A. Dransfeld et al., THE AROMATICITY OF POLYPHOSPHAPHOSPHOLES DECREASES WITH THE PYRAMIDALITY OF THE TRICOORDINATE PHOSPHORUS, Inorganic chemistry, 37(17), 1998, pp. 4413-4420
The aromaticity of phospholes, P-n(CH)(4-n)PH with n = 0-4, becomes la
rger as the number of dicoordinate atoms (n) in the ring increases. Th
is is due to the decreasing pyramidality of the tricoordinate phosphor
us. The aromaticity is shown by various criteria, e.g., the nucleus-in
dependent chemical shifts (NICS), and the separated pi contribution NI
CS(pi), as well as the Bird aromaticity (BI) and the bond-shortening (
BDSHRT) indexes. The change of the aromaticity is correlated with the
bond angle sum (the degree of planarity) about the tricoordinate phosp
horus (e.g., correlation coefficient = 0.968 with NICS). The aromatici
ties of the planar P-n(CH)(4-n)PH phospholes (first-order saddle point
s for n = 0-3, minimum for n = 4) (NICS values between -15.4 and -17.4
ppm) all are similar. The same is true for the NICS (-12.3 to -15.9)
aromaticity of the phospholyl anions (P-n(CH)(4-n)P-), n = 0-4, which
are close to those of the planar phospholes. According to NICS, BI and
BDSHRT planar phospholes are more aromatic than pyrrol or thiophene.
Isodesmic reaction energies do not correlate with the bond angle sum,
since the energy cost of the increasing planarization of the tricoordi
nate phosphorus consumes the stabilization achieved by aromaticity.