Evidence for a SN2-type pathway for phosphine exchange in phosphine-phosphenium cations, [R2P--PR'3]+

Abstraction of a Cl- ion from the P-chlorophospholes, R4C4PCl (R=Me, Et), produced the PP bonded cations [R4C4PP(Cl)C4R4]+, which reacted with PPh3 to afford X-ray crystallographically characterised phosphine-phosphenium cations [R4C4P(PPh3)]+ (R=Me, Et). Examination of the 31P-{1H} NMR spectrum of a solution (CH2Cl2) of [Et4C4P(PPh3)]+ and PPh3 revealed broadening of the resonances due to both free and coordinated PPh3, and importantly it proved possible to measure the rate of exchange between PPh3 and [Et4C4P(PPh3)]+ by line shape analysis (gNMR programmes). The results established second-order kinetics with S=(-106.3±6.7) J mol-1 K-1, H=(14.9±1.6) kJ mol-1 and G (298.15 K)=(46.6±2.6) kJ mol-1, values consistent with a SN2-type pathway for the exchange process. This result contrasts with the dominant dissociative (SN1-type) pathway reported for the analogous exchange reactions of the [Ar[bar NCH2CH2N(Ar)P ](PMe3)]+ 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 [Et4C4P(PPh3)]+ and [Ar[bar NCH2CH2N(Ar)P ](PMe3)]+ is superficially similar, the two cations differ significantly in both their electronic and steric requirements. The high electrophilicity of the phosphorus center in [Et4C4P]+, combined with strong - interactions between the ring and the incoming and outgoing phenyl groups of PPh3, favours the SN2-type over the SN1-type pathway in [Et4C4P(PPh3)]+. Effective -donation from the amide groups reduces the intrinsic electrophilicity of [Ar[bar NCH2CH2N(Ar)P ]]+, which, when combined with the steric bulk of the aryl groups, shifts the mechanism in favour of a dissociative SN1-type pathway.