Free Cyclooctatetraene Dianion: Planarity, Aromaticity, and Theoretical Challenges

The planarity and 10 π-electron aromaticity of the free cyclooctatetraene dianion (C8H8 2–, COT2–) 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 COT2–. Like many multiply charged anions, COT2– exists in isolation only as a short-lived resonance state lying above neutral COT. Wave function stability analysis demonstrates that predictions of nonplanar COT2– rings are artifacts of using overly diffuse basis sets. The resulting broken-symmetry wave functions are not characteristic of COT2– 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 COT2– structure. Final focal point analyses place the COT2– resonance state 61.6 kcal mol–1 above neutral COT. Nonetheless, COT2– exhibits structural, magnetic, and energetic properties characteristic of aromatic compounds. Comparison with all-trans octatetraene indicates that COT2– 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 COT2– with two sodium cations results in a thermodynamically stable Na2COT compound (D 8h symmetry), for which high-level structures are also presented.