A Parent Iron Amido Complex in Catalysis of Ammonia Oxidation

Electrocatalytic Oxidation of Ammonia by (Salen)ruthenium(III) Ammine Complexes: Direct Evidence for a Ruthenium(VI) Nitrido Active Intermediate

The electrocatalytic oxidation of ammonia using molecular catalysts has attracted much attention recently due to its potential for fuel cell applications. In this study, we report the electrocatalytic ammonia oxidation (AO) by [RuIII(salchda)(NH3)(CH3CN)]+ (RuNH3, salchda = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion) and its bromo derivative. Controlled potential electrolysis at 0.65 V versus Fc+/0 for 3.2 h of a solution of RuNH3 and NH3 in CH3CN produced N2 with a TON of 26 and Faradaic efficiency (FE) close to 100%. The TON was increased to 79 and 147 when electrolysis was carried out at 0.7 and 0.80 V vs Fc+/0, respectively, with FE maintained at >99%, which are the highest among molecular ruthenium catalysts. An active intermediate was detected and shown to be the corresponding ruthenium(VI) nitrido complex [RuVI(salchda)(N)]+ (RuN) by direct comparison with an authentic sample of RuN, which we have previously synthesized and fully characterized. Direct kinetic studies on the oxidation of NH3 to N2 have also been carried out and the results are consistent with parallel electrophilic attack of NH3 by RuN and bimolecular N···N coupling of RuN to produce N2. DFT calculations have also been performed to support the proposed mechanism.

Understanding the factors governing the ammonia oxidation reaction by a mononuclear ruthenium complex

Precise regulation of the active site of molecular catalysts is appealing because it could provide insights into the catalytic mechanism and possibly provide a new strategy for catalyst design. A ruthenium complex, [Ru(dppMe, COMe)(bipy)(Cl)] (CSU-3), containing -Me and -COMe substituted dipyridylpyrrole as a pincer ligand, was designed and synthesized. The CSU-3 complex featured a Cl- ligand at the axial position as the active site for ammonia oxidation (AO), and is structurally analogous to AO catalyst [Ru(trpy)(dmabpy)(NH3)][PF6]2 (1) bearing a terpyridine ligand, but different from AO catalyst [Ru(dpp)(bipy)(NH3)] (CSU-2) containing unsubstituted dipyridylpyrrole as a hemilabile ligand with the active site at an equatorial position. To gain insight into the role of active-site and ligand regulation in the AO reaction, the structure and electrochemical properties of CSU-3 and its catalytic performance and mechanism for the AO reaction were comparably studied. Complex CSU-3 has good selective catalytic performance for the oxidation of ammonia to hydrazine with a turnover frequency (TOF) of 258.8 h-1 and N2H4 formation selectivity of 84.7% at E app of 1.0 V. The DFT calculations reveal that N2H4 as a dominant product is generated via an ammonia nucleophilic attack of ruthenium(iv)-imide to form N2H4 followed by N2H4-by-NH3 substitution.

Electrocatalytic Ammonia Oxidation by a Ruthenium Complex Bearing a 2,6-Pyridinedicarboxylate Ligand

Molecular catalysts for the electrocatalytic ammonia oxidation reaction (eAOR) have much to offer in terms of mechanistic investigations and practical energy issues. This work reports the use of complex [Ru(pdc-κ-N1O2)(bpy)(NH3)] (Ru-NH 3 ) (H2pdc = 2, 6-pyridinedicarboxylic acid; bpy = 2,2'-bipyridine) bearing a readily accessible pdc2- ligand to catalyze ammonia oxidation under electrochemical conditions. The rich structural variations of Ru-NH 3 in coordinating solvents and an ammonia atmosphere were fully characterized by cyclic voltammograms (CVs), NMR, and XRD. CV experiments showed that Ru-NH 3 promotes electrocatalytic ammonia oxidation at a low overpotential of 0.85 V with a calculated catalytic rate (k obs) of 18.9 s-1. Controlled potential electrolysis (CPE) at an applied potential of 0.3 V vs Fc+/0 achieves 76.1 equiv of N2 with a faradaic efficiency of 89.8%. Experimental and computational analyses indicated that oxidation of Ru-NH 3 generates a reactive Ru III -NH 3 intermediate, which undergoes sequential electron and proton transfer steps to form a Ru VI ≡N species. N-N bond formation occurs via the nucleophilic attack of an ammonia molecule on the Ru VI ≡N moiety with a facile barrier of 8.6 kcal/mol. Eventually, N2 evolved as the product after releasing two electrons and three protons.

