The "anomalous" basicity of P(NHCH2CH2)3N relative to P(NMeCH2CH2)3N and p(NBzCH2CH2)3N: a chemical consequence of orbital charge balance?

Five-Coordinate and Quasi-Five-Coordinate Phosphorus

“…Indeed the reaction was over within an hour at room temperature, but to our surprise, the azaphosphatranes 4-6 formed in virtually quantitative yield (14,15 …”

“…It is interesting that the 1 JPH couplings of these azaphosphatrane cations decrease in the same order as the acidity: 6(C1) (506 Hz) > 5(C1) (491 Hz) > 4(C1) (453 Hz) (14). Since the structures of 4(C1) and 5(C1) are so similar that hybridzational changes are likely to be small (74) the coupling trend indicates a buildup of negative charge on the phosphorus, in accord with the decrease in acidity.…”

“…Thus the order expected would be 2 > 1 > 3. The anomalous basicity of 1 cannot be attributed to a shorter (stabilizing) transannular P-N bond length in 4 + than in 5 + , because it is in fact longer (14). It is tempting to ascribe the higher acidités of 5 + and 6 + to sterically assisted proton departure by the Me and CH2PI1 groups, respectively.…”

“…,^f D U • 1 + 2 (27) -K + -r-BuOH azaphosphatranes (14). From the NMR data, a pK a in DMSO of 29.6 was calculated for 4(C1) and an upper limit of pK a = 26.8 was estimated for 5(C1) and 6(C1).…”

“…Another possibility is that equilibria such as those in Scheme I are involved, which are expected to strengthen the P-H bond. The unusual stability of the P-H bond in 4(C1) may also be due to relatively little phosphorus orbital imbalance, which is signalled by its strongly negative 31 P chemical shift (-42 ppm) compared with that in 5(C1) (-10.1 ppm) and 6(C1) (-11.0 ppm) (14).…”