Computational Analysis of Low Overpotential Ammonia Oxidation by Metal-Metal Bonded Ruthenium Catalysts, and Predictions for Related Osmium Catalysts

The catalyzed electrochemical oxidation of ammonia to nitrogen (AOR) is an important fuel-cell half-reaction that underpins a future nitrogen-based energy economy. Our laboratory has reported spontaneous chemical and electrochemical oxidation of ammonia to dinitrogen via reaction of ammonia with the metal-metal bonded diruthenium complex Ru2(chp)4OTf (chp- = 2-chloro-6-hydroxypyridinate, TfO- = trifluoromethanesulfonate). This complex facilitates electrocatalytic ammonia oxidation at mild applied potentials of -255 mV vs ferrocene, which is the [Ru2(chp)4(NH3)]0/+ redox potential. We now report a comprehensive computational investigation of possible mechanisms for this reaction and electronic structure analysis of key intermediates therein. We extend this analysis to proposed second-generation electrocatalysts bearing structurally similar fhp and hmp (2-fluoro-6-hydroxypyridinate and 2-hydroxy-6-methylpyridinate, respectively) equatorial ligands, and we further expand this study from Ru2 to analogous Os2 cores. Predicted M24+/5+ redox potentials, which we expect to correlate with experimental AOR overpotential, depend strongly on the identity of the metal center, and to a lesser degree on the nature of the equatorial supporting ligand. Os2 complexes are easier to oxidize than analogous Ru2 complexes by ∼640 mV, on average. In contrast to mono-Ru catalysts, which oxidize ammonia via a rate-limiting activation of the strong N-H bond, we find lowest-energy reaction pathways for Ru2 and Os2 complexes that involve direct N-N bond formation onto electrophilic intermediates having terminal amido, imido, or nitrido groups. While transition state energies for Os2 complexes are high, those for Ru2 complexes are moderate and notably lower than those for mono-Ru complexes. We attribute these lower barriers to enhanced electrophilicity of the Ru2 intermediates, which is a consequence of their metal-metal bonded structure. Os2 intermediates are found to be, surprisingly, less electrophilic, and we suggest that Os2 complexes may require access to oxidation states higher than Os25+ in order to perform AOR at reasonable reaction rates.

Formation of the N≡N Triple Bond from Reductive Coupling of a Paramagnetic Diruthenium Nitrido Compound

Construction of nitrogen-nitrogen triple bonds via homocoupling of metal nitrides is an important fundamental reaction relevant to a potential Nitrogen Economy. Here, we report that room temperature photolysis of Ru₂(chp)₄N₃ (chp^- = 2-chloro-6-hydroxypyridinate) in CH₂Cl₂ produces N₂ via reductive coupling of Ru₂(chp)₄N nitrido species. Computational analysis reveals that the nitride coupling transition state (TS) features an out-of-plane "zigzag" geometry instead of the anticipated planar zigzag TS. However, with intentional exclusion of dispersion correction, the planar zigzag TS geometry can also be found. Both the out-of-plane and planar zigzag TS geometries feature two important types of orbital interactions: (1) donor-acceptor interactions involving intermolecular donation of a nitride lone pair into an empty Ru-N π* orbital and (2) Ru-N π to Ru-N π* interactions derived from coupling of nitridyl radicals. The relative importance of these two interactions is quantified both at and after the TS. Our analysis shows that both interactions are important for the formation of the N-N σ bond, while radical coupling interactions dominate the formation of N-N π bonds. Comparison is made to isoelectronic Ru₂-oxo compounds. Formation of an O-O bond via bimolecular oxo coupling is not observed experimentally and is calculated to have a much higher TS energy. The major difference between the nitrido and oxo systems stems from an extremely large driving force, ∼-500 kJ/mol, for N-N coupling vs a more modest driving force for O-O coupling, -40 to -140 kJ/mol.

