Substituent effects on the aromaticity of carbocyclic five-membered rings

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Abstract
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 (dj) for all compounds have been estimated using the neural network developed previously. It has been demonstrated that the dj as a scale of aromaticity overcomes the particular limitations of the ASE, Λ, 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.