Evidence for a S_N2‐Type Pathway for Phosphine Exchange in Phosphine–Phosphenium Cations, [R₂PPR′₃]⁺

Abstract

Abstraction of a Cl(-) ion from the P-chlorophospholes, R4C4PCl (R=Me, Et), produced the P--P bonded cations [R4C4P--P(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 (CH2Cl(2)) 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 DeltaS( not equal)=(-106.3+/-6.7) J mol(-1) K(-1), DeltaH( not equal)=(14.9+/-1.6) kJ mol(-1) and DeltaG( not equal) (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 (S(N)1-type) pathway reported for the analogous exchange reactions of the [ArNCH2CH2N(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 [ArNCH2CH2N(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 pi-pi 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 pi-donation from the amide groups reduces the intrinsic electrophilicity of [ArNCH2CH2N(Ar)P]+, which, when combined with the steric bulk of the aryl groups, shifts the mechanism in favour of a dissociative SN1-type pathway.

Dative Bonds in Main‐Group Compounds: A Case for Fewer Arrows!

“…PR' 3 ] + . [11][12][13][14][15] The Lewis acid R 2 P + , the basis for the dative bond description, is well defined and a known stable compound with substituents like N(iPr) 2 . [16] As such, the description is certainly bold but makes sense, and in the cited articles a balanced description with dative bond arrows and also as onium ions is used.…”

Structure–property-reactivity studies on dithiaphospholes

“…In this context Russell's work on the related phosphenium ion {(RC) 4 P} + led to isolation of the [{(RC) 4 P} 2 Cl] + cation. 42 Despite the uncertainty in the exact nature of the active species present in solution, it exhibits some Lewis acid character, based on in situ 31 P NMR data. For example addition of Ph 3 P afforded two doublets (δ = 52 ppm, 1 J PP = 444 Hz; δ = 8 ppm, 1 J PH = 444 Hz) whereas addition of Ph 3 As afforded a singlet at 47 ppm.…”

Phosphine complexes of lone pair bearing Lewis acceptors

“…For instance, displacement of the PPh 3 ligand from the phospholophosphonium cation [(PPh 3 )PC 4 Et 4 ] 1+ shows second-order kinetics consistent with an S N 2-type mechanism. 42 The most likely transition state is calculated to be only 18 kJ mol −1 above the starting materials, and features a tetracoordinate geometry at the acceptor centre, with two identical and mutually-trans PPh 3 interactions. In contrast, a dissociative pathway is evidenced in the reaction of PMe 3 with bis-(arylamino)phosphenium cations 43 and a computational analysis found no hypercoordinate transition state for these types of substitution reactions.…”