The Aromaticity of Polyphosphaphospholes Decreases with the Pyramidality of the Tricoordinate Phosphorus

The aromaticity of phospholes, P(n)()(CH)(4)(-)(n)()PH with n = 0-4, becomes larger as the number of dicoordinate atoms (n) in the ring increases. This is due to the decreasing pyramidality of the tricoordinate phosphorus. The aromaticity is shown by various criteria, e.g., the nucleus-independent chemical shifts (NICS), and the separated pi contribution NICS(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 phosphorus (e.g., correlation coefficient = 0.968 with NICS). The aromaticities of the planar P(n)()(CH)(4)(-)(n)()PH phospholes (first-order saddle points 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 tricoordinate phosphorus consumes the stabilization achieved by aromaticity.