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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.
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Demystifying Cp2Ti(H)Cl and its enigmatic role in the reactions of epoxides with Cp2TiCl
The role of Cp2Ti(H)Cl in the reactions of Cp2TiCl with trisubstituted epoxides has been investigated in a combined exptl. and computational study. Although Cp2Ti(H)Cl has generally been regarded as a robust species, its decompn. to Cp2TiCl and mol. hydrogen was found to be exothermic (ΔG = -11 kcal/mol when the effects of THF solvation are considered). In lab. studies, Cp2Ti(H)Cl was generated using the reaction of 1,2-epoxy-1-methylcyclohexane with Cp2TiCl as a model. Rapid evolution of hydrogen gas was measured, indicating that Cp2Ti(H)Cl is indeed a thermally unstable mol., which undergoes intermol. reductive elimination of hydrogen under the reaction conditions. The stoichiometry of the reaction (Cp2TiCl:epoxide = 1:1) and the quantity of hydrogen produced (1 mol per 2 mol of epoxide) is consistent with this assertion. The diminished yield of allylic alc. from these reactions under the conditions of protic vs. aprotic catalysis can be understood in terms of the predominant titanium(III) present in soln. Under the conditions of protic catalysis, Cp2TiCl complexes with collidine hydrochloride and the titanium(III) center is less available for "cross-disproportionation" with carbon-centered radicals; this leads to byproducts from radical capture by hydrogen atom transfer, resulting in a satd. alc. (2-methylcyclohexan-1-ol). [on SciFinder(R)] CAPLUS AN 2020:290383(Conference; Meeting Abstract; Online Computer File)
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- Award ID(s):
- 1900141
- PAR ID:
- 10279868
- Date Published:
- Journal Name:
- 259th ACS National Meeting & Exposition Abstract of Papers
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1021/acs.organomet.8b00793
