Conspectus In recent years, synthetic transformations based on aryne chemistry have become particularly popular, mostly due to the spread of methods to generate these highly reactive intermediates in ...a controlled manner under mild reaction conditions. In fact, aryne cycloadditions such as the Diels–Alder reaction are nowadays widely used for the efficient preparation of polycyclic aromatic compounds. In 1998, our group discovered that arynes can undergo transition metal-catalyzed reactions, a finding that opened new perspectives in aryne chemistry. In particular, Pd-catalyzed 2 + 2 + 2 cycloaddition of arynes allowed the straightforward synthesis of triphenylene derivatives such as starphenes or cloverphenes. We found that this reaction is compatible with different substituents and sterically demanding arynes as starting materials. This transformation is especially useful to increase the molecular complexity in one single step, transforming molecules formed by n cycles in structures with 3n + 1 cycles. In fact, we took advantage of this protocol to prepare a large variety of sterically congested polycyclic aromatic hydrocarbons such as helicenes or twisted polyarenes. Soon after the discovery of the reaction, the co-cyclotrimerization of arynes with other reaction partners, such as electron deficient alkynes, significantly expanded the potential of this transformation. Also the use of catalysts based on alternative metals besides Pd (e.g., Ni, Cu, Au) or the use of other strained intermediates such as cycloalkynes or cycloallenes added value to this reaction. In addition, we realized that the Pd-catalyzed aryne cyclotrimerization reaction is particularly useful for the bottom-up preparation of well-defined nanographenes by chemical methods. Although the extreme insolubility of these planar nanographenes hampered their manipulation and characterization by conventional methods, recent advances in single molecule on-surface characterization by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) with functionalized tips under ultrahigh vacuum (UHV) conditions, permitted the impressive visualization of these nanographenes with submolecular resolution, together with the examination of the corresponding molecular orbital densities. Moreover, arynes have been shown to possess a rich on-surface chemistry. In particular, arynes have been generated and studied on-surface, showing that the reactivity is preserved even at cryogenic temperatures. On-surface aryne cyclotrimerization was also demonstrated to obtain large starphene derivatives. Therefore, it is expected that the combination of aryne cycloadditions and on-surface synthesis will provide notable findings in the near future, including the “à la carte” preparation of graphene materials or the synthesis of elusive molecules with unique properties.
Cycloaddition reactions involving arynes provide privileged strategies for the convergent synthesis of polycyclic compounds containing aromatic rings. This review focuses on the application of aryne ...chemistry, in particular 4+2 and palladium‐catalysed 2+2+2 cycloaddition reactions, in the synthesis of large polycyclic aromatic hydrocarbons (PAHs) containing five or more fused benzene rings. Many of the polycyclic compounds included in this report display properties of interest in the field of materials science and some can be considered as nano‐sized graphene substructures or nanographenes.
The cycloaddition of arynes is a powerful tool for the convergent synthesis of polycyclic aromatic compounds (PACs). The controlled synthesis of large PACs is an important challenge due to the interest in these molecules as functional materials and in graphene chemistry. This microreview covers the synthesis of a variety of large PACs, which are of interest in the field of materials science.
Contributing to the need for new graphene nanoribbon (GNR) structures that can be synthesized with atomic precision, we have designed a reactant that renders chiral (3,1)-GNRs after a multistep ...reaction including Ullmann coupling and cyclodehydrogenation. The nanoribbon synthesis has been successfully proven on different coinage metals, and the formation process, together with the fingerprints associated with each reaction step, has been studied by combining scanning tunneling microscopy, core-level spectroscopy, and density functional calculations. In addition to the GNR’s chiral edge structure, the substantial GNR lengths achieved and the low processing temperature required to complete the reaction grant this reactant extremely interesting properties for potential applications.
The acene series represents a model system to investigate the intriguing electronic properties of extended π-electron structures in the one-dimensional limit, which are important for applications in ...electronics and spintronics and for the fundamental understanding of electronic transport. Here, we present the on-surface generation of the longest acene obtained so far: dodecacene. Scanning tunneling spectroscopy gives access to the energy position and spatial distribution of its electronic states on the Au(111) surface. We observe that, after a progressive closing of the gap and a stabilization to about 1 eV at the length of decacene and undecacene, the energy gap of dodecacene unexpectedly increases to 1.4 eV. Considering the acene series as an exemplary general case, we discuss the evolution with length of the single tunneling resonances in comparison with ionization energy, electronic affinity, and optical gap.
