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Terminal iron nitrides (Fe≡N) have been proposed as intermediates of Fe-mediated nitrogen fixation, and well-defined synthetic iron nitrides have been characterized in high oxidation states, including Fe(IV), Fe(V), and Fe(VI). This study reports the generation and low temperature characterization of a terminally bound iron(III) nitride, P₃ᴮFe(N) (P₃ᴮ = tris(o-diisopropylphosphinophenyl)borane), which is a proposed intermediate of iron-mediated nitrogen fixation by the P₃ᴮFe-catalyst system. CW- and pulse EPR spectroscopy (HYSCORE and ENDOR), supported by DFT calculations, help to define a 2A ground state electronic structure of this C₃-symmetric nitride species, placing the unpaired spin in a sigma orbital along the B-Fe-N vector; this electronic structure is distinct for an iron nitride. The unusual d⁵-configuration is stabilized by significant delocalization (~50%) of the unpaired electron onto the axial boron and nitrogen ligands, with a majority of the spin residing on boron.

We report the use of electron rich iron complexes supported by a dianionic diborate pentadentate ligand system, B2Pz4Py, for the coordination and activation of ammonia (NH3) and hydrazine (NH2NH2). For ammonia, coordination to neutral (B2Pz4Py)Fe(ii) or cationic [(B2Pz4Py)Fe(iii)]+ platforms leads to well characterized ammine complexes from which hydrogen atoms or protons can be removed to generate, fleetingly, a proposed (B2Pz4Py)Fe(iii)–NH2 complex (3Ar-NH2). DFT computations suggest a high degree of spin density on the amido ligand, giving it significant aminyl radical character. It rapidly traps the H atom abstracting agent 2,4,6-tri-tert-butylphenoxy radical (ArO˙) to form a C–N bond in a fully characterized product (2Ar), or scavenges hydrogen atoms to return to the ammonia complex (B2Pz4Py)Fe(ii)–NH3 (1Ar-NH3). Interestingly, when (B2Pz4Py)Fe(ii) is reacted with NH2NH2, a hydrazine bridged dimer, (B2Pz4Py)Fe(ii)–NH2NH2–Fe(ii)(B2Pz4Py) ((1Ar)2-NH2NH2), is observed at −78 °C and converts to a fully characterized bridging diazene complex, 4Ar, along with ammonia adduct 1Ar-NH3 as it is allowed to warm to room temperature. Experimental and computational evidence is presented to suggest that (B2Pz4Py)Fe(ii) induces reductive cleavage of the N–N bond in hydrazine to produce the Fe(iii)–NH2 complex 3Ar-NH2, which abstracts H˙ atoms from (1Ar)2-NH2NH2 to generate the observed products. All of these transformations are relevant to proposed steps in the ammonia oxidation reaction, an important process for the use of nitrogen-based fuels enabled by abundant first row transition metals., Synopsis: a highly reactive Fe(iii)–NH2 complex is generated via activation of ammonia or hydrazine in reactions of relevance to fundamental steps in ammonia oxidation processes mediated by an abundant, first row transition metal.

A variety of neutral alkyl-cobalt(III) complexes bearing a dianionic tetrapodal pendadentate ligand B2Pz4Py are reported. Compounds 2-R (R = CH3, CH2SiMe3, CH2SiMe2Ph, i Bu, CH2(c-C5H9) and (CH2)4CH=CH2) are synthesized in 58-90% yield. These diamagnetic, octahedral complexes are thermally stable up to 110˚C and are also remarkably stable to ambient atmosphere. They were fully characterized by spectroscopic techniques, and in three cases, X-ray crystallography. Evidence for reversible homolytic cleavage of the Co-C bonds was found in their reactions with the hydrogen atom donor 1,4-cyclohexadiene and the radical trap TEMPO, as well as the observed cyclization of the 5-hexenyl group to the methylcyclopentyl derivative over the course of several hours. Despite these observations, it can be concluded that the diborate B2Pz4Py ligand provides a very stable platform for these Co(III) alkyls. Reduction by one electron to a Co(II) alkyl can accelerate bond homolysis, but in this instance, using cobaltocene as the reducing agent, leads to ejection of an alkide anion through bond heterolysis, an unusual reaction for Co(III) alkyls. Finally, protonation of compound 2-Me with the strong acid HNTf2 leads to divergent reactivity in which the major protonation site is the pyridyl nitrogen of the ligand as opposed to protonation of the methyl group. The produce of protonation at nitrogen is the dimeric species 4 which was prepared via separate synthesis and characterized by Xray crystallography.

