Synthesis and ligand modification chemistry of a molybdenum dinitrogen complex: redox and chemical activity of a bis(imino)pyridine ligand

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.

Activation of Ammonia by a Carbene-Stabilized Dithiolene Zwitterion

A carbene-stabilized dithiolene zwitterion (3) activates ammonia, affording 4^• and 5, through both single-electron transfer (SET) and hydrogen atom transfer (HAT). Reaction products were characterized spectroscopically and by single-crystal X-ray diffraction. The mechanism of the formation of 4^• and 5 was probed by experimental and computational methods. This discovery is the first example of metal-free ammonia activation via HAT.

A Parent Iron Amido Complex in Catalysis of Ammonia Oxidation

Parent amido complexes are crucial intermediates in ammonia-based transformations. We report a well-defined ferric ammine system [Cp*Fe(1,2-Ph₂PC₆H₄NH)(NH₃)]⁺ ([1-NH₃]⁺), which processes electrocatalytic ammonia oxidation to N₂ and H₂ at a mild potential. Through establishing elementary e^-/H⁺ conversions with the ferric ammine, a formal Fe(IV)-amido species, [1-NH₂]⁺, together with its conjugated Lewis acid, [1-NH₃]²⁺, was isolated and structurally characterized for the first time. Mechanism studies indicated that further oxidation of [1-NH₂]⁺ induces the reaction of the parent amido unit with NH₃. The formation of hydrazine is realized by the non-innocent nature of the phenylamido ligand that facilitates the concerted transfer of one proton and two electrons.

A thiolate-bridged ruthenium-molybdenum complex featuring terminal nitrido and bridging amido ligands derived from N−H and N−N bond cleavage of hydrazine

Biomimetic di- or multimetallic complexes featuring NxHy species in a sulfur-rich coordination sphere have attracted considerable attention in modelling the possible scenarios of biological nitrogen fixation by nitrogenases. Although the...

Fixation of Dinitrogen at an Asymmetric Binuclear Titanium Complex

A new type of dititanium dinitrogen complex supported by a triphenolamine (TPA) ligand is reported. Analysis by single-crystal X-ray diffraction and Raman and NMR spectroscopy reveals different coordination geometries for the two titanium centers. Hence, coordination of TPA and a nitrogen ligand results in trigonal-bipyramidal geometry, while an octahedral titanium center is obtained upon additional coordination of an ethoxide generated upon C-O bond cleavage in a diethyl ether solvent molecule. The titanium complex successfully generates ammonia in the presence of an excess amount of PCy₃HI and KC₈ in 154% yield (per titanium atom). A titanium complex with a bulkier TPA does not form a dinitrogen complex, and mononuclear titanium dinitrogen complexes were not accessible, presumably because of the high tendency of early transition metals to form binuclear dinitrogen complexes.

Synthesis of Unprecedented 4d/4f-Polypnictogens

A series of 4d/4f-polyarsenides, -polyarsines and -polystibines was obtained by reduction of the Mo-pnictide precursor complexes [{Cpt Mo(CO)2 }2 (μ,η2:2 -E2 )] (E=As, Sb; Cpt =tBu substituted cyclopentadienyl) with two different divalent samarocenes [Cp*2 Sm] and [(CpMe4nPr )2 Sm]. For the reductive conversion of the Mo-stibide only one product was isolated, featuring a planar tetrastibacyclobutadiene moiety as an unprecedented ligand for organometallic compounds. For the corresponding Mo-arsenide a tetraarsacyclobutadiene and a second species with a side-on coordinated As2 2- anion was isolated. The latter can be considered as reaction intermediate for the formation of the tetraarsacyclobutadiene.

Activation of ammonia and hydrazine by electron rich Fe(ii) complexes supported by a dianionic pentadentate ligand platform through a common terminal Fe(iii) amido intermediate

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 (3_Ar-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 (2_Ar), or scavenges hydrogen atoms to return to the ammonia complex (B₂Pz₄Py)Fe(ii)–NH₃ (1_Ar-NH₃). Interestingly, when (B₂Pz₄Py)Fe(ii) is reacted with NH₂NH₂, a hydrazine bridged dimer, (B₂Pz₄Py)Fe(ii)–NH₂NH₂–Fe(ii)(B₂Pz₄Py) ((1_Ar)₂-NH₂NH₂), is observed at −78 °C and converts to a fully characterized bridging diazene complex, 4_Ar, along with ammonia adduct 1_Ar-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 3_Ar-NH₂, which abstracts H˙ atoms from (1_Ar)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.

