Instituto de Química Orgánica General, C.S.I.C., Juan de la Cierva 3, 28006 Madrid, Spain.
Phys Chem Chem Phys. 2010 Feb 14;12(6):1305-17. doi: 10.1039/b917343a. Epub 2009 Oct 21.
The influence of the substituent nature, as well as the number and position of the substituents, on the aromaticity/antiaromaticity of a comprehensive set of derivatives of cyclopentadienyl anion, cyclopentadiene and cyclopentadienyl cation has been analyzed. The aromaticity/antiaromaticity of substituted cyclopentadienyl derivatives has been measured using energetic, magnetic and structural criteria to take into account the multidimensional character of aromaticity. Furthermore, the Euclidean distance values (d(j)) for all compounds have been estimated using the neural network developed previously. It has been demonstrated that the d(j) as a scale of aromaticity overcomes the particular limitations of the ASE, Lambda, NICS and HOMA indices to describe the changes in the aromaticity of cyclopentadiene ring due to substituent effects. Additionally, it is shown that neural networks are useful tools for establishing structure-property relationships. The results indicate that mostly the aromaticity of cyclopentadiene ring and cyclopentadienyl anion ring decrease upon substitution irrespective of the electronic character of the substituent. The electron-donating groups, especially hydroxyl groups, destabilize carbanion to a larger extent than electron-accepting ones. On the other hand, all the substituents reduce strongly the antiaromaticity of the cyclopentadienyl cation. These results can be useful for the design of new cationic ligands with lowered instability and new cyclopentadienyl ligands with improved stability and reactivity.
已经分析了取代基的性质以及取代基的数量和位置对一系列环戊二烯阴离子、环戊二烯和环戊二烯阳离子衍生物的芳香性/反芳香性的影响。使用能量、磁性和结构标准来衡量取代的环戊二烯衍生物的芳香性/反芳香性，以考虑芳香性的多维性质。此外，还使用先前开发的神经网络估计了所有化合物的欧几里得距离值（d(j)）。结果表明，d(j)作为芳香性的标度克服了 ASE、Lambda、NICS 和 HOMA 指数的特定局限性，能够描述取代效应对环戊二烯环芳香性的变化。此外，结果表明神经网络是建立结构-性质关系的有用工具。结果表明，环戊二烯环和环戊二烯阴离子环的芳香性主要在取代后降低，而与取代基的电子性质无关。供电子基团，尤其是羟基，使碳负离子更加不稳定，比吸电子基团更不稳定。另一方面，所有取代基都强烈降低了环戊二烯阳离子的反芳香性。这些结果可用于设计具有降低的不稳定性的新型阳离子配体和具有提高的稳定性和反应性的新型环戊二烯配体。