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O

Diprotonation of a remarkably stable, toluene soluble cobalt peroxo complex supported by a neutral, dianionic pentadentate ligand leads to facile O-O bond cleavage and production of a highly reactive Co(IV) oxyl cation intermediate that dimerizes and releases O2. These processes are relevant to both O2 reduction and O2 evolution and the mechanism was probed in detail both experimentally and computationally.

International audience; In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O-2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O-2 to water via Co(III)-O-O-Co(III) peroxo intermediates.

Neutral cobalt(II) complexes of the dianionic tetrapodal pentadentate ligand B2Pz4Py, in which borate linkers supply the anionic charges, are reported. Both the six-coordinate THF adduct 1-THF and the five-coordinate THF-free complex 1 are in a high-spin S = 3/2 configuration in the ground state and have been structurally characterized by X-ray crystallography. These two Co(II) starting materials react rapidly with aryl azides of moderate steric bulk. The thermodynamic products of these reactions are low-spin, diamagnetic, Co(III) amido complexes that are either monomeric, when an external hydrogen atom source such as 1,4-cyclohexadiene is present, or dimeric products formed via C-C coupling of the azide aryl group and internal transfer of H• to the nitrogen. These products are fully characterized and are rare examples of octahedral Co amido compounds; structural determinations reveal significant pyramidalization of the amido nitrogens due to π-π repulsion wherein the amido ligand is primarily a σ donor. The amido products arise from highly reactive Co(III) imido radical intermediates that are the kinetic products of the reactions of 1 or 1-THF with the azide reagents. The imido radicals can be detected by X-band EPR spectroscopy and have been probed by density functional theory computations, which indicate that this doublet species is characterized by a high degree of spin localization on the imido ligand, accounting for the reactivity with hydrogen atom sources and dimerization chemistry observed. The high coordination number and the electron-rich nature of the dianionic B2Pz4Py ligand framework render the imido ligand formed highly reactive.

International audience; We describe the first method to control, in a single microfluidic device, the generation, fusion, and mixing of pico- to nanoliter water-in-oil drops using a straightforward LED light stimulus. This is achieved by implementing a photosensitive surfactant in the water phases of a novel dual flow-focusing microfluidic configuration. UV illumination at 365 nm enables dynamic switch of the flow behavior between a stable dual jet regime (−UV) and a stable dual drop regime (+UV), thus allowing us to generate on-demand two distinct populations of water drops. These drops are fused and mixed downstream inside expansion chambers, offering both a dynamic control by light stimulation and a controllable degree of fusion and mixing by the positions of chambers. The mixing inside the fused drops is found to be much faster than diffusion due to the chaotic advection inside the moving drops.; Grâce à l’introduction d’un tensioactif photosensible dans les phases aqueuses d’un nouveau type de dispositif microfluidique à double focalisation de flux, nous décrivons pour la première fois le contrôle de la génération, de la fusion et du mélange de microgouttes aqueuses dispersées dans une phase d’huile, à l’aide d’un stimulus lumineux produit par une simple diode électroluminescente. Une illumination UV à 365 nm fait basculer réversiblement l’écoulement entre un régime à double jet (−UV) et un régime à double train de gouttes (+UV), nous permettant ainsi de générer à la demande, et de manière dynamique, deux populations de gouttes de compositions distinctes. Des chambres d’expansion permettent d’induire la fusion des gouttes générées par la lumière et de contrôler ainsi le mélange de leurs composants. Nous montrons que la position des chambres définit le degré de fusion et de mélange. Ce mélange s’opère de manière bien plus rapide que le temps caractéristique de diffusion des composants, grâce à l’advection chaotique créée au sein des gouttes en mouvement.