January 2025
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6 Reads
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 1TR²⁻ dianions. As a result, the highly contorted 1TR²⁻ 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(1TR²⁻)] (K2‐1TR²⁻). The use of a secondary ligand facilitates the isolation of discrete complexes with heavy alkali metals, [Rb⁺(18‐crown‐6)]2[1TR²⁻] (Rb2‐1TR²⁻) and [Cs⁺(18‐crown‐6)]2[1TR²⁻] (Cs2‐1TR²⁻). Both internal and external coordination are observed in K2‐1TR²⁻, while the bulky 18‐crown‐6 ligand only allows external metal binding in Rb2‐1TR²⁻ and Cs2‐1TR²⁻. 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 1TR²⁻TBCOT dianion, however, the aromaticity of the polycyclic ligand remains largely unaffected.