Synthesis and Reactivity of Phosphine-Stabilized Phosphoranimine Cations, [R₃P·PR′₂NSiMe₃]⁺

A series of phosphine-stabilized phosphoranimine cations [R3P·PR′2NSiMe3]+, which can be regarded as derivatives of the proposed transient reactive intermediate [PR′2NSiMe3]+ in the thermal condensation polymerization of phosphoranimines (R′′O)PR′2NSiMe3 to form poly(alkyl/arylphosphazenes) [PR′2N]n at 180−200 °C, have been prepared. The bromide salts [R3P·PR′2NSiMe3]Br [R′ = Me ([6]+), OCH2CF3 ([8]+); R3P = Me3P (a), Et3P (b), nBu3P (c), dmpm (d, dmpm = dimethylphosphinomethane), dmpe (e, dmpe = dimethylphosphinoethane)] were prepared from the direct reactions between BrMe2PNSiMe3 (5) and Br(CF3CH2O)2PNSiMe3 (7) and the corresponding tertiary phosphines R3P or the diphosphines Me2P(CH2)nPMe2 (n = 1, 2). Cations of the type [6]+ and [8]+, with electron-donating and -withdrawing groups at the phosphoranimine phosphorus center, respectively, undergo facile phosphine ligand substitution with the strong N-donor 4-dimethylaminopyridine (DMAP) to yield the corresponding DMAP-stabilized salts [DMAP·PR2NSiMe3]Br [R = Me ([9]+), OCH2CF3 ([10]+)]. Cations [6]+ with Br− anions are particularly labile: for example, [6a]Br slowly releases PMe3, BrSiMe3, and forms cyclic phosphazenes such as [Me2PN]4. Anion exchange reactions between the salts [6b]Br or [8c]Br and AgOTf (OTf = CF3SO3) quantitatively afforded the corresponding and more stable triflate salts [6b]OTf and [8c]OTf. Phosphine ligand abstraction reactions with B(C6F5)3 were observed for the bromide salts [6b]Br and [8c]Br, which regenerated the phosphoranimines 5 and 7, respectively, and formed the adduct R3P·B(C6F5)3. In contrast, the triflate salts [6b]OTf and [8c]OTf were unreactive under the same conditions. X-ray structural analysis of the P-donor stabilized cations revealed longer P−P and P−N bond lengths and smaller P−N−Si bond angles for cations [6]+ compared to analogs [8]+. These structural differences were rationalized using the negative hyperconjugation bonding model. In addition, the 1JPP coupling constants for the cations [6]+ observed by both solution and solid-state 31P NMR are remarkably small (13−25 Hz), whereas those for [8]+ are substantially larger and positive (276−324 Hz) and are as expected for P(IV)+−P(V) systems. DFT studies suggest that the significant difference in 1JPP couplings observed for [6]+ and [8]+ appears to be related to the electronegativity of the R′ substituents at the phosphoranimine phosphorus center rather than the strength of the donor−acceptor P−P bond, which is slightly weaker in [6]+ relative to that in [8]+, as indicated by the X-ray data and reactivity studies.