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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 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 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. 

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. 

Abstract Chemical reduction of pentacene (C₂₂H₁₄,1) with Group 1 metals ranging from Li to Cs revealed that1readily undergoes a two‐fold reduction to afford a doubly‐reduced1²⁻anion in THF. With the help of 18‐crown‐6 ether used as a secondary coordinating agent, five π‐complexes of1²⁻with different alkali metal counterions have been isolated and fully characterized. This series of complexes enables the first evaluation of alkali‐metal ion binding patterns and structural changes of the1²⁻dianion based on the crystallographically confirmed examples. The difference in coordination of the smallest Li⁺ion vs. heavier Group 1 congeners has been demonstrated. In addition, the use of benzo‐15‐crown‐5 in the reaction of1with Na metal allowed the isolation of the unique solvent‐separated ion product with a “naked” dianion,1²⁻. The detailed structural analyses of the series revealed the C−C bond alteration and core deformation of pentacene upon two‐fold reduction and complexation. The negative charge localization at the central six‐membered ring of1²⁻identified by theoretical calculations corroborates with the X‐ray crystallographic results. Subsequent in‐depth theoretical analysis provided a detailed description of changes in the electronic structure and aromaticity of pentacene upon reduction. 

Abstract The electronic, optical, and solid state properties of a series of monoradicals, anions and cations obtained from starting neutral diradicals have been studied. Diradicals based ons‐indacene and indenoacenes, with benzothiophenes fused and in different orientations, feature a varying degree of diradical character in the neutral state, which is here related with the properties of the radical redox forms. The analysis of their optical features in the polymethine monoradicals has been carried out in the framework of the molecular orbital and valence bond theories. Electronic UV‐Vis‐NIR absorption, X‐ray solid‐state diffraction and quantum chemical calculations have been carried out. Studies of the different positive‐/negative‐charged species, both residing in the same skeletalπ‐conjugated backbone, are rare for organic molecules. The key factor for the dual stabilization is the presence of the starting diradical character that enables to indistinctively accommodate a pseudo‐hole and a pseudo‐electron defect with certainly small reorganization energies for ambipolar charge transport.