Mapping the Catalytic Cycle of Ru-Catalyzed Ammonia Oxidation

This study presents a mechanism of ammonia oxidation catalyzed by [RuII(NH3)][PF6]2 (Ru=Ru(tpy)(dmabpy)) complexes. All intermediates, including the elusive [RuIII(NH2)]2+ and [RuIV(NH)]2+, were detected, with several intermediates characterized spectroscopically. The catalytic cycle follows multiple reaction pathways branching at the key intermediates [RuIII(NH2)]2+ and [RuII(N2H4)]2+. The competition between the pathways is influenced by ammonia concentration and the local proton concentration generated during electrooxidation at the anode. In the N-N bond-forming steps, dimerization of [RuIII(NH2)]2+ and nucleophilic attack of [RuIV(NH)]2+ by ammonia compete, while in the subsequent [RuII(N2H4)]2+ oxidation, direct oxidation competes with proton-catalyzed disproportionation. These findings provide molecular-level insights into the catalytic cycle and offer guidance for developing more efficient ruthenium-based ammonia oxidation catalysts.

Electrocatalytic Ammonia Oxidation by Pyridyl-Substituted Ferrocenes

Ammonia (NH3) is a promising carbon-free fuel when prepared from sustainable resources. First-row transition metal electrocatalysts for ammonia oxidation are an enabling technology for sustainable energy production. We describe electrocatalytic ammonia oxidation using robust molecular complexes based on Earth-abundant iron. Electrochemical studies of ferrocenes with covalently attached pyridine arms reveal facile ammonia oxidation in DMSO (2.4 M NH3) with modest overpotentials (η = 770-820 mV) and turnover frequencies (125-560 h-1). Experimental and computational studies indicate that the pendant pyridyl base serves as an H-bond acceptor with an N-H bond of ammonia that transfers a proton to the pyridine following oxidation by the attached ferrocenium moiety in a proton-coupled electron transfer (PCET) step. This generates an amidyl (•NH2) radical stabilized via H-bonding to a pendant pyridinium moiety that rapidly dimerizes to hydrazine (H2N-NH2), which is easily oxidized to nitrogen (N2) at the glassy carbon working electrode. This report identifies a general strategy to oxidize ammonia via H-bonding to a base (B:), thereby activating [B···H-NH2] toward PCET by a proximal oxidant to form [BH···NH2]+/• radical cations, which are susceptible to dimerization to form easily oxidized hydrazine.

pH-Dependent Electrocatalytic Aqueous Ammonia Oxidation to Nitrite and Nitrate by a Copper(II) Complex with an Oxidation-Resistant Ligand

The electrocatalytic aqueous ammonia oxidation (AO) represents a more sustainable alternative to accessing nitrite (NO2-) and nitrate (NO3-). We now report that Cu(pyalk)2 {pyalk = 2-(pyridin-2-yl)propan-2-oate}, previously employed as a homogeneous water oxidation (WO) catalyst, is also active for selective AO in aqueous environments. The traditional Griess analytical test for NO2-/NO3- was modified to permit the operation in the presence of the otherwise interfering Cu2+ ion. Choosing the right pH is crucial for achieving high AO selectivity, with optimal formation of NO2- occurring at pH 9 (faradaic efficiency 62%). Electrochemical analysis reveals a monometallic reaction pathway and offers a plausible explanation for the chemoselectivity: at pH 9, AO is dominant, while at elevated pH 13, WO dominates.

Selective Electrocatalytic Oxidation of Ammonia by Ru-dpp Complexes Containing Aromatic Nitrogen Donor as Axial Ligand

