Free Cyclooctatetraene Dianion: Planarity, Aromaticity, and Theoretical Challenges

The planarity and 10 π-electron aromaticity of the free cyclooctatetraene dianion (C₈H₈^2–, COT^2–) have been questioned recently on the basis of conflicting density functional and second-order Møller–Plesset perturbation computations. Rigorous coupled-cluster methods are employed here to establish the structure and properties of COT^2–. Like many multiply charged anions, COT^2– exists in isolation only as a short-lived resonance state lying above neutral COT. Wave function stability analysis demonstrates that predictions of nonplanar COT^2– rings are artifacts of using overly diffuse basis sets. The resulting broken-symmetry wave functions are not characteristic of COT^2– but mainly describe COT in a continuum of free electrons. All-electron coupled cluster theory extended through triple excitations [AE-CCSD­(T)] yields a planar D_8h symmetry COT^2– structure. Final focal point analyses place the COT^2– resonance state 61.6 kcal mol^–1 above neutral COT. Nonetheless, COT^2– exhibits structural, magnetic, and energetic properties characteristic of aromatic compounds. Comparison with all-trans octatetraene indicates that COT^2– has a substantial aromatic stabilization energy (25 kcal mol^–1) approaching that of benzene (33 kcal mol^–1), but this favorable influence is swamped by Coulomb repulsion. Charge-compensating complexation of COT^2– with two sodium cations results in a thermodynamically stable Na₂COT compound (D_8h symmetry), for which high-level structures are also presented.