Four decades after the first (and only) reported synthesis of kekulene, this emblematic cycloarene has been obtained again through an improved route involving the construction of a key synthetic ...intermediate, 5,6,8,9-tetrahydrobenzomtetraphene, by means of a double Diels–Alder reaction between styrene and a versatile benzodiyne synthon. Ultra-high-resolution AFM imaging of single molecules of kekulene and computational calculations provide additional support for a molecular structure with a significant degree of bond localization in accordance with the resonance structure predicted by the Clar model.
Decacene: On‐Surface Generation Krüger, Justus; García, Fátima; Eisenhut, Frank ...
Angewandte Chemie International Edition,
September 18, 2017, Volume:
56, Issue:
39
Journal Article
Peer reviewed
Acenes are intriguing molecules with unique electronic properties. The difficulties in their preparation owing to low stability under ambient conditions are apparent because successful syntheses of ...long unsubstituted acenes are still scarce, in spite of the great attention they have attracted. Only unsubstituted acenes up to heptacene have been isolated in bulk, with nonacene being the largest acene detected to date. Herein we use on‐surface assisted reduction of tetraepoxy decacene precursors on Au(111) as the key step to generate unprecedented decacene which is visualized and its electronic resonances studied by scanning tunneling microscopy (STM) and spectroscopy (STS).
Decathletes: Combined efforts of organic synthesis and on‐surface chemistry have led to the preparation of decacene for the first time. An iterative sequence of aryne cycloadditions was used to synthesize stable tetraepoxy precursors which were reduced on Au(111) to obtain unprecedented decacene as visualized by scanning tunneling microscopy (STM).
Bond-Order Discrimination by Atomic Force Microscopy Gross, Leo; Mohn, Fabian; Moll, Nikolaj ...
Science (American Association for the Advancement of Science),
09/2012, Volume:
337, Issue:
6100
Journal Article
Peer reviewed
We show that the different bond orders of individual carbon-carbon bonds in polycyclic aromatic hydrocarbons and fullerenes can be distinguished by noncontact atomic force microscopy (AFM) with a ...carbon monoxide (CO)—functionalized tip. We found two different contrast mechanisms, which were corroborated by density functional theory calculations: The greater electron density in bonds of higher bond order led to a stronger Pauli repulsion, which enhanced the brightness of these bonds in high-resolution AFM images. The apparent bond length in the AFM images decreased with increasing bond order because of tilting of the CO molecule at the tip apex.
Large aromatic carbon nanostructures are cornerstone materials due to their increasingly active role in functional devices, but their synthesis in solution encounters size and shape limitations. New ...on-surface strategies facilitate the synthesis of large and insoluble planar systems with atomic-scale precision. While dehydrogenation is usually the chemical zipping reaction building up large aromatic carbon structures, mostly benzenoid structures are being produced. Here, we report on a new cyclodehydrogenation reaction transforming a sterically stressed precursor with conjoined cove regions into a planar carbon platform by incorporating azulene moieties in their interior. Submolecular resolution STM is used to characterize this exotic large polycyclic aromatic compound on Au(111) yielding unprecedented insight into a dehydrogenative intramolecular aryl–aryl coupling reaction. The resulting polycyclic aromatic carbon structure shows a 18annulene core hosting peculiar pore states confined at the carbon cavity.
Reactive intermediates are involved in many chemical transformations. However, their characterization is a great challenge because of their short lifetimes and high reactivities. Arynes, formally ...derived from arenes by the removal of two hydrogen atoms from adjacent carbon atoms, are prominent reactive intermediates that have been hypothesized for more than a century. Their rich chemistry enables a widespread use in synthetic chemistry, as they are advantageous building blocks for the construction of polycyclic compounds that contain aromatic rings. Here, we demonstrate the generation and characterization of individual polycyclic aryne molecules on an ultrathin insulating film by means of low-temperature scanning tunnelling microscopy and atomic force microscopy. Bond-order analysis suggests that a cumulene resonance structure is the dominant one, and the aryne reactivity is preserved at cryogenic temperatures. Our results provide important insights into the chemistry of these elusive intermediates and their potential application in the field of on-surface synthesis.
We describe the generation of a meta-aryne at low temperature (T = 5 K) using atomic manipulation on Cu(111) and on bilayer NaCl on Cu(111). We observe different voltage thresholds for dehalogenation ...of the precursor and different reaction products depending on the substrate surface. The chemical structure is resolved by atomic force microscopy with CO-terminated tips, revealing the radical positions and confirming a diradical rather than an anti-Bredt olefin structure for this meta-aryne on NaCl.
PDF
