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The rectangular cyclobutadiene (CBD, C₄H₄) is a unique moiety for building nonbenzenoid polycyclic conjugated hydrocarbons with interesting electron‐accepting properties. Herein, the investigation on chemical reduction of several CBD‐containing polycyclic hydrocarbons with increasing conjugation length is reported: biphenylene (C₁₂H₈), dimethyl[2]naphthalene (C₂₂H₁₆), and tetramethyl‐dibenzo‐[3]phenylene (C₃₀H₂₂). The two‐step sequential reduction is first demonstrated by in situ spectroscopic investigation and then confirmed by the isolation of single crystals of the reduced products. The X‐ray crystallographic analysis reveals the formation of several mono‐ and doubly reduced products in solvent‐separated and complexed forms. The crystal structures for both neutral parents and corresponding reduced products unravel the changes in bond alternation in each ring of the fused systems. Density functional theory (DFT) and nucleus‐independent chemical shift (NICS) scan calculations reveal that the two‐electron addition reduces the aromatic character in the benzenoid rings but has minor influence on the antiaromatic CBD rings. 

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 Octalenobisterphenylene1(also known as terphenylene dimer) was synthesized from 3,3′,5,5′‐tetraaryl‐substituted biaryl bytert‐butyllithium‐mediated cyclization followed by oxidative coupling. This one‐pot two‐step protocol facilitated the successive formation of four four‐membered and two eight‐membered rings. Treatment of1with sodium metal, followed by crystallization from THF, yielded the remarkable diradical dianion [(1^•–)₂]²⁻, where the two molecular halves are connected by four σ bonds. The cyclodimerization is driven by the pronounced reactivity and strain of the central six‐membered ring within the [3]phenylene subunit. The structure and diradical nature of [(Na⁺)₂(1^•–)₂] were confirmed through X‐ray crystallography, DFT computations, and¹H NMR and ESR spectra. These investigations revealed that the two spins, one on each molecular half, exhibit minimal mutual interaction. 

Abstract The chemical reduction of a bilayer spironanographene,spiro‐NG(C₁₃₇H₁₂₀), with Na and K metals in the presence of [2.2.2]cryptand to yield [Na⁺(2.2.2‐cryptand)](C₁₃₇H₁₂₁⁻) (1) and [K⁺(2.2.2‐cryptand)](C₁₃₇H₁₂₁⁻) (2), respectively, is reported. X‐ray crystallography reveals the formation of a new “naked” anion (spiro‐NG_H⁻), in which spirocyclic ring cleavage and subsequent hydrogenation have occurred. Density Functional Theory (DFT) calculations suggest that the generation of the radical anion of the parent nanographene (spiro‐NG^•−), upon electron acceptance from Na and K metals, induces the cleavage of the strained spirobifluorene core. The resulting spin density localizes on a particular carbon atom, previously attached to the spiranic sp³carbon atom, facilitating a site‐specific hydrogenation to afford (spiro‐NG_H⁻). The electrostatic potential map of this anion reveals electron density concentrated at the five‐membered ring of the readily formed indenyl fragment, thus enhancing the aromaticity of the system. Furthermore, nuclear magnetic resonance (NMR) and UV–vis absorption spectroscopy experiments allowed to follow the in situ reduction and hydrogenation processes in detail. 

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. 

Abstract Chemical reduction of a [4]cumulene with cesium metal was explored, and the structural changes stemming from electron acquisition are detailed using X‐ray crystallography. It is found that the [4]cumulene undergoes dramatic geometric changes upon stepwise reduction, including bending of the cumulenic core and twisting of the endgroups from orthogonal to planar. The structural deformation is consistent with early theoretical reports that suggest that the twisting should occur upon reduction of both even and odd [n]cumulenes. The current results, on the other hand, are inconsistent with a previous experimental study of a [3]cumulene in which the predicted twisting is not observed upon reduction. DFT calculations reveal that the barrier to deformation is an order of magnitude lower in a [3]cumulene than a [4]cumulene, allowing the barrier to be overcome in the solid‐state.