Synthesis and reactivity of phosphine-stabilized phosphoranimine cations, [R3P x PR'2=NSiMe3]+.

A series of phosphine-stabilized phosphoranimine cations R(3)P x PR'(2)=NSiMe(3), which can be regarded as derivatives of the proposed transient reactive intermediate PR'(2)=NSiMe(3) in the thermal condensation polymerization of phosphoranimines (R''O)PR'(2)=NSiMe(3) to form poly(alkyl/arylphosphazenes) PR'(2)=N at 180-200 degrees C, have been prepared. The bromide salts [R(3)P x PR'(2)=NSiMe(3)]Br [R' = Me (6), OCH(2)CF(3) (8); R(3)P = Me(3)P (a), Et(3)P (b), (n)Bu(3)P (c), dmpm (d, dmpm = dimethylphosphinomethane), dmpe (e, dmpe = dimethylphosphinoethane)] were prepared from the direct reactions between BrMe(2)P=NSiMe(3) (5) and Br(CF(3)CH(2)O)(2)P=NSiMe(3) (7) and the corresponding tertiary phosphines R(3)P or the diphosphines Me(2)P(CH(2))(n)PMe(2) (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 x PR(2)=NSiMe(3)]Br [R = Me (9), OCH(2)CF(3) (10)]. Cations 6 with Br(-) anions are particularly labile: for example, [6a]Br slowly releases PMe(3), BrSiMe(3), and forms cyclic phosphazenes such as Me(2)P=N. Anion exchange reactions between the salts [6b]Br or [8c]Br and AgOTf (OTf = CF(3)SO(3)) quantitatively afforded the corresponding and more stable triflate salts [6b]OTf and [8c]OTf. Phosphine ligand abstraction reactions with B(C(6)F(5))(3) were observed for the bromide salts [6b]Br and [8c]Br, which regenerated the phosphoranimines 5 and 7, respectively, and formed the adduct R(3)P x B(C(6)F(5))(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 (1)J(PP) coupling constants for the cations 6 observed by both solution and solid-state (31)P 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 (1)J(PP) 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.