Stabilizing Contorted Doubly-Reduced Tetraphenylene with Heavy Alkali Metal Complexation: Crystallographic and Theoretical Evidence

The two-fold reduction of tetrabenzo[a,c,e,g]cyclooctatetraene (TBCOT, or tetraphenylene, 1) with K, Rb, and Cs metals reveals a distinctive core transformation pathway: a newly formed C-C bond converts the central eight-membered ring into a twisted core with two fused five-membered rings. This C-C bond of 1.589(3)-1.606(6) Å falls into a single σ-bond range and generates two perpendicular π-surfaces with dihedral angles of 110.3(9)°-117.4(1)° in the 1_TR ^2- dianions. As a result, the highly contorted 1_TR ^2- ligand exhibits a "butterfly" shape and could provide different coordination sites for metal-ion binding. The K-induced reduction of 1 in THF affords a polymeric product with low solubility, namely [{K⁺(THF)}₂(1_TR ^2-)] (K₂-1_TR ^2-). The use of a secondary ligand facilitates the isolation of discrete complexes with heavy alkali metals, [Rb⁺(18-crown-6)]₂[1_TR ^2-] (Rb₂-1_TR ^2-) and [Cs⁺(18-crown-6)]₂[1_TR ^2-] (Cs₂-1_TR ^2-). Both internal and external coordination are observed in K₂-1_TR ^2-, while the bulky 18-crown-6 ligand only allows external metal binding in Rb₂-1_TR ^2- and Cs₂-1_TR ^2-. The reversibility of the two-fold reduction and bond rearrangement is demonstrated by NMR spectroscopy. Computational analysis shows that the heavier alkali metals enable effective charge transfer from the 1_TR ^2-TBCOT dianion, however, the aromaticity of the polycyclic ligand remains largely unaffected.