A Synthetic Cycle for Nitrogen Atom Transfer Featuring a Diruthenium Nitride Intermediate

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.

Electronic Structure of Ru26+ Complexes with Electron-Rich Anilinopyridinate Ligands

Diruthenium paddlewheel complexes supported by electron-rich anilinopyridinate (Xap) ligands were synthesized in the course of the first in-depth structural and spectroscopic interrogation of monocationic [Ru₂(Xap)₄Cl]⁺ species in the Ru₂⁶⁺ oxidation state. Despite paramagnetism of the compounds, ¹H NMR spectroscopy proved highly informative for determining the isomerism of the Ru₂⁵⁺ and Ru₂⁶⁺ compounds. While most compounds are found to have the polar (4,0) geometry, with all four Xap ligands in the same orientation, some synthetic procedures resulted in a mixture of (4,0) and (3,1) isomers, most notably in the case of the parent compound Ru₂(ap)₄Cl. The isomerism of this compound has been overlooked in previous reports. Electrochemical studies demonstrate that oxidation potentials can be tuned by the installation of electron donating groups to the ligands, increasing accessibility of the Ru₂⁶⁺ oxidation state. The resulting Ru₂⁶⁺ monocations were found to have the expected (π*)² ground state, and an in-depth study of the electronic transitions by Vis/NIR absorption and MCD spectroscopies with the aid of TD-DFT allowed for the assignment of the electronic spectra. The empty δ* orbital is the major acceptor orbital for the most prominent electronic transitions. Both Ru₂⁵⁺ and Ru₂⁶⁺ compounds were studied by Ru K-edge X-ray absorption spectroscopy; however, the rising edge energy is insensitive to redox changes in the compounds due to the broad line shape observed for 4d transition metal K-edges. DFT calculations indicate the presence of ligand orbitals at the frontier level, suggesting that further oxidation beyond Ru₂⁶⁺ will be ligand-centered rather than metal-centered.

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.

Spontaneous N2 formation by a diruthenium complex enables electrocatalytic and aerobic oxidation of ammonia

The electrochemical conversion of ammonia to dinitrogen in a direct ammonia fuel cell (DAFC) is a necessary technology for the realization of a nitrogen economy. Previous efforts to catalyse this reaction with molecular complexes required the addition of exogenous oxidizing reagents or application of potentials greater than the thermodynamic potential for the oxygen reduction reaction-the cathodic process of a DAFC. We report a stable metal-metal bonded diruthenium complex that spontaneously produces dinitrogen from ammonia under ambient conditions. The resulting reduced diruthenium material can be reoxidized with oxygen for subsequent reactions with ammonia, demonstrating its ability to spontaneously promote both half-reactions necessary for a DAFC. The diruthenium complex also acts as a redox mediator for the electrocatalytic oxidation of ammonia to dinitrogen at potentials as low as -255 mV versus Fc^0/+ and operates below the oxygen reduction reaction potential in alkaline conditions, thus achieving a thermodynamic viability relevant for the future development of DAFCs.

A metastable RuIII azido complex with metallo-Staudinger reactivity

The metastable purple [(Py5Me2)RuIII(N3)]2+ ion reacts with PPh3 at room temperature to form the phosphinimine complex [(Py5Me2)RuII(N(H)PPh3)]2+ and free [H2NPPh3]+ in a combined 23% conversion. Mechanistic studies suggest that this is the first metallo-Staudinger reaction of a late transition metal that bypasses the nitrido mechanism and instead utilizes a Ru-N[double bond, length as m-dash]N[double bond, length as m-dash]N-PPh3 phosphazide intermediate.

N-Atom transfer via thermal or photolytic activation of a Co-azido complex with a PNP pincer ligand

Thermal or photolytic activation of well-defined mononuclear [Co(N₃)(PNP)] (PNP = 2,2'-bis(diisopropylphosphino)-4,4'-ditolylamido) results in the structurally characterized dinuclear species [Co(μ-N;κ³-P,N,N-PN^PN)]₂ (3), with two N-bridging phosphiniminato bridgeheads. Density Functional Theory (DFT) calculations indicate the intermediacy of a mononuclear cobalt-nitrido complex, followed by N-migratory insertion into a Co-P bond. Reaction of 3 with two equiv. HCl leads to rupture of the dimer with formation of mononuclear [CoCl(PN^PN^H)] (4) by protonation of the N-bridges.

Migratory insertion and hydrogenation of a bridging azide in a thiolate-bridged dicobalt reaction platform

A novel well-defined thiolate-bridged dicobalt azido complex is converted to a rare sulfilimide-bridged dicobalt complex via nitrogen atom migratory insertion into the Co-S bond upon thermolysis. Intriguingly, the homolytic cleavage of hydrogen is achieved by this azide under mild conditions to furnish a partially hydrogenated azido complex.

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.

Anilinopyridinate-supported Ru2x+ (x = 5 or 6) paddlewheel complexes with labile axial ligands

Five new metal–metal bonded Ru₂ amidinate compounds with labile axial ligands are presented and discussed.

A Synthetic Oxygen Atom Transfer Photocycle from a Diruthenium Oxyanion Complex

Three new diruthenium oxyanion complexes have been prepared, crystallographically characterized, and screened for their potential to photochemically unmask a reactive Ru-Ru═O intermediate. The most promising candidate, Ru2(chp)4ONO2 (4, chp = 6-chloro-2-hydroxypyridinate), displays a set of signals centered around m/z = 733 amu in its MALDI-TOF mass spectrum, consistent with the formation of the [Ru2(chp)4O](+) ([6](+)) ion. These signals shift to 735 amu in 4*, which contains an (18)O-labeled nitrate. EPR spectroscopy and headspace GC-MS analysis indicate that NO2(•) is released upon photolysis of 4, also consistent with the formation of 6. Photolysis of 4 in CH2Cl2 at room temperature in the presence of excess PPh3 yields OPPh3 in 173% yield; control experiments implicate 6, NO2(•), and free NO3(-) as the active oxidants. Notably, Ru2(chp)4Cl (3) is recovered after photolysis. Since 3 is the direct precursor to 4, the results described herein constitute the first example of a synthetic cycle for oxygen atom transfer that makes use of light to generate a putative metal oxo intermediate.

Redox-Active-Ligand-Mediated Formation of an Acyclic Trinuclear Ruthenium Complex with Bridging Nitrido Ligands

Coordination of a redox-active pyridine aminophenol ligand to Ru(II) followed by aerobic oxidation generates two diamagnetic Ru(III) species [1 a (cis) and 1 b (trans)] with ligand-centered radicals. The reaction of 1 a/1 b with excess NaN3 under inert atmosphere resulted in the formation of a rare bis(nitrido)-bridged trinuclear ruthenium complex with two nonlinear asymmetrical Ru-N-Ru fragments. The spontaneous reduction of the ligand centered radical in the parent 1 a/1 b supports the oxidation of a nitride (N(3-) ) to half an equivalent of N2 . The trinuclear omplex is reactive toward TEMPO-H, tin hydrides, thiols, and dihydrogen.

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.

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.