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.一系列经膦稳定的膦亚胺阳离子（phosphoranimine cation）[R₃P·PR'₂=NSiMe₃]⁺，可被视为膦亚胺(R''O)PR'₂=NSiMe₃在180~200℃下热缩聚生成聚(烷基/芳基磷腈) [PR'₂=N]ₙ时，所提出的瞬态活性中间体[PR'₂=NSiMe₃]⁺的衍生物，该类阳离子已被成功合成。该类阳离子对应的溴化物盐[R₃P·PR'₂=NSiMe₃]Br [R'=Me（记为[6]⁺）、OCH₂CF₃（记为[8]⁺）；R₃P=Me₃P (a)、Et₃P (b)、nBu₃P (c)、dmpm (d，dmpm=二甲基膦甲烷（dimethylphosphinomethane）)、dmpe (e，dmpe=二甲基膦乙烷（dimethylphosphinoethane）)]，可通过BrMe₂P=NSiMe₃ (5)与Br(CF₃CH₂O)₂P=NSiMe₃ (7)分别与相应的叔膦R₃P或双膦Me₂P(CH₂)ₙPMe₂（n=1,2）直接反应制得。带有分别位于膦亚胺磷中心的给电子与吸电子基团的[6]⁺和[8]⁺型阳离子，可与强N给体4-二甲氨基吡啶（4-dimethylaminopyridine, DMAP）发生易发生的膦配体取代反应，生成对应的DMAP稳定盐[DMAP·PR₂=NSiMe₃]Br [R=Me（记为[9]⁺）、OCH₂CF₃（记为[10]⁺）]。携带Br⁻阴离子的[6]⁺型阳离子尤为不稳定：例如[6a]Br会缓慢释放PMe₃、BrSiMe₃，并生成环状磷腈如[Me₂P=N]₄。溴化物盐[6b]Br或[8c]Br与三氟甲磺酰银（AgOTf，OTf=CF₃SO₃）发生阴离子交换反应，可定量得到相应的更稳定的三氟甲磺酸盐[6b]OTf与[8c]OTf。溴化物盐[6b]Br和[8c]Br可与B(C₆F₅)₃发生膦配体抽提反应，分别再生膦亚胺5与7，并生成加合物R₃P·B(C₆F₅)₃；与之相反，三氟甲磺酸盐[6b]OTf与[8c]OTf在相同条件下无反应性。对P给体稳定的阳离子进行X射线结构分析后发现，相较于[8]⁺的同类物，[6]⁺型阳离子的P-P键与P-N键更长，且P-N-Si键角更小。上述结构差异可通过负超共轭成键模型进行合理解释。此外，通过溶液态与固态³¹P核磁共振波谱（NMR）观测到的[6]⁺型阳离子的¹J_PP耦合常数极小（13~25 Hz），而[8]⁺型阳离子的耦合常数则显著更大且为正值（276~324 Hz），这符合P(IV)⁺-P(V)体系的预期结果。密度泛函理论（DFT, Density Functional Theory）研究表明，[6]⁺与[8]⁺之间观测到的¹J_PP耦合常数的显著差异，似乎与膦亚胺磷中心上R'取代基的电负性相关，而非与给体-受体P-P键的强度有关；X射线数据与反应性研究显示，[6]⁺中的P-P键相较于[8]⁺中的略弱。