An official website of the United States government
Abstract The chemical reduction of π‐conjugated bilayer nanographene1(C138H120) with K and Rb in the presence of 18‐crown‐6 affords [K+(18‐crown‐6)(THF)2][{K+(18‐crown‐6)}2(THF)0.5][C138H1223−] (2) and [Rb+(18‐crown‐6)2][{Rb+(18‐crown‐6)}2(C138H1223−)] (3). Whereas K+cations are fully solvent‐separated from the trianionic core thus affording a “naked”1.3−anion, Rb+cations are coordinated to the negatively charged layers of1.3−. According to DFT calculations, the localization of the first two electrons in the helicene moiety leads to an unprecedented site‐specific hydrogenation process at the carbon atoms located on the edge of the helicene backbone. This uncommon reduction‐induced site‐specific hydrogenation provokes dramatic changes in the (electronic) structure of1as the helicene backbone becomes more compressed and twisted upon chemical reduction, which results in a clear slippage of the bilayers.
more »
« less
Negative Charge as a Lens for Concentrating Antiaromaticity: Using a Pentagonal “Defect” and Helicene Strain for Cyclizations
Abstract Incorporation of a five‐membered ring into a helicene framework disrupts aromatic conjugation and provides a site for selective deprotonation. The deprotonation creates an anionic cyclopentadienyl unit, switches on conjugation, leads to a >200 nm red‐shift in the absorbance spectrum and injects a charge into a helical conjugated π‐system without injecting a spin. Structural consequences of deprotonation were revealed via analysis of a monoanionic helicene co‐crystallized with {K+(18‐crown‐6)(THF)} and {Cs+2(18‐crown‐6)3}. UV/Vis‐monitoring of these systems shows a time‐dependent formation of mono‐ and dianionic species, and the latter was isolated and crystallographically characterized. The ability of the twisted helicene frame to delocalize the negative charge was probed as a perturbation of aromaticity using NICS scans. Relief of strain, avoidance of antiaromaticity, and increase in charge delocalization assist in the additional dehydrogenative ring closures that yield a new planarized decacyclic dianion.
more »
« less
- PAR ID:
- 10127289
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Angewandte Chemie International Edition
- Volume:
- 59
- Issue:
- 3
- ISSN:
- 1433-7851
- Page Range / eLocation ID:
- p. 1256-1262
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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 the1TR2−dianions. As a result, the highly contorted1TR2−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(1TR2−)] (K2‐1TR2−). The use of a secondary ligand facilitates the isolation of discrete complexes with heavy alkali metals, [Rb+(18‐crown‐6)]2[1TR2−] (Rb2‐1TR2−) and [Cs+(18‐crown‐6)]2[1TR2−] (Cs2‐1TR2−). Both internal and external coordination are observed inK2‐1TR2−, while the bulky 18‐crown‐6 ligand only allows external metal binding inRb2‐1TR2−andCs2‐1TR2−. 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 the1TR2−TBCOT dianion, however, the aromaticity of the polycyclic ligand remains largely unaffected.more » « less
-
Abstract Mono‐ and dianions of 2‐tert‐butyl‐3a2‐azapentabenzo[bc,ef,hi,kl,no]corannulene (1 a) were synthesized by chemical reduction with sodium and cesium metals, and crystallized as the corresponding salts in the presence of 18‐crown‐6 ether. X‐ray diffraction analysis of the sodium salt, [{Na+(18‐crown‐6)(THF)2}3{Na+(18‐crown‐6)(THF)}(1 a2−)2], revealed the presence of a naked dianion. In contrast, controlled reaction of1 awith Cs allowed the isolation of singly and doubly reduced forms of1 a, both forming π‐complexes with cesium ions in the solid state. In [{Cs+(18‐crown‐6)}(1 a−)]⋅THF, asymmetric binding of the Cs+ion to the concave surface of1 a−is observed, whereas in [{Cs+(18‐crown‐6)}2(1 a2−)], two Cs+ions bind to both the concave and convex surfaces of the dianion. The present study provides the first successful isolation and characterization of the reduced products of heteroatom‐containing buckybowl molecules.more » « less
-
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.more » « less