Inspired by the rapid growth of Ru-based complexes as molecular ammonia oxidation catalysts, we propose novel Ru-dpp complexes bearing a nitrogen donor as the axial ligand into the ammonia oxidation catalysts family. Herein, a series of Ru-dpp complexes [Ru(K3-N,N',N″-dpp)(bpy)(L)]·PF6 (where Hdpp = 2-[5-(pyridin-2-yl)-1H-pyrrol-2-yl]pyridine; bpy = 2,2'-bipyridine; L = pyridine (Ru-py); 4-methylpyridine (Ru-pic); pyrimidine (Ru-pmd); isoquinoline (Ru-isoq)) containing aromatic nitrogen donor axial ligand are synthesized and fully characterized by NMR, IR, and ESI-MS. The structural analysis displays that dpp- as a pincer ligand coordinates to ruthenium, and nitrogen donor L binds to ruthenium at an axial ligand. The reaction of Ru-L with ammonia generates ammonia-ligated Ru-L-NH3 complexes, which are monitored by NMR and UV-vis spectra. The electrochemical properties are studied by cyclic voltammetry and density functional theory calculation. These titled complexes have good electrocatalytic performances for selective oxidation of ammonia into hydrazine at 0.6 V versus Cp2Fe+/0 onset potential. The turnover frequency of N2H4 formation is 326.4-393.4 h-1, and the faraday efficiency of generating hydrazine exceeds 97%. The N-N formation mechanism via bimolecular coupling of ruthenium amide/imide is proposed. The aminyl character of ruthenium amide intermediate is confirmed by EPR spectra.

Electrodegradation of nitrogenous pollutants in sewage: from reaction fundamentals to energy valorization applications

The excessive accumulation of nitrogen pollutants (mainly nitrate, nitrite, ammonia nitrogen, hydrazine, and urea) in water bodies seriously disrupts the natural nitrogen cycle and poses a significant threat to human life and health. Electrolysis is considered a promising method to degrade these nitrogenous pollutants in sewage, with the advantages of high efficiency, wide generality, easy operability, retrievability, and environmental friendliness. For particular energy devices, including metal-nitrate batteries, direct fuel cells, and hybrid water electrolyzers, the realization of energy valorization from sewage purification processes (e.g., valuable chemical generation, electricity output, and hydrogen production) becomes feasible. Despite the progress in the research on pollutant electrodegradation, the development of electrocatalysts with high activity, stability, and selectivity for pollutant removal, coupled with corresponding energy devices, remains a challenge. This review comprehensively provides advanced insights into the electrodegradation processes of nitrogenous pollutants and relevant energy valorization strategies, focusing on the reaction mechanisms, activity descriptors, electrocatalyst design, and actuated electrodes and operation parameters of tailored energy conversion devices. A feasibility analysis of electrodegradation on real wastewater samples from the perspective of pollutant concentration, pollutant accumulation, and electrolyte effects is provided. Challenges and prospects for the future development of electrodegradation systems are also discussed in detail to bridge the gap between experimental trials and commercial applications.

Oxidation of Ammonia Catalyzed by a Molecular Iron Complex: Translating Chemical Catalysis to Mediated Electrocatalysis

Ammonia is a promising candidate in the quest for sustainable, clean energy. With its capacity to serve as an energy carrier, the oxidation of ammonia opens avenues for carbon-neutral approaches to address worldwide growing energy needs. We report the catalytic chemical oxidation of ammonia by an Earth-abundant transition metal complex, trans-[LFeII(MeCN)2][PF6]2, where L is a macrocyclic ligand bearing four N-heterocyclic carbene (NHC) donors. Using triarylaminium radical cations in MeCN, up to 182 turnovers of N2 per Fe were obtained from chemical catalysis with an extremely low loading of the Fe catalyst (0.043 mM, 0.004 mol % catalyst). This chemical catalysis was successfully transitioned to mediated electrocatalysis for the oxidation of ammonia. Molecular electrocatalysis by the Fe catalyst and the mediator (p-MeOC6H4)3N exhibited a catalytic half-wave potential (Ecat/2) of 0.18 V vs [Cp2Fe]+/0 in MeCN, and achieved 9.3 turnovers of N2 at an applied potential of 0.20 V vs [Cp2Fe]+/0 at -20 °C in controlled-potential electrolysis, with a Faradaic efficiency of 75 %. Based on computational results, the catalyst undergoes sequential oxidation and deprotonation steps to form [LFeIV(NH2)2]2+, and thereafter bimetallic coupling to form an N-N bond.

