What a Difference an Electron Makes: Structural Response of Saddle-Shaped Tetraphenylene to One and Two Electron Uptake

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.