N2 Reduction versus H2 Evolution in a Molybdenum- or Tungsten-Based Small-Molecule Model System of Nitrogenase

Chemocatalytic Conversion of Dinitrogen to Ammonia Mediated by a Tungsten Complex

Whereas molybdenum dinitrogen complexes have played a major role as catalytic model systems of nitrogenase, corresponding tungsten complexes were in most cases found to be catalytically inactive. Herein, we present a modified pentadentate tetrapodal (pentaPod) phosphine ligand in which two dimethylphosphine groups of the pentaPodMe (P5Me) ligand have been replaced with phospholanes (Pln). The derived molybdenum complex [Mo(N2)P5Pln] generates 22 and the analogous tungsten complex [W(N2)P5Pln] 7 equivalents of NH3 from N2 in the presence of 180 equivalents of SmI2(THF)2/H2O, rendering the latter the first tungsten complex chemocatalytically converting N2 to NH3. In contrast, the tungsten complex [W(N2)P5Me] generates ammonia from N2 only in a slightly overstoichiometric fashion. The reasons for these reactivity differences are investigated with the help of spectroscopic and electrochemical methods.

Synthesis and electrochemical properties of molybdenum nitrido complexes supported by redox-active NHC and MIC ligands

We report the synthesis of a series of molybdenum nitrido complexes supported by bis-phenolate N-heterocyclic and mesoionic carbenes (NHC & MIC). The reaction between MoN(OtBu)3 and the corresponding azolium salts [H3L1]Cl and [H3L2]Cl (with L1 = bis-phenolate triazolylidene and L2 = bis-phenolate benzimidazolylidene) gives clean access to the corresponding NHC/MIC complexes 1-Cl and 2-Cl. Electrochemical investigations of these complexes showed that they can be reversibly reduced at potentials of -1.13 and -1.01 V vs. Fc/[Fc]+ and the reduced complexes [1-Cl]- and [2-Cl]- can be cleanly isolated after chemical reduction with one equivalent of decamethylcobaltocene. Exchange of the halide atoms is furthermore reported to give a series of nitrido complexes supported by tert-butanolate (1-OtBu and 2-OtBu), perfluoro-tert-butanolate (1-OtBuF9 and 2-OtBuF9), tritylate (1-OCPh3 and 2-OCPh3), mesitolate (1-OMes and 2-OMes), thio-tert-butanolate (1-StBu), thiotritylate (1-SCPh3 and 2-SCPh3) and thiomesitolate complexes (1-SMes). The electrochemical properties of all complexes were evaluated and compared. All isolated complexes were characterized by multinuclear and multidimensional NMR spectroscopy and (if applicable) by EPR spectroscopy. Furthermore, the reactivity of 1-Cl and 2-Cl in the presence of protons and decamethylcobaltocene was investigated, which shows facile extrusion of ammonia, yielding diamagnetic bis-molybdenum(III) complexes 3 and 4.

Unveiling the potential of aluminum-decorated 3D phosphorus graphdiyne as a catalyst for N2O reduction

Interest in single-atom catalysts (SACs) has surged due to their potential to mitigate greenhouse N2O gas from the environment. In this study, we explore the potential of N2O reduction using porous 3D phosphorus graphdiyne decorated with an Al atom (3D-Al/PGDYN) through density functional theory. Results confirm the energetic stability of Al decorations on 3D-PGDYN and indicate that the Al atom plays an active role in catalysis. The N2O molecule undergoes spontaneous dissociation on the surface of the 3D-Al/PGDYN, initiating from the O-end, with a dissociation energy of -2.93 eV. In parallel, N2O dissociation through the N-end involves chemisorption onto the 3D-Al/PGDYN surface, with an adsorption energy (Ead) of -1.74 eV. The negative Ead values (-2.47 and -2.64 eV) indicate that CO and O2 species chemisorb onto the 3D-Al/PGDYN surface, but these energies are lower than that of N2O, suggesting that CO and O2 molecules do not hinder the N2O reduction process. Furthermore, the reaction CO + O* → CO2, which is vital for catalyst regeneration, proceeds swiftly on the 3D-Al/PGDYN catalyst with a low energy barrier of 0.11 eV, highlighting the catalyst's exceptional reactivity. This work holds significance in the design of catalysts and could be instrumental in developing new and efficient solutions for effectively removing harmful N2O from the environment.

Catalytic Nitrogen Fixation Using Well-Defined Molecular Catalysts under Ambient or Mild Reaction Conditions

Ammonia (NH3) is industrially produced from dinitrogen (N2) and dihydrogen (H2) by the Haber-Bosch process, although H2 is prepared from fossil fuels, and the reaction requires harsh conditions. On the other hand, microorganisms have fixed nitrogen under ambient reaction conditions. Recently, well-defined molecular transition metal complexes have been found to work as catalyst to convert N2 into NH3 by reactions with chemical reductants and proton sources under ambient reaction conditions. Among them, involvement of both N2-splitting pathway and proton-coupled electron transfer is found to be very effective for high catalytic activity. Furthermore, direct electrocatalytic and photocatalytic conversions of N2 into NH3 have been recently achieved. In addition to catalytic formation of NH3, selective catalytic conversion of N2 into hydrazine (NH2NH2) and catalytic silylation of N2 into silylamines have been reported. Catalytic C-N bond formation has been more recently established to afford cyanate anion (NCO-) under ambient reaction conditions. Further development of direct conversion of N2 into nitrogen-containing compounds as well as green ammonia synthesis leading to the use of ammonia as an energy carrier is expected.

Oxidative Decarbonylation of an Azacalixpyridine-Supported Mo(0)-Tricarbonyl to a Mo(VI)-Trioxo Complex with Dioxygen in Solution and on Au(111): Determination of Molecular Mechanism

The conversion of an azacalixpyridine-supported Mo(0) tricarbonyl into a Mo(VI) trioxo complex with dioxygen (O2) is investigated in homogeneous solution and in a molecular film adsorbed on Au(111) using a variety of spectroscopic and analytical methods. These studies in particular show that the dome-shaped carbonyl complex adsorbed on the metal surface has the ability to bind and activate gaseous oxygen, overcoming the so-called surface trans-effect. Furthermore, the rate of the conversion dramatically increases by irradiation with light. This observation is explained with the help of complementary DFT calculations and attributed to two different pathways, a thermal and a photochemical one. Based on the experimental and theoretical findings, a molecular mechanism for the conversion of the carbonyl to the oxo complex is derived.

Ammonia synthesis by the reductive N-N bond cleavage of hydrazine using an air-stable, phosphine-free ruthenium catalyst

The development of an effective molecular catalyst to reduce hydrazine efficiently to ammonia using a suitable reductant and proton source is demanding. Herein, an unprecedented air-stable, phosphine-free ruthenium complex is used as a potent catalyst for hydrazine hydrate reduction to generate ammonia using SmI2 and water under ambient reaction conditions. Maximizing the flow of electrons from the reductant to the hydrazine hydrate via the metal centre results in a greater yield of ammonia while minimizing the evolution of H2 gas as a competing product.