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Evidence for a S N 2‐Type Pathway for Phosphine Exchange in Phosphine–Phosphenium Cations, [R 2 PPR′ 3 ] +
Author(s) -
Slattery John M.,
Fish Cheryl,
Green Michael,
Hooper Thomas N.,
Jeffery John C.,
Kilby Richard J.,
Lynam Jason M.,
McGrady John E.,
Pantazis Dimitrios A.,
Russell Christopher A.,
Willans Charlotte E.
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200601742
Subject(s) - chemistry , phosphine , density functional theory , ion , crystallography , stereochemistry , medicinal chemistry , computational chemistry , catalysis , organic chemistry
Abstraction of a Cl − ion from the P ‐chlorophospholes, R 4 C 4 PCl (R=Me, Et), produced the PP bonded cations [R 4 C 4 PP(Cl)C 4 R 4 ] + , which reacted with PPh 3 to afford X‐ray crystallographically characterised phosphine–phosphenium cations [R 4 C 4 P(PPh 3 )] + (R=Me, Et). Examination of the 31 P‐{ 1 H} NMR spectrum of a solution (CH 2 Cl 2 ) of [Et 4 C 4 P(PPh 3 )] + and PPh 3 revealed broadening of the resonances due to both free and coordinated PPh 3 , and importantly it proved possible to measure the rate of exchange between PPh 3 and [Et 4 C 4 P(PPh 3 )] + by line shape analysis (gNMR programmes). The results established second‐order kinetics with Δ S ≠ =(−106.3±6.7) J mol −1 K −1 , Δ H ≠ =(14.9±1.6) kJ mol −1 and Δ G ≠ (298.15 K)=(46.6±2.6) kJ mol −1 , values consistent with a S N 2‐type pathway for the exchange process. This result contrasts with the dominant dissociative (S N 1‐type) pathway reported for the analogous exchange reactions of the [Ar NCH 2 CH 2 N(Ar)P (PMe 3 )] + ion, and to understand in more detail the factors controlling these two different reaction pathways, we have analysed the potential energy surfaces using density functional theory (DFT). The calculations reveal that, whilst phosphine exchange in [Et 4 C 4 P(PPh 3 )] + and [Ar NCH 2 CH 2 N(Ar)P (PMe 3 )] + is superficially similar, the two cations differ significantly in both their electronic and steric requirements. The high electrophilicity of the phosphorus center in [Et 4 C 4 P] + , combined with strong π–π interactions between the ring and the incoming and outgoing phenyl groups of PPh 3 , favours the S N 2‐type over the S N 1‐type pathway in [Et 4 C 4 P(PPh 3 )] + . Effective π‐donation from the amide groups reduces the intrinsic electrophilicity of [Ar NCH 2 CH 2 N(Ar)P ] + , which, when combined with the steric bulk of the aryl groups, shifts the mechanism in favour of a dissociative S N 1‐type pathway.