shows that this system is capable of catalytically silylating dinitrogen.

Proton–electron shuttling between a metal–ligand bifunctional iron complex and hydrazine, which enables catalytic disproportionation of hydrazine into dinitrogen and ammonia, was investigated by DFT calculations. The proton-coupled electron transfer processes are associated with singlet–triplet interconversion.

Cubane-type RuIr₃S₄ cluster forms a dimer in which two Ru centers are triply bridged by iodido ligands each other. Two-electron reduction induces fusion of two RuIr₃S₄ units accompanied by deformation of one cubane core to give a unique sulfido-bridged octanuclear framework. The cubane dimer is catalytically active in reduction of N₂H₄ with Cp₂Co and [HNEt₃][BF₄].

Nitrogen fixation catalyzed by dinitrogen-bridged dimolybdenum complexes is theoretically investigated. In the newly-proposed reaction pathway, the dimolybdenum structure is maintained during the entire catalytic cycle and N₂ coordinated at the Mo^I center bearing the electron-withdrawing triflate group exhibits higher reactivity for protonation than that at the Mo⁰ center.

Nitride is functionalized by bis-Si–H bonds to (bis-silyl)amine, revealing a new route of stepwise functionalization of a nitride complex to silylamine under mild conditions.

The nitrogen molecule cannot only be activated thermally and electrochemically, but also photochemically. A study of the electronic structure of four Mo and W complexes with similar ligand spheres that are either photochemically or thermally active shows that ligand-to-metal charge transfer excitations appear more often and with higher intensity in complexes capable of nitrogen photoactivation.

Catalytic silylation of dinitrogen to tris(trimethylsilyl)amine by a series of trinuclear first row transition metal complexes (M = Cr, Mn, Fe, Co, Ni) is reported. Yields depend on metal ion type ranging from 14 to 199 equiv. NH₄⁺/complex after protonolysis for the Mn to Co congeners, respectively.

The picture compares a molecular orbital located mostly on the bridging S or Se in [Fe₂X₂]²⁺ dimers. The 3p- and 4p-orbitals of S and Se, respectively, show a different mixing with the terminal ligands and with each other.