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Chemical reduction of highly-twisted 9,10,11,20,21,22-hexaphenyltetrabenzo[a,c,l,n]pentacene (C74H46, 1) was investigated using Li and Cs metals as the reducing agents. The Cs-induced reduction of 1 in the presence of 18-crown-6 ether enabled the isolation of a solvent-separated ion pair (SSIP) with a “naked” monoanion. Upon reduction with Li metal, a double reductive dehydrogenative annulation of 1 was observed to afford a new C74H422– dianion. The latter was shown to undergo a further reduction to C74H424– without additional core transformation. All products were characterized by single-crystal X-ray diffraction and spectroscopic methods. Subsequent in-depth theoretical analysis of one vs. two and four electron uptake by 1 provided insights into how the changes of geometry, aromaticity and charge facilitated the core transformation of twistacene observed upon two-fold reduction. These experimental and theoretical results pave the way to understanding of the reduction-induced core transformations of highly twisted and strained π-systems. 

Abstract 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 the1_TR²⁻dianions. As a result, the highly contorted1_TR²⁻ligand exhibits a “butterfly” shape and could provide different coordination sites for metal‐ion binding. The K‐induced reduction of1in THF affords a polymeric product with low solubility, namely [{K⁺(THF)}₂(1_TR²⁻)] (K₂‐1_TR²⁻). The use of a secondary ligand facilitates the isolation of discrete complexes with heavy alkali metals, [Rb⁺(18‐crown‐6)]₂[1_TR²⁻] (Rb₂‐1_TR²⁻) and [Cs⁺(18‐crown‐6)]₂[1_TR²⁻] (Cs₂‐1_TR²⁻). Both internal and external coordination are observed inK₂‐1_TR²⁻, while the bulky 18‐crown‐6 ligand only allows external metal binding inRb₂‐1_TR²⁻andCs₂‐1_TR²⁻. 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 the1_TR²⁻TBCOT dianion, however, the aromaticity of the polycyclic ligand remains largely unaffected. 

Abstract Cyclooctatetraene (COT) and COT²⁻dianion are well‐known as archetypical non‐aromatic and aromatic systems, respectively. However, despite a wealth of studies the effect of one electron addition to the eight‐membered ring remains equivocal. Herein, we report the first stepwise electron addition to tetrabenzo[a,c,e,g]cyclooctatetraene (TBCOT or tetraphenylene), accompanied by isolation and structural characterization of the mono‐ and doubly‐reduced anions. The X‐ray crystallographic study reveals only a small asymmetric distortion of the saddle‐shaped core upon one electron uptake. In contrast, the doubly‐reduced product exhibits a severely twisted conformation, with a new C−C bond separating the COT ring into two fused 5‐membered rings. The reversibility of the two‐fold reduction and bond rearrangement is demonstrated by NMR spectroscopy. In agreement with experimental results, computational analysis confirms that the reduction‐induced core rearrangement requires the addition of the second electron.