Oxidation of Ammonia with Molecular Complexes

Oxidation of ammonia by molecular complexes is a burgeoning area of research, with critical scientific challenges that must be addressed. A fundamental understanding of individual reaction steps is needed, particularly for cleavage of N-H bonds and formation of N-N bonds. This Perspective evaluates the challenges of designing molecular catalysts for oxidation of ammonia and highlights recent key contributions to realizing the goals of viable energy storage and retrieval based on the N-H bonds of ammonia in a carbon-free energy cycle.

Assembly and Redox-Rich Hydride Chemistry of an Asymmetric Mo2S2 Platform

Although molybdenum sulfide materials show promise as electrocatalysts for proton reduction, the hydrido species proposed as intermediates remain poorly characterized. We report herein the synthesis, reactions and spectroscopic properties of a molybdenum-hydride complex featuring an asymmetric Mo₂S₂ core. This molecule displays rich redox chemistry with electrochemical couples at E_½ = −0.45, −0.78 and −1.99 V vs. Fc/Fc⁺. The corresponding hydrido-complexes for all three redox levels were isolated and characterized crystallographically. Through an analysis of solid-state bond metrics and DFT calculations, we show that the electron-transfer processes for the two more positive couples are centered predominantly on the pyridinediimine supporting ligand, whereas for the most negative couple electron-transfer is mostly Mo-localized.

Activation of Dinitrogen by Polynuclear Metal Complexes

Activation of dinitrogen plays an important role in daily anthropogenic life, and the processes by which this fixation occurs have been a longstanding and significant research focus within the community. One of the major fields of dinitrogen activation research is the use of multimetallic compounds to reduce and/or activate N2 into a more useful nitrogen-atom source, such as ammonia. Here we report a comprehensive review of multimetallic-dinitrogen complexes and their utility toward N2 activation, beginning with the d-block metals from Group 4 to Group 11, then extending to Group 13 (which is exclusively populated by B complexes), and finally the rare-earth and actinide species. The review considers all polynuclear metal aggregates containing two or more metal centers in which dinitrogen is coordinated or activated (i.e., partial or complete cleavage of the N2 triple bond in the observed product). Our survey includes complexes in which mononuclear N2 complexes are used as building blocks to generate homo- or heteromultimetallic dinitrogen species, which allow one to evaluate the potential of heterometallic species for dinitrogen activation. We highlight some of the common trends throughout the periodic table, such as the differences between coordination modes as it relates to N2 activation and potential functionalization and the effect of polarizing the bridging N2 ligand by employing different metal ions of differing Lewis acidities. By providing this comprehensive treatment of polynuclear metal dinitrogen species, this Review aims to outline the past and provide potential future directions for continued research in this area.

Diversion of Catalytic C-N Bond Formation to Catalytic Oxidation of NH3 through Modification of the Hydrogen Atom Abstractor

We report that (TMP)Ru(NH₃)₂ (TMP = tetramesitylporphryin) is a molecular catalyst for oxidation of ammonia to dinitrogen. An aryloxy radical, tri-tert-butylphenoxyl (ArO·), abstracts H atoms from a bound ammonia ligand of (TMP)Ru(NH₃)₂, leading to the discovery of a new catalytic C-N coupling to the para position of ArO· to form 4-amino-2,4,6-tri-tert-butylcyclohexa-2,5-dien-1-one. Modification of the aryloxy radical to 2,6-di-tert-butyl-4-tritylphenoxyl radical, which contains a trityl group at the para position, prevents C-N coupling and diverts the reaction to catalytic oxidation of NH₃ to give N₂. We achieved 125 ± 5 turnovers at 22 °C for oxidation of NH₃, the highest turnover number (TON) reported to date for a molecular catalyst.

Dinitrogen activation by a penta-pyridyl molybdenum complex

A new dinitrogen (N₂) molybdenum(0) complex supported exclusively by pyridine ligands was synthesized.

