Exploring the structure-aromaticity relationship in Huckel and Mobius N-fused pentaphyrins using DFT

摘要

N-fused pentaphyrins (NFP) are the stable forms of fully meso-aryl pentaphyrins(1.1.1.1.1). In order to determine the optimum conditions for viable Mobius topologies of these porphyrinoids, the conformational preferences, Huckel-Mobius interconversion pathways and aromaticity of [221 and [24]NFP have been investigated using density functional theory calculations. The conformation of the macrocycle is shown to be strongly dependent on the oxidation state and the macrocyclic aromaticity. [22]NFP prefers a highly aromatic and relatively strain-free Huckel conformation. However, antiaromatic Huckel and weakly aromatic Mobius conformers coexist in dynamic equilibrium in [24]NFP. The Huckel-Mobius aromaticity switch requires very low activation energy barriers (E_a = 3-4 kcal mol~(-1)). Interestingly, the balance between Mobius and Huckel conformations in [24]NFP can be controlled by meso-substituents. The structure-property relationship between the molecular conformation, number of π electrons and aromaticity has been established in our study using energetic, magnetic, structural, and reactivity descriptors of aromaticity. Although the Mobius topology is indeed accessible for [24]NFP, it does not exhibit a distinct macrocyclic aromaticity mainly due to the large dihedral angles around the molecular twist. Regarding the computational methodology, B3LYP and M06 show the best overall performance for describing the experimental geometries of NFP and, importantly, our computational results support the experimental evidence available for N-fused pentaphyrins.