Rh2(II)-Catalyzed Intermolecular N-Aryl Aziridination of Olefins Using Nonactivated N Atom Precursors

The development of the first intermolecular Rh₂(II)-catalyzed aziridination of olefins using anilines as nonactivated N atom precursors and an iodine(III) reagent as the stoichiometric oxidant is reported. This reaction requires the transfer of an N-aryl nitrene fragment from the iminoiodinane intermediate to a Rh₂(II) carboxylate catalyst; in the absence of a catalyst only diaryldiazene formation was observed. This N-aryl aziridination is general and can be successfully realized by using as little as 1 equiv of the olefin. Di-, tri-, and tetrasubstituted cyclic or acylic olefins can be employed as substrates, and a range of aniline and heteroarylamine N atom precursors are tolerated. The Rh₂(II)-catalyzed N atom transfer to the olefin is stereospecific as well as chemo- and diastereoselective to produce the N-aryl aziridine as the only amination product. Because the chemistry of nonactivated N-aryl aziridines is underexplored, the reactivity of N-aryl aziridines was explored toward a range of nucleophiles to stereoselectively access privileged 1,2-stereodiads unavailable from epoxides, and removal of the N-2,4-dinitrophenyl group was demonstrated to show that functionalized primary amines can be constructed.

Ring-Size-Modulated Reactivity of Putative Dicobalt-Bridging Nitrides: C-H Activation versus Phosphinimide Formation

Dicobalt complexes supported by flexible macrocyclic ligands were used to target the generation of the bridging nitrido species [(ⁿ PDI₂ )Co₂ (μ-N)(PMe₃ )₂ ]³⁺ (PDI=2,6-pyridyldiimine; n=2, 3, corresponding to the number of catenated methylene units between imino nitrogen atoms). Depending on the size of the macrocycle and the reaction conditions (solution versus solid-state), the thermolysis of azide precursors yielded bridging phosphinimido [(² PDI₂ )Co₂ (μ-NPMe₃ )(PMe₃ )₂ ]³⁺ , amido [(ⁿ PDI₂ )Co₂ (μ-NH₂ )(PMe₃ )₂ ]³⁺ (n=2, 3), and C-H amination [(³ PDI₂ *-μ-NH)Co₂ (PMe₃ )₂ ]³⁺ products. All results are consistent with the initial formation of [(ⁿ PDI₂ )Co₂ (μ-N)(PMe₃ )₂ ]³⁺ , followed by 1) PMe₃ attack on the nitride, 2) net hydrogen-atom transfer to form N-H bonds, or 3) C-H amination of the alkyl linker of the ⁿ PDI₂ ligand.

Synthesis, characterization and solution behavior of a systematic series of pentapyridyl-supported RuII complexes: comparison to bimetallic analogs

A series of RuII complexes stabilized with the pentapyridyl ligand Py5Me2 (Py5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine) and with an axial X ligand (X = Cl-, H2O, N3-, MeCN) were prepared and characterized in the solid state and in non-aqueous solution. The cyclic voltammograms of these complexes in MeCN reflect a reversible substitution of the axial X ligand with MeCN. Irreversible ligand substitution of [(Py5Me2)RuN3]+ is also observed in propylene carbonate, but only at oxidizing potentials that decompose the azide ligand. The monometallic chloride and azide species are compared with analogous Ru2 metal-metal bonded complexes, which have been reported to undergo irreversible chloride dissociation upon reduction.

Not all density functionals are created equal: the case of the missing electron in the oxidized [W-W≡O](7+) core

The location of the unpaired electron in the new mixed-valent (W2)(IV,V) trication [W2O(dpa)4](3+) presents a challenge for DFT methods. EPR spectroscopy confirms the unpaired electron to be in the W(V)-oxo unit, in agreement with the predictions of hybrid functionals B3LYP and TPSSh, but contrary to the predictions of non-hybrid functionals.

Mixed-ligand complexes of paddlewheel dinuclear molybdenum as hydrodehalogenation catalysts for polyhaloalkanes

A mixed-ligated dimolybdenum complex Mo₂(OAc)₂[CH(NAr)₂]₂ in combination with 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene and ⁿBu₄NCl exhibited high catalytic activity for hydrodehalogenation reactions.