Dual Pathways in Catalytic Ammonia Oxidation by a Ruthenium Complex Bearing a Tetradentate Bipyridine-Bipyrazole Ligand: Isolation of a Diruthenium Intermediate with a μ-Hexazene Derivative

We report herein chemical and electrochemical ammonia oxidation (AO) catalyzed by a Ru complex, [RuII(H2L)(pic)2]2+ [1, H2L = 6,6'-di(1H-pyrazol-3-yl)-2,2'-bipyridine, pic = 4-picoline], where H2L is a tetradentate ligand with a bipyridyl unit connected to two pyrazoles. 1 functions as an efficient electrocatalyst for the oxidation of NH3 to N2, with a low overpotential of 0.51 V vs Fc+/0 and a Faradaic efficiency of 96%. 1 also undergoes catalytic chemical AO using (4-BrPh)3N•+ as an oxidant, with a turnover number for N2 reaching 41. A novel binuclear complex, [RuIII(L)(pic)2(N2)RuIII(L)(pic)2]4+ (2), was isolated and structurally characterized in the catalytic chemical AO by 1. Complex 2 possesses a zigzag dianionic μ-hexazene unit where the N2 derived from ammonia oxidation is bonded to the pyrazoles, with an NN2-NN2 bond length of 1.3091(70) Å. 2 readily releases N2 upon treating with NH3. Based on experimental and DFT studies, in chemical AO the formation of an N-N bond is proposed to occur via bimolecular coupling of a ruthenium pyrazole imido intermediate to give 2; while in electrochemical AO the N-N bond is formed by nucleophilic attack of NH3 on the 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.

Oxidation-induced ambiphilicity triggers N-N bond formation and dinitrogen release in octahedral terminal molybdenum(v) nitrido complexes

Coupling of octahedral, terminal d1 molybdenum(v) nitrido complexes supported by a dianionic pentadentate ligand via N-N bond formation to give μ-dinitrogen complexes was found to be thermodynamically feasible but faces significant kinetic barriers. However, upon oxidation, a kinetically favored nucleophilic/electrophilic N-N bond forming mechanism was enabled to give monocationic μ-dinitrogen dimers. Computational and experimental evidence for this "oxidation-induced ambiphilic nitrido coupling" mechanism is presented. The factors influencing release of dinitrogen from the resulting μ-dinitrogen dimers were also probed and it was found that further oxidation to a dicationic species is required to induce (very rapid) loss of dinitrogen. The mechanistic path discovered for N-N bond formation and dinitrogen release follows an ECECC sequence (E = "electrochemical step"; C = "chemical step"). Experimental evidence for the intermediacy of a highly electrophilic, cationic d0 molybdenum(vi) nitrido in the N-N bond forming mechanism via trapping with an isonitrile reagent is also discussed. Together these results are relevant to the development of molecular catalysts capable of mediating ammonia oxidation to dihydrogen and dinitrogen.

DFT Study of N-modified Co3Mo3C Electrocatalyst with Separated Active Sites for Enhanced Ammonia Oxidation

Since the facile oxidation of ammonia is one key for its utilization as a zero-carbon fuel in a direct ammonia fuel cell, developing the ammonia oxidation reaction (AOR) catalysts with cost-effective and higher activity is urgently required. However, the catalytic activity of AOR is limited by the scaling relationship of the intermediate adsorption. Based on the density functional theory, the N-modified Co3Mo3C with separated active sites of NH3 dehydrogenation and N-N coupling has been designed and investigated, which is a promising strategy to circumvent the scaling relationship, achieving improved AOR catalytic performance with a lower theoretical overpotential of 0.59 V under fast reaction kinetics condition. The calculation results show that the hollow site (Co-Mo-Mo and Co-Co-Mo) and Co site in N-modified Co3Mo3C play essential roles in NH3 dehydrogenation and N-N coupling, respectively. This work not only benefits for understanding the mechanism of AOR, but also provides a fundamental guidance for rational design of AOR catalysts.

Synthesis and Reactivity of Fe(II) Complexes Containing Cis Ammonia Ligands

The development of earth-abundant transition-metal complexes for electrocatalytic ammonia oxidation is needed to facilitate a renewable energy economy. Important to this goal is a fundamental understanding of how ammonia binds to complexes as a function of ligand geometry and electronic effects. We report the synthesis and characterization of a series of Fe(II)-NH3 complexes supported by tetradentate, facially binding ligands with a combination of pyridine and N-heterocyclic carbene donors. Electronic modification of the supporting ligand led to significant shifts in the FeIII/II potential and variations in NH bond acidities. Finally, investigations of ammonia oxidation by cyclic voltammetry, controlled potential bulk electrolysis, and through addition of stoichiometric organic radicals, TEMPO and tBu3ArO• are reported. No catalytic oxidation of NH3 to N2 was observed, and 15N2 was detected only in reactions with tBu3ArO•.

