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

Abstract

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](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](4). 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.