Are the Six-Membered λ5-Phosphorins Aromatic or Ylidic?
✍ Scribed by Zhi-Xiang Wang; Paul von Ragué Schleyer
- Publisher
- John Wiley and Sons
- Year
- 2001
- Tongue
- German
- Weight
- 286 KB
- Volume
- 84
- Category
- Article
- ISSN
- 0018-019X
No coin nor oath required. For personal study only.
✦ Synopsis
Dedicated with admiration to Professor Edgar Heilbronner on the occasion of his 80th birthday
The aromaticity of a series of substituted six-membered l 5 -phosphorins, (CH) 5 PX 2 , (X F, Cl, Br, OH, Me, H, and SiH 3 ) 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 l 5 -phosphorins with electronegative substituents (XF, OH, Cl, and Br) show aromatic character, e.g., as characterized by NICS computed 1 above the ring centers: NICS(1) À 7.8, À 7.3, À 7.1, and À 6.6 ppm, respectively, vs. À 10.8 for phosphabenzene. The l 5 -phosphorins with electropositive substituents (X H, Me, and SiH 3 ) have small NICS(1) values, À 2.6, À 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 l 5phosphorins with strongly electronegative substituents may be described as hybrids of internal zwitterion (ylid) and Hückel 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 p electron system and the PÀX(2) bonds which serve as pseudo p-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 l 4 -S compounds, (CH) 5 SX, also is related to the electronegativity of the S-substituents, X. The (CH) 5 SX derivatives with XF, Cl, Br, OH, H, and Me have NICS(1), of À 10.1, À 10.0, À 9.7, À 8.0, À 5.2, and À 4.2 ppm, respectively. The same generalizations extend to the six-membered l 5 -As and l 4 -Se compounds, as well as to cyclohexadienyl anions, (CH) 5 (CX À 2 (X H and F) and (CH) 5 SiX À 2 (X H and F). In the four-membered ring compounds, l 5 -(CH) 3 P(As)X 2 and l 4 -(CH) 3 S(Se)X (XF 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.