Electrocatalytic Ammonia Oxidation with a Tailored Molecular Catalyst Heterogenized via Surface Host-Guest Complexation

Macrocyclic host molecules bound to electrode surfaces enable the complexation of catalytically active guests for molecular heterogeneous catalysis. We present a surface-anchored host-guest complex with the ability to electrochemically oxidize ammonia in both organic and aqueous solutions. With an adamantyl motif as the binding group on the backbone of the molecular catalyst [Ru(bpy-NMe2)(tpada)(Cl)](PF6) (1) (where bpy-NMe2 is 4,4'-bis(dimethylamino)-2,2'-bipyridyl and tpada is 4'-(adamantan-1-yl)-2,2':6',2″-terpyridine), high binding constants with β-cyclodextrin were observed in solution (in DMSO-d6:D2O (7:3), K11 = 492 ± 21 M-1). The strong binding affinities were also transferred to a mesoporous ITO (mITO) surface functionalized with a phosphonated derivative of β-cyclodextrin. The newly designed catalyst (1) was compared to the previously reported naphthyl-substituted catalyst [Ru(bpy-NMe2)(tpnp)(Cl)](PF6) (2) (where tpnp is 4'-(naphthalene-2-yl)-2,2':6',2″-terpyridine) for its stability during catalysis. Despite the insulating nature of the adamantyl substituent serving as the binding group, the stronger binding of this unit to the host-functionalized electrode and the resulting shorter distance between the catalytic active center and the surface led to better performance and higher stability. Both guests are able to oxidize ammonia in both organic and aqueous solutions, and the host-anchored electrode can be refunctionalized multiple times (>3) following the loss of the catalytic activity, without a reduction in performance. Guest 1 exhibits significantly higher stability in comparison to guest 2 toward basic conditions, which often constitutes a challenge for anchored molecular systems. Ammonia oxidation in water led to the selective formation of NO3- with Faradaic efficiencies of up to 100%.

Deciphering the Mechanism of Base-Triggered Conversion of Ammonia to Molecular Nitrogen and Methylamine to Cyanide

We report an in-depth investigation into the ammonia oxidation mechanism by the catalyst [RuIII(tpy)(dmabpy)NH3]3+ ([Ru(NH3)]3+). Stoichiometric reactions of [Ru(NH3)]3+ were carried out with exogenous noncoordinating bases to trigger a proposed redox disproportionation reaction, which was followed using variable-temperature NMR spectroscopy. An intermediate species was identified as a dinitrogen-bridged complex using 15N NMR and Raman spectroscopy on isotopically labeled complexes. This intermediate is proposed to derive from coupling of nitridyl species formed upon sequential redox disproportion reactions. Acetonitrile displaces the dinitrogen bridge to yield free N2. DFT calculations support this lower-energy pathway versus that previously reported for ammonia oxidation by the parent [RuIII(tpy)(bpy)NH3]3+ complex. These experimental and computational results are consistent with the interpretation of redox disproportionation involving sequential hydrogen atom transfer reactions by an amide/aminyl intermediate, [Ru(NH2)-]+ ⇔ [Ru(NH2)•]+, formed upon deprotonation of the parent complex. Control experiments employing a large excess of ammonia as a base indicate this new proposed lower-energy pathway contributes to the oxidation of ammonia to dinitrogen in conditions relevant to electrocatalysis. In addition, analogous methylamine complexes, [Ru(NH2CH3)]2+/3+, were prepared to further test the proposed mechanism. Treating [Ru(NH2CH3)]3+ with a base cleanly yields two products [Ru(NH2CH3)]2+ and [Ru(CN)]+ in an ∼3:1 ratio, fully consistent with the proposed cascade of hydrogen atom transfer reactions by an intermediate.