Catalytic Ammonia Oxidation to Dinitrogen by Hydrogen Atom Abstraction

Catalysts for the oxidation of NH₃ are critical for the utilization of NH₃ as a large-scale energy carrier. Molecular catalysts capable of oxidizing NH₃ to N₂ are rare. This report describes the use of [Cp*Ru(P^tBu ₂ N^Ph ₂ )(¹⁵ NH₃ )][BAr^F ₄ ], (P^tBu ₂ N^Ph ₂ =1,5-di(phenylaza)-3,7-di(tert-butylphospha)cyclooctane; Ar^F =3,5-(CF₃ )₂ C₆ H₃ ), to catalytically oxidize NH₃ to dinitrogen under ambient conditions. The cleavage of six N-H bonds and the formation of an N≡N bond was achieved by coupling H⁺ and e^- transfers as net hydrogen atom abstraction (HAA) steps using the 2,4,6-tri-tert-butylphenoxyl radical (^t Bu₃ ArO^. ) as the H atom acceptor. Employing an excess of ^t Bu₃ ArO^. under 1 atm of NH₃ gas at 23 °C resulted in up to ten turnovers. Nitrogen isotopic (¹⁵ N) labeling studies provide initial mechanistic information suggesting a monometallic pathway during the N⋅⋅⋅N bond-forming step in the catalytic cycle.

Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex

Among the many metal-dinitrogen complexes synthesized, the end-on bridging (μ2, η1, η1-N2) coordination mode is notoriously unreactive for nitrogen fixation. This is principally due to the large activation energy for ground-state nitrogen-element bond formation and motivates exploration of the photoexcited reactivity of this coordination mode. To provide the foundation for this concept, the photophysics of a dinitrogen-bridged molybdenum complex was explored by ultrafast electronic spectroscopies. The complex absorbs light from the UV to near-IR, and the transitions are predominantly of metal-to-ligand charge transfer (MLCT) character. Five excitation wavelengths (440, 520, 610, 730, and 1150 nm) were employed to access MLCT bands, and the dynamics were probed between 430 and 1600 nm. Despite the large energy space occupied by electronic states (ca. 1.2 eV), the dynamics were independent of the excitation wavelength. In the proposed kinetic model, photoexcitation from a Mo-N═N-Mo centered ground state populates the π*-state delocalized over two terpyridine ligands. Due to a large terpyridine-terpyridine spatial separation, electronic localization occurs within 100 fs, augmented by symmetry breaking. The subsequent interplay of internal conversion and intersystem crossing (ISC) populates the lowest 3MLCT state in 2-3 ps. Decay to the ground state occurs either directly or via a thermally activated metal-centered (3MC) trap state having two time constants (10-15 ps, 23-26 ps [298 K]; 103 ps, 612 ps [77 K]). ISC between 1MLCT and 3MLCT involves migration of energized electron density from the terpyridine π* orbitals to the Mo-N═N-Mo core. Implication of the observed dynamics for the potential N-H bond forming reactivity are discussed.

Interconversion of Molybdenum Imido and Amido Complexes by Proton-Coupled Electron Transfer

Interconversion of the molybdenum amido [(^Ph Tpy)(PPh₂ Me)₂ Mo(NHtBuAr)][BArF²⁴ ] (^Ph Tpy=4'-Ph-2,2',6',2"-terpyridine; tBuAr=4-tert-butyl-C₆ H₄ ; ArF²⁴ =(C₆ H₃ -3,5-(CF₃ )₂ )₄ ) and imido [(^Ph Tpy)(PPh₂ Me)₂ Mo(NtBuAr)][BArF²⁴ ] complexes has been accomplished by proton-coupled electron transfer. The 2,4,6-tri-tert-butylphenoxyl radical was used as an oxidant and the non-classical ammine complex [(^Ph Tpy)(PPh₂ Me)₂ Mo(NH₃ )][BArF²⁴ ] as the reductant. The N-H bond dissociation free energy (BDFE) of the amido N-H bond formed and cleaved in the sequence was experimentally bracketed between 45.8 and 52.3 kcal mol^-1 , in agreement with a DFT-computed value of 48 kcal mol^-1 . The N-H BDFE in combination with electrochemical data eliminate proton transfer as the first step in the N-H bond-forming sequence and favor initial electron transfer or concerted pathways.

Scalable Syntheses of 4-Substituted Pyridine-Diimines

A concise benchtop and scalable synthesis of pyridine-diimine (PDI) ligand frameworks is presented using inexpensive commercial starting materials as opposed to previous syntheses of these ligands, which have confronted long and tedious routes that employ toxic and often difficult to scale pyrophoric reagents. The streamlined synthesis is derived from the facile delivery of 4-functionalized diacetylpyridines from a Minisci reaction using pyruvic acid, silver nitrate, and persulfate. As the PDI ligand scaffold has been adopted for a range of catalytic applications, the ability to modulate the electronic properties of the ligand with facility may be useful for optimizing a variety of catalytic transformations.

