| 19449868 |
Huynh K, Lough AJ, Forgeron MA, Bendle M, Soto AP, Wasylishen RE, Manners I: Synthesis and reactivity of phosphine-stabilized phosphoranimine cations, [R3P x PR'2=NSiMe3]+. J Am Chem Soc. 2009 Jun 10;131(22):7905-16.
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. |
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