Electrocatalytic Interconversions of CO2 and Formate on a Versatile Iron-Thiolate Platform

Exploring bidirectional CO2/HCO2- catalysis holds significant potential in constructing integrated (photo)electrochemical formate fuel cells for energy storage and applications. Herein, we report selective CO2/HCO2- electrochemical interconversion by exploiting the flexible coordination modes and rich redox properties of a versatile iron-thiolate platform, Cp*Fe(II)L (L = 1,2-Ph2PC6H4S-). Upon oxidation, this iron complex undergoes formate binding to generate a diferric formate complex, [(L-)2Fe(III)(μ-HCO2)Fe(III)]+, which exhibits remarkable electrocatalytic performance for the HCO2--to-CO2 transformation with a maximum turnover frequency (TOFmax) ∼103 s-1 and a Faraday efficiency (FE) ∼92(±4)%. Conversely, this iron system also allows for reduction at -1.85 V (vs Fc+/0) and exhibits an impressive FE ∼93 (±3)% for the CO2-to-HCO2- conversion. Mechanism studies revealed that the HCO2--to-CO2 electrocatalysis passes through dicationic [(L2)-•Fe(III)(μ-HCO2)Fe(III)]2+ generated by unconventional oxidation of the diferric formate species taking place at ligand L, while the CO2-to-HCO2- reduction involves a critical intermediate of [Fe(II)-H]- that was independently synthesized and structurally characterized.

Transformation of Acetylene to Ethenylidene, Carbene, Acetylide, Vinyl, and Olefin Groups with Cp*Fe(1,2-Cy2PC6H4S)

Tautomerization of C2H2 at half-sandwich compound Cp*Fe(1,2-Cy2PC6H4S) exclusively produces an iron ethenylidene, Cp*Fe(=C=CH2)(1,2-Cy2PC6H4S) (2). Protonation of the ethenylidene causes nucleophilic attack of the Cα by sulfur, affording a sulfur-tethered carbene complex, [Cp*Fe=C(CH3)SC6H4PCy2]+ (3+). This Fischer-type carbene complex undergoes an unusual isomerization by migrating a hydrogen atom from the β-CH3 group to the α-C, leading to the formation of an olefin complex [Cp*Fe(η4-CH=CH2SC6H4PCy2]+ (4+). Compound 2 also displays diverse redox reactivities. It transforms to a neutral acetylide ferric complex (5) when reacting with free radical scavengers and to a cationic vinyl complex [Cp*Fe(η3-C(=CH2)SC6H4PCy2]+ (6+) upon 1e- oxidation. The interconversion between the vinyl and acetylide complexes can be realized through protonation/deprotonation reactions.

Improving Molecular Iron Ammonia Oxidation Electrocatalysts via Substituent Effects That Modulate Standard Potential and Stability

Molecular ammonia oxidation (AO) catalysis is a rapidly evolving research area. Among the catalysts studied, featuring metals including ruthenium, iron, manganese, nickel, and copper, polypyridyl iron complexes are attractive owing to fast catalytic rates and significant turnover numbers (TON). Building upon our previous work on AO using [(TPA)Fe(MeCN)2]2+ and [(BPM)Fe(MeCN)2]2+, this study investigates factors that impact rate and TON within and across catalyst series based on polypyridyl ligand frameworks. The synthesis and analysis of derivatives functionalized in the 4-pyridyl position with electron-donating and electron-withdrawing groups (NMe2, OMe, CF3) are described; a combination of electroanalytical, UV-vis, and NMR analyses provide insights into the relative importance of catalyst standard potential (E°) and 4-pyridyl substituent to rate and stability. These findings constrain hypotheses rationalizing the nature of improved catalysis comparing two classes of polypyridyl ligands for [(Laux)Fe(MeCN)2]2+ species, and help define a roadmap for future catalyst development. For the most active catalyst studied herein, [(BPMOMe)Fe(MeCN)2]2+, a TON of 381 is demonstrated after 48 h of sustained catalysis.

Heterogeneous Electrochemical Ammonia Oxidation with a Ru-bda Oligomer Anchored on Graphitic Electrodes via CH-π Interactions

Molecular catalysts can promote ammonia oxidation, providing mechanistic insights into the electrochemical N₂ cycle for a carbon-free fuel economy. We report the ammonia oxidation activity of carbon anodes functionalized with the oligomer {[Ru^II(bda-κ-N ² O ²)(4,4'-bpy)]₁₀(4,4'-bpy)}, Rubda-10, where bda is [2,2'-bipyridine]-6,6'-dicarboxylate and 4,4'-bpy is 4,4'-bipyridine. Electrocatalytic studies in propylene carbonate demonstrate that the Ru-based hybrid anode used in a 3-electrode configuration transforms NH₃ to N₂ and H₂ in a 1:3 ratio with near-unity faradaic efficiency at an applied potential of 0.1 V vs Fc^+/0, reaching turnover numbers of 7500. X-ray absorption spectroscopic analysis after bulk electrolysis confirms the molecular integrity of the catalyst. Based on computational studies together with electrochemical evidence, ammonia nucleophilic attack is proposed as the primary pathway that leads to critical N-N bond formation.

