172539-50-3Relevant academic research and scientific papers
Homogeneous models for hydrodenitrogenation catalysis
Weller, Keith J.,Fox, Peter A.,Gray, Steven D.,Wigley, David E.
, p. 3139 - 3163 (1997)
Hydrodenitrogenation (HDN) catalysis is the process of removing nitrogen from petroleum feed-stocks in the form of NH3 to provide more processable and environmentally compatible liquid fuels. In practice, HDN is carried out simultaneously with other catal
Mechanistic aspects of carbon-nitrogen bond cleavage in an η2(N,C)-pyridine complex: Intimate details of metal to ligand aryl migrations and their relevance to hydrodenitrogenation catalysis
Weller, Keith J.,Gray, Steven D.,Briggs, Paula M.,Wigley, David E.
, p. 5588 - 5597 (2008/10/09)
The reaction of the η2(N,C)-pyridine complex [η2(N,C)-2,4,6-NC5tBu3H 2]Ta(OAr)2Cl (1, Ar = 2,6-C6H3iPr2) with PhLi affords the phenyl complex [η2(N,C)-2,4,6-NC5tBu3H 2]Ta(OAr)2-Ph (2) in moderate yield. The para-substituted phenyl derivatives [η2(N,C)-2,4,6-NC5tBu3H 2]-Ta(OAr)2(4-C6H4X), where X = OMe (3), Me (4), Cl (5), and CF3 (6), are prepared similarly from 1 and the appropriate aryl Grignard or lithium reagent. Complexes 2-6 represent the kinetic products of this reaction since, upon their thermolysis, aryl migration from metal to ligand occurs and the ring-opened compounds Ta[=NCtBu=CHCtBu=CHCtBu(4-C6H 4X)]-(OAr)2, where X = H (7), OMe (8), Me (9), Cl (10), and CF3 (11), are formed by C-N bond scission. Kinetic and mechanistic studies of the [η2(N,C)-2,4,6-NC5tBu3H 2]Ta(OAr)2(4-C6H4X) → Ta[=NCtBu=CHCtBu=CHCtBu(4-C6H 4X)](OAr)2 rearrangement reveal that aryl migration follows clean first-order kinetics, implying an intramolecular, endo attack of the migrating aryl group on the η2(N,C)-pyridine ligand. An Eyring plot provides activation parameters for the [η2(N,C)-2,4,6-NC5tBu3H 2]Ta(OAr)2(4-C6H4OMe) (3) → Ta[=NCtBu=CHC-tBu=CHCtBu(4-C6H 4OMe)](OAr)2 (8) rearrangement of ΔH? = +19.2 ± 1.3 kcal mol-1 and ΔS? = -27 ± 4 cal K-1 mol-1, for a ΔG? = +27.3 ± 1.3 kcal mol-1 at 25°C, suggesting a highly ordered transition state leading to aryl transfer. Hammett considerations for the transformation of complexes 2-6 into 7-11 provide ρ = -0.58 ± 0.10 for the aryl migration reaction. While the sign of ρ is in agreement with an electrophilic aromatic substitution at the migrating phenyl ligand, the small value of ρ argues against this pathway and against the involvement of the aryl π framework during migration. These data support the direct attack of the formal [C6H4X]- σ bond on Cα of the η2(N,C)-pyridine ligand and C-N bond scission resulting from migration of the aryl group as a σ-nucleophile. Unlike their alkyl congeners, these ring-opened aryl derivatives Ta[=NCtBu=CHCtBu=CHCtBu(4-C6H 4X)]-(OAr)2 (7-11) are stable to subsequent rearrangement. The structure of Ta[=NCtBu=CHC-tBu=CHCtBuPh](OAr)2 (7) indicates a highly localized metallacycle with an imido nitrogen linkage characterized by a Ta-N-C angle of 145.3(3)° and Ta-N bond distance of 1.771(3) A. Although an endo attack of the phenyl group is evident from mechanistic studies, the solid-state structure intimates an exo addition, and a simple pathway that interconverts endo and exo isomers is described to account for this observation. The reactions and structures of this model system are discussed in terms of a fundamental mechanistic knowledge of C-N bond scission that is relevant to hydrodenitrogenation catalysis.
