Abstract
Rhenium complexes with aliphatic PNP pincer ligands have been shown to be capable of reductive N2 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(NH2)2Cl]+, or results in dehydrohalogenation to the rhenium(iii) amido complex, (PNP)Re(NH2)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 NH3 generation.
Highlights
The interconversion of N2 and NH3 is important in elds that range from agriculture to sustainable energy.[1,2] The heavy use of NH3 in fertilizer manufacturing has resulted in an extensive global infrastructure for its transportation and storage.[3]
Rhenium complexes with aliphatic PNP pincer ligands have been shown to be capable of reductive N2 splitting to nitride complexes
Our analysis demonstrates that addition of the first H atom to the nitride complex is a thermochemical bottleneck for NH3 generation
Summary
The interconversion of N2 and NH3 is important in elds that range from agriculture to sustainable energy.[1,2] The heavy use of NH3 in fertilizer manufacturing has resulted in an extensive global infrastructure for its transportation and storage.[3]. Additional challenges are that the high energy of the lowest unoccupied molecular orbital (LUMO) of 2 prevents a reduction- rst pathway, and that 2 is unreactive towards organic hydrogenatom transfer (HAT) reagents or H2.59 In this manuscript, we evaluate the reverse reactions (Scheme 1, blue arrows) to elucidate the factors that prevent PCET nitride reduction in this system. This fundamental information may help to improve NH3 oxidation catalysis and to avoid bottlenecks in NH3 generation by future N2-cleaving systems, and importantly provides a thermochemical framework for nitrogen xation products beyond ammonia. The (PNP)–Re and Re–NH2 amide bond lengths in 3 are within 0.02 Aof each other with
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