Stretching [8]cycloparaphenylene with encapsulated potassium cations: structural and theoretical insights into core...

The consequences of four-electron addition to [8]cycloparaphenylene ([8]CPP, 1) have been evaluated crystallographically, revealing a significant core deformation. The structural analysis exposes an elliptical distortion observed upon electron transfer, with the deformation parameter (D.P.) increased by 28% in comparison with neutral [8]CPP. The C-C bond length alteration pattern also indicates a quinoidal structural rearrangement upon four-fold reduction. The large internal cavity of [8]CPP^4- allows the encapsulation of two {K⁺(THF)₂} cationic moieties with two additional cations bound externally in the solid-state structure of [{K⁺(THF)₂}₄([8]CPP^4-)]. The experimental structural data have been used as a benchmark for the comprehensive theoretical description of the geometric changes and electronic properties of the highly-charged [8]CPP^4- nanohoop in comparison with its neutral parent. While neutral [8]CPP and the [8]CPP^2- anion clearly show aromatic behavior of all six-membered rings, subsequent addition of two more electrons completely reverses their aromatic character to afford the highly-antiaromatic [8]CPP^4- anion, as evidenced by structural, topological, and magnetic descriptors. The disentanglement of electron transfer from metal binding effects allowed their contributions to the overall core perturbation of the negatively-charged [8]CPP to be revealed. Consequently, the internal coordination of potassium cations is identified as the main driving force for drastic elliptic distortion of the macrocyclic framework upon reduction.