Are the Six-Memberedλ5-Phosphorins Aromatic or Ylidic?

Abstract

The aromaticity of a series of substituted six-membered λ5-phosphorins, (CH)5PX2, (X=F, Cl, Br, OH, Me, H, and SiH3) has been evaluated by using magnetic criteria (nucleus-independent chemical shifts (NICS), magnetic susceptibility anisotropies and exaltations), as well structural and energetic considerations. The nature of the substituents influences the extent of cyclic electron delocalization significantly. The λ5-phosphorins with electronegative substituents (X=F, OH, Cl, and Br) show aromatic character, e.g., as characterized by NICS computed 1 A above the ring centers: NICS(1)=−7.8, −7.3, −7.1, and −6.6 ppm, respectively, vs. −10.8 for phosphabenzene. The λ5-phosphorins with electropositive substituents (X=H, Me, and SiH3) have small NICS(1) values, −2.6, −2.5, and 1.4 ppm, respectively, and are nonaromatic or only weakly aromatic. Based on these findings as well as geometrical and energetic results, the electronic structures of six-membered λ5-phosphorins with strongly electronegative substituents may be described as hybrids of internal zwitterion (ylid) and `Huckel' aromatic contributors, whereas the compounds having more electropositive substituents may be considered to be basically ylidic in character. The substituent effect on the aromaticity is due to the hyperconjugation (or to the negative hyperconjugation) involving the ring π electron system and the P−X(2) bonds which serve as pseudo π-electron donors (or as acceptors). The more electronegative the substituents X, the more aromatic the molecule. Contradicting early suggestions, no evidence was found for d-orbital participation of phosphorus in cyclic electron delocalization. Similarly, the aromaticity of six-membered λ4-S compounds, (CH)5SX, also is related to the electronegativity of the S-substituents, X. The (CH)5SX derivatives with X=F, Cl, Br, OH, H, and Me have NICS(1), of −10.1, −10.0, −9.7, −8.0, −5.2, and −4.2 ppm, respectively. The same generalizations extend to the six-membered λ5-As and λ4-Se compounds, as well as to cyclohexadienyl anions, (CH)5 (CX (X=H and F) and (CH)5SiX (X=H and F). In the four-membered ring compounds, λ5-(CH)3P(As)X2 and λ4-(CH)3S(Se)X (X=F and H), the substituents weaken the antiaromaticity of cyclobutadiene significantly. Unlike the six-membered ring cases, the electronegativity of X has no significant influence on the degree of antiaromaticity in the cyclobutadienyl analogs.