From Corannulene to Indacenopicene: Effect of Carbon Framework Topology on Aromaticity and Reduction Limits

The electronic structure, reduction limits, and coordination abilities of a bowl-shaped polycyclic aromatic hydrocarbon, indacenopicene (C₂₆H₁₂, 1), have been investigated for the first time using a combination of theoretical and experimental tools. A direct comparison with the prototypical corannulene bowl (C₂₀H₁₀, 2) revealed the effects of carbon framework topology and symmetry change on the electronic properties and aromaticity of indacenopicene. The accessibility of two reduction steps for 1 was predicted theoretically and then confirmed experimentally. Two reversible one-electron reduction processes with the formal reduction potentials at −1.92 and −2.29 V vs Fc^+/0 were detected by cyclic voltammetry measurements, demonstrating the stability of the corresponding mono- and dianionic states of 1. The products of the doubly reduced indacenopicene have been isolated as rubidium and cesium salts and fully characterized. Their X-ray diffraction study revealed the formation of tetranuclear organometallic building blocks with the [M₂(18-crown-6)]²⁺ (M = Rb (3) and Cs (4)) cations occupying the concave cavities of two C₂₆H₁₂^2– anions. The coordination of two outside exo-bound rubidium ions is terminated by crown ether molecules in 3 to form the discrete [Rb⁺₄(18-crown-6)₃­(C₂₆H₁₂^2–)₂] tetramer. In contrast, the larger cesium ions allow the 1D polymeric chain propagation in 4 to afford [Cs⁺₂(18-crown-6)₂­(THF)­(C₂₆H₁₂^2–)]_∞.