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Porphyrin complexes are well-known in O 2 and CO 2 reduction, but their application to N 2 reduction is less developed. Here, we show that oxo and nitrido complexes of molybdenum supported by tetramesitylporphyrin (TMP) are effective precatalysts for catalytic N 2 reduction to ammonia, verified by 15 N 2 labeling studies and other control experiments. Spectroscopic and electrochemical studies illuminate some relevant thermodynamic parameters, including the N–H bond dissociation free energy of (TMP)MoNH (43 ± 2 kcal mol −1 ). We place these results in the context of other work on homogeneous N 2 reduction catalysis. 

Abstract Biological N₂reduction occurs at sulfur‐rich multiiron sites, and an interesting potential pathway is concerted double reduction/ protonation of bridging N₂through PCET. Here, we test the feasibility of using synthetic sulfur‐supported diiron complexes to mimic this pathway. Oxidative proton transfer from μ‐η¹ : η¹‐diazene (HN=NH) is the microscopic reverse of the proposed N₂fixation pathway, revealing the energetics of the process. Previously, Sellmann assigned the purple metastable product from two‐electron oxidation of [{Fe²⁺(PPr₃)L¹}₂(μ‐η¹ : η¹‐N₂H₂)] (L¹=tetradentate SSSS ligand) at −78 °C as [{Fe²⁺(PPr₃)L¹}₂(μ‐η¹ : η¹‐N₂)]²⁺, which would come from double PCET from diazene to sulfur atoms of the supporting ligands. Using resonance Raman, Mössbauer, NMR, and EPR spectroscopies in conjunction with DFT calculations, we show that the product is not an N₂complex. Instead, the data are most consistent with the spectroscopically observed species being the mononuclear iron(III) diazene complex [{Fe(PPr₃)L¹}(η²‐N₂H₂)]⁺. Calculations indicate that the proposed double PCET has a barrier that is too high for proton transfer at the reaction temperature. Also, PCET from the bridging diazene is highly exergonic as a result of the high Fe^3+/2+redox potential, indicating that the reverse N₂protonation would be too endergonic to proceed. This system establishes the “ground rules” for designing reversible N₂/N₂H₂interconversion through PCET, such as tuning the redox potentials of the metal sites. 

Rhenium complexes with aliphatic PNP pincer ligands have been shown to be capable of reductive N 2 splitting to nitride complexes. However, the conversion of the resulting nitride to ammonia has not been observed. Here, the thermodynamics and mechanism of the hypothetical N–H bond forming steps are evaluated through the reverse reaction, conversion of ammonia to the nitride complex. Depending on the conditions, treatment of a rhenium( iii ) precursor with ammonia gives either a bis(amine) complex [(PNP)Re(NH 2 ) 2 Cl] + , or results in dehydrohalogenation to the rhenium( iii ) amido complex, (PNP)Re(NH 2 )Cl. The N–H hydrogen atoms in this amido complex can be abstracted by PCET reagents which implies that they are quite weak. Calorimetric measurements show that the average bond dissociation enthalpy of the two amido N–H bonds is 57 kcal mol −1 , while DFT computations indicate a substantially weaker N–H bond of the putative rhenium( iv )-imide intermediate (BDE = 38 kcal mol −1 ). Our analysis demonstrates that addition of the first H atom to the nitride complex is a thermochemical bottleneck for NH 3 generation. 

Hydrogen atom transfer from a metal hydride (MHAT) has emerged as a powerful, if puzzling, technique in chemical synthesis. In catalytic MHAT reactions, earth-abundant metal complexes generate stabilized and unstabilized carbon-centered radicals from alkenes of various substitution patterns with robust chemoselectivity. This perspective combines organic and inorganic perspectives to outline challenges and opportunities, and to propose working models to assist further developments. We attempt to demystify the putative intermediates, the basic elementary steps, and the energetic implications, especially for cage pair formation, collapse and separation. Distinctions between catalysts with strong-field (SF) and weak-field (WF) ligand environments may explain some differences in reactivity and selectivity, and provide an organizing principle for kinetics that transcends the typical thermodynamic analysis. This blueprint should aid practitioners who hope to enter and expand this exciting area of chemistry. 

Kang et al . (Reports, 19 June 2020, p. 1381) report a structure of the nitrogenase MoFe protein that is interpreted to indicate binding of N 2 or an N 2 -derived species to the active-site FeMo cofactor. Independent refinement of the structure and consideration of biochemical evidence do not support this claim.