We developed a hydrodehalogenation reaction of polyhaloalkanes catalyzed by paddlewheel dimolybdenum complexes in combination with 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (MBTCD) as a non-toxic H-atom source as well as a salt-free reductant. A mixed-ligated dimolybdenum complex Mo₂(OAc)₂[CH(NAr)₂]₂ (3a, Ar = 4-MeOC₆H₄) having two acetates and two amidinates exhibited high catalytic activity in the presence of ⁿBu₄NCl, in which [ⁿBu₄N]₂[Mo₂{CH(NAr)₂}₂Cl₄] (9a), derived by treating 3a with ClSiMe₃ and ⁿBu₄NCl, was generated as a catalytically-active species in the hydrodehalogenation. All reaction processes, oxidation and reduction of the dimolybdenum complex, were clarified by control experiments, and the oxidized product, [ⁿBu₄N][Mo₂{CH(NAr)₂}₂Cl₄] (10a), was characterized by EPR and X-ray diffraction studies. Kinetic analysis of the hydrodehalogenation reaction as well as a deuterium-labelling experiment using MBTCD-d₈ suggested that the H-abstraction was the rate-determining step for the catalytic reaction.

Electronic tuning of Mo2(thioamidate)4 complexes through π-system substituents and cis/trans isomerism

We report an exploration of the coordination chemistry of a systematic series of cyclic thioamidate ligands with the quadruply-bonded Mo2(4+) core. In addition to the S and N donor atoms that bind to Mo, the ligands utilized in this study have an additional O or S atom in conjugation with the thioamidate π system. The preparation of four new Mo2 complexes is described, and these compounds are characterized by X-ray crystallography, NMR and UV-vis spectroscopy, electrochemistry, and DFT calculations. These complexes provide a means to interrogate the electronics of Mo2(thioamidate)4 systems. Notably, we describe the first two examples of Mo2(thioamidate)4 complexes in their cis-2,2-regioisomer. By varying the π-system substituent and regioisomerism of these compounds, the electronics of the dimolybdenum core is shown to be altered with varying degrees of effect. Cyclic voltammetry results show that changing the π-system substituent from O to S results in an increase in the Mo2(4+/5+) oxidation potential by 170 mV. Changing the arrangement of ligands around the dimolybdenum core from trans-2,2 to cis-2,2 slightly weakens the metal-ligand bonds, raising the oxidation potential by a more modest 30-100 mV. MO diagrams of each compound derived from DFT calculations support these conclusions as well; the identity of the π-system substituent alters the δ-δ* (HOMO-LUMO) gap by up to 0.4 eV, whereas regioisomerism yields smaller changes in the electronic structure.

Lewis acid enhanced axial ligation of [Mo2]4+ complexes

We report here the syntheses, X-ray crystal structures, electrochemistry, and density functional theory (DFT) single-point calculations of three new complexes: tetrakis(monothiosuccinimidato)dimolybdenum(II) [Mo2(SNO5)4, 1a], tetrakis(6-thioxo-2-piperidinonato)dimolybdenum(II) [Mo2(SNO6)4, 1b], and chlorotetrakis(monothiosuccinimidato)pyridinelithiumdimolybdenum(II) [pyLiMo2(SNO5)4Cl, 2-py]. X-ray crystallography shows unusually short axial Mo2-Cl bond lengths in 2-py, 2.6533(6) Å, and dimeric 2-dim, 2.644(1) Å, which we propose result from an increased Lewis acidity of the Mo2 unit in the presence of the proximal Li(+) ion. When 2-py is dissolved in MeCN, the lithium reversibly dissociates, forming an equilibrium mixture of (MeCNLiMo2(SNO5)4Cl) (2-MeCN) and [Li(MeCN)4](+)[Mo2(SNO5)4Cl](-) (3). Cyclic voltammetry was used to determine the equilibrium lithium binding constant (room temperature, K(eq) = 95 ± 1). From analysis of the temperature dependence of the equilibrium constant, thermodynamic parameters for the formation of 2-MeCN from 3 (ΔH° = -6.96 ± 0.93 kJ mol(-1) and ΔS° = 13.9 ± 3.5 J mol(-1) K(-1)) were extracted. DFT calculations indicate that Li(+) affects the Mo-Cl bond length through polarization of metal-metal bonding/antibonding molecular orbitals when lithium and chloride are added to the dimolybdenum core.