Search for: All records

Two-fold reduction of a vertically expanded functionalized pentacene with Na metal is accompanied by a loss of diatropicity in the resulting dianion and engagement of side groups in sodium ion coordination.

 

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. 

We describe reductive dehydrogenative cyclizations that form hepta-, nona-, and decacyclic anionic graphene subunits from mono- and bis-helicenes with an embedded five-membered ring. The reaction of bis-helicenes can either proceed to the full double annulation or be interrupted by addition of molecular oxygen at an intermediate stage. The regioselectivity of the interrupted cyclization cascade for bis-helicenes confirms that relief of antiaromaticity is a dominant force for these facile ring closures. Computational analysis reveals the unique role of the preexisting negatively charged cyclopentadienyl moiety in directing the second negative charge at a specific remote location and, thus, creating a localized antiaromatic region. This region is the hotspot that promotes the initial cyclization. Computational studies, including MO analysis, molecular electrostatic potential maps, and NICS(1.7)ZZ calculations, evaluate the interplay of the various effects including charge delocalization, helicene strain release, and antiaromaticity. The role of antiaromaticity relief is further supported by efficient reductive closure of the less strained monohelicenes where the relief of antiaromaticity promotes the cyclization even when the strain is substantially reduced. The latter finding significantly expands the scope of this reductive alternative to the Scholl ring closure. 

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.