Department of General Chemistry (ALGC), Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050, Brussels, Belgium.
Laboratoire de Chimie Théorique (LCT), Sorbonne Université, place Jussieu 4, 75052, Paris, France.
Chemistry. 2023 Jan 27;29(6):e202202264. doi: 10.1002/chem.202202264. Epub 2022 Nov 28.
Despite the central role of aromaticity in the chemistry of expanded porphyrins, the evaluation of aromaticity remains difficult for these extended macrocycles. The presence of multiple conjugation pathways and different planar and nonplanar π-conjugation topologies makes the quantification of global and local aromaticity even more challenging. In neutral expanded porphyrins, the predominance of the aromatic conjugation pathway passing through the imine-type nitrogens and circumventing the amino NH groups is established. However, for charged macrocycles, the question about the main conjugation circuit remains open. Accordingly, different conjugation pathways in a set of neutral, anionic, and cationic expanded porphyrins were investigated by means of several aromaticity indices rooted in the structural, magnetic, and electronic criteria. Overall, our results reveal the predominance of the conjugation pathway that passes through all nitrogen atoms to describe the aromaticity of deprotonated expanded porphyrins, while the outer pathway through the perimeter carbon atoms becomes the most aromatic in protonated macrocycles. In nonplanar and charged macrocycles, a discrepancy between electronic and magnetic descriptors is observed. Nevertheless, our work demonstrates AV remains the best tool to determine the main conjugation pathway of expanded porphyrins.
尽管芳香性在扩展卟啉的化学中起着核心作用，但对于这些扩展的大环来说，评估芳香性仍然很困难。存在多种共轭途径以及不同的平面和非平面π-共轭拓扑结构，使得对全局和局部芳香性的量化更加具有挑战性。在中性扩展卟啉中，通过亚胺型氮原子并绕过氨基 NH 基团的芳香共轭途径占主导地位。然而，对于带电大环，主要共轭电路的问题仍然存在。因此，通过几种基于结构、磁和电子标准的芳香性指数，研究了一组中性、阴离子和阳离子扩展卟啉中的不同共轭途径。总的来说，我们的结果表明，通过所有氮原子传递的共轭途径在描述去质子化扩展卟啉的芳香性方面占主导地位，而在外围通过周长碳原子的途径在质子化大环中成为最芳香的。在非平面和带电大环中，观察到电子和磁性描述符之间存在差异。然而，我们的工作表明，AV 仍然是确定扩展卟啉主要共轭途径的最佳工具。