Ammonia Activation, H2 Evolution and Nitride Formation from a Molybdenum Complex with a Chemically and Redox Noninnocent Ligand

Treatment of the bis(imino)pyridine molybdenum η⁶-benzene complex (^iPrPDI)Mo(η⁶-C₆H₆) (^iPrPDI, 2,6-(2,6-iPr₂C₆H₃N═CMe)₂C₅H₃N) with NH₃ resulted in coordination induced haptotropic rearrangement of the arene to form (^iPrPDI)Mo(NH₃)₂(η²-C₆H₆). Analogous η²-ethylene and η²-cyclohexene complexes were also synthesized, and the latter was crystallographically characterized. All three compounds undergo loss of the η²-coordinated ligand followed by N-H bond activation, bis(imino)pyridine modification, and H₂ loss. A dual ammonia activation approach has been discovered whereby reversible M-L cooperativity and coordination induced bond weakening likely contribute to dihydrogen formation. Significantly, the weakened N-H bonds in (^iPrPDI)Mo(NH₃)₂(η²-C₂H₄) enabled hydrogen atom abstraction and synthesis of a terminal nitride from coordinated ammonia, a key step in NH₃ oxidation.

Stepwise N-H Bond Formation From N2-Derived Iron Nitride, Imide and Amide Intermediates to Ammonia

Reduction of N₂ to ammonia in nature and in electrocatalysis takes place through 1-proton/1-electron steps, motivating efforts to experimentally study the steps during proton/electron transfer to well-characterized N₂-derived species with bridging nitrides. We report here the protonation and reduction reactions of an N₂-derived iron bis(nitride) complex (Rodriguez et al., Science, 2011, 334, 780). We isolate and definitively characterize triiron imido and amido intermediates that lie along the path to ammonia formation, and Mössbauer spectroscopy shows the oxidation level of iron atoms in these mixed-valence clusters. The first two H atoms add to one of the two nitrides of the bis(nitride) complex, and the proton-coupled electron transfer in the second step can be concerted or stepwise depending on the sources of protons and electrons. The characterization of partially protonated nitrides and their mechanisms of formation are expected to guide efforts to convert N₂ to ammonia with mild acids.

RhCl(PPh3)3-mediated C–H oxyfunctionalization of pyrrolido-functionalized bisazoaromatic pincers: a combined experimental and theoretical scrutiny of redox-active and spectroscopic properties

Non-trivial coordination mode of symmetrical NNN ligands with Rh(iii) leads to redox-active NNO-scaffolds via C(sp²)–H oxyfunctionalization at rt, opening an opportunity to juxtapose different redox-active domains.

Nitrogen fixation catalyzed by ferrocene-substituted dinitrogen-bridged dimolybdenum-dinitrogen complexes: unique behavior of ferrocene moiety as redox active site

A series of dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing metallocene-substituted PNP-pincer ligands is synthesized by the reduction of the corresponding monomeric molybdenum-trichloride complexes under 1 atm of molecular dinitrogen. Introduction of ferrocene as a redox-active moiety to the pyridine ring of the PNP-pincer ligand increases the catalytic activity for the formation of ammonia from molecular dinitrogen, up to 45 equiv. of ammonia being formed based on the catalyst (22 equiv. of ammonia based on each molybdenum atom of the catalyst). The time profile for the catalytic reaction reveals that the presence of the ferrocene unit in the catalyst increases the rate of ammonia formation. Electrochemical measurement and theoretical studies indicate that an interaction between the Fe atom of the ferrocene moiety and the Mo atom in the catalyst may play an important role to achieve a high catalytic activity.

Recent progress in transition-metal-catalyzed reduction of molecular dinitrogen under ambient reaction conditions

This paper describes our recent progress in catalytic nitrogen fixation by using transition-metal-dinitrogen complexes as catalysts. Two reaction systems for the catalytic transformation of molecular dinitrogen into ammonia and its equivalent such as silylamine under ambient reaction conditions have been achieved by the molybdenum-, iron-, and cobalt-dinitrogen complexes as catalysts. Many new findings presented here may provide new access to the development of economical nitrogen fixation in place of the Haber-Bosch process.