Catalytic Ammonia Oxidation to Dinitrogen by a Nickel Complex

We report a nickel complex for catalytic oxidation of ammonia to dinitrogen under ambient conditions. Using the aryloxyl radical 2,4,6-tri-tert-butylphenoxyl (^t Bu₃ ArO⋅) as a H atom acceptor to cleave the N-H bond of a coordinated NH₃ ligand up to 56 equiv of N₂ per Ni center can be generated. Employing the N-oxyl radical 2,2,6,6-(tetramethylpiperidin-1-yl)oxyl (TEMPO⋅) as the H-atom acceptor, up to 15 equiv of N₂ per Ni center are formed. A bridging Ni-hydrazine product identified by isotopic nitrogen (¹⁵ N) studies and supported by computational models indicates the N-N bond forming step occurs by bimetallic homocoupling of two paramagnetic [Ni]-NH₂ fragments. Ni-mediated hydrazine disproportionation to N₂ and NH₃ completes the catalytic cycle.

Hydrogen Atom Abstraction from an OsII(NH3)2 Complex Generates an OsIV(NH2)2 Complex: Experimental and Computational Analysis of the N-H Bond Dissociation Free Energies and Reactivity

Double hydrogen atom abstraction from (TMP)Os^II(NH₃)₂ (TMP = tetramesitylporphyrin) with phenoxyl or nitroxyl radicals leads to (TMP)Os^IV(NH₂)₂. This unusual bis(amide) complex is diamagnetic and displays an N-H resonance at 12.0 ppm in its ¹H NMR spectrum. ¹H-¹⁵N correlation experiments identified a ¹⁵N NMR spectroscopic resonance signal at -267 ppm. Experimental reactivity studies and density functional theory calculations support relatively weak N-H bonds of 73.3 kcal/mol for (TMP)Os^II(NH₃)₂ and 74.2 kcal/mol for (TMP)Os^III(NH₃)(NH₂). Cyclic voltammetry experiments provide an estimate of the pK_a of [(TMP)Os^III(NH₃)₂]⁺. In the presence of Barton's base, a current enhancement is observed at the Os(III/II) couple, consistent with an ECE event. Spectroscopic experiments confirmed (TMP)Os^IV(NH₂)₂ as the product of bulk electrolysis. Double hydrogen atom abstraction is influenced by π donation from the amides of (TMP)Os^IV(NH₂)₂ into the d orbitals of the Os center, favoring the formation of (TMP)Os^IV(NH₂)₂ over N-N coupling. This π donation leads to a Jahn-Teller distortion that splits the energy levels of the d_xz and d_yz orbitals of Os, results in a low-spin electron configuration, and leads to minimal aminyl character on the N atoms, rendering (TMP)Os^IV(NH₂)₂ unreactive toward amide-amide coupling.

Spontaneous Ammonia Activation Through Coordination-Induced Bond Weakening in Molybdenum Complexes of a Dianionic Pentadentate Ligand Platform

Ammonia oxidation catalyzed by molecular compounds is of current interest as a carbon-free source of dihydrogen. Activation of N-H bonds through coordination to transition metal centers is a key reaction in this process. We report the substantial activation of ammonia via reaction with low-valent molybdenum complexes of a diborate pentadentate ligand system. Spontaneous loss of dihydrogen from (B₂ Pz₄ Py)Mo^II -NH₃ at room temperature to produce the dinuclear μ-nitrido compound (B₂ Pz₄ Py)Mo-N-Mo(B₂ Pz₄ Py) is observed due to substantial N-H bond weakening upon coordination to Mo. Mechanistic details are supported through the experimental observation/characterization of terminal amido, imido and nitrido complexes and density functional theory computations. The generally under-appreciated role of bridging nitrido intermediates is revealed and discussed, providing guidance for further catalyst development for this process.