Benzo-fused Tri[8]annulenes as Molecular Models of Cubic Graphite

Synthesis, structure and π-expansion of tris(4,5-dehydro-2,3:6,7-dibenzotropone)

The polycyclic skeleton of tris(4,5-dehydro-2,3:6,7-dibenzotropone) is a key structural fragment in carbon schwarzites, a theoretical form of negatively curved carbon allotrope. This report presents a new synthesis of this compound using a Ni-mediated Yamamoto coupling reaction and structural analysis of it with X-ray crystallography. Interestingly, it is observed that tris(4,5-dehydro-2,3:6,7-dibenzotropone) crystallized from its solution in hexane resulting in colorless and yellow crystal polymorphs, where it adopts conformations of approximate C s and C 2 symmetry, respectively. Furthermore, expanding its π-skeleton through the Barton-Kellogg and Scholl reactions led to the successful synthesis of a curved polycyclic arene containing three heptagons and two pentagons.

K+-Mediated vs Pd-Catalyzed Cyclotrimerization of 9,10-Didehydrotribenzo[8]annulene (TribenzoCOTyne): Stereodivergent Access to (α,α,α)- and (α,α,β)-Fragments of Cubic Graphite

A remarkable stereodivergent cyclotrimerization of 9,10-didehydrotribenzo[8]annulene (tribenzoCOTyne) to the corresponding (α,α,α)- and (α,α,β)-benzofused derivatives has been developed by controlling the reaction conditions. While the K+-mediated cyclotrimerization afforded the (α,α,α) stereoisomer, using Pd as a catalyst resulted in the (α,α,β)-derivative. Both stereoisomers were evidenced by spectroscopic data and crystal X-Ray analysis. The (α,α,α) stereoisomer is a fragment of cubic graphite (CG), an elusive 3D carbon allotrope that contains carbon cages, since all of its sixty carbons are part of the structure of CG, and 36 constitute a part of the C48 molecular cage of CG. Experimental and computational mechanistic studies revealed that the potassium ion would play a key role as a template to favor the formation of the (α,α,α) stereoisomer.

Pentacosacyclenes: cruciform molecular nanocarbons based on cyclooctatetraene

Pentacosacyclene (PC) and pentacosacyclene tetraimide (PCTI) were obtained in concise syntheses involving radial extension of tridecacyclene. PC is an electron-rich hydrocarbon with a C88 π-conjugated framework, whereas PCTI is electron-deficient and contains a C96N4 core. PC and PCTI both have non-planar saddle-shaped conformations, and PC was found to self-assemble with C60 to produce a uniquely structured supramolecular crystalline phase. In solution, PCTI undergoes eight single-electron reductions, while PC exhibits two reversible oxidations and three reversible reduction events. Chemically generated anions of PC and PCTI showcase extended near-infrared to infrared absorptions, with the lowest energy bands observed at >3200 nm for the PCTI monoanion and ca. 2800 nm for the PCTI dianion. The electronic and redox properties of pentacosacyclenes can be explained using molecular orbital and valence bond theories as originating from changes in the local aromaticity of five- and eight-membered rings.

A Negatively Curved Nanographene with Four Embedded Heptagons

Negatively curved nanographenes are considered as cutouts of three-dimensional fully sp2-hybridized carbon allotropes such as Schwarzites. Here we present the synthesis of a C76 cut-out of the Schwarzite 8-4-1-p proposed by Lenosky et al. and investigate its optical as well as electrochemical properties. Furthermore, supramolecular interactions with fullerenes C60 and C70 were studied.

Thermolysis of Biphenylene toward Cyclo-ortho-phenylenes

The solvent and catalyst free thermolysis of biphenylenes at 350 °C furnishes [n]cyclo-ortho-phenylenes ([n]COPs, n=4-10) in one step and in high yields. At 400 °C biphenylene dimerizes into tetraphenylene, but lower reaction temperatures produce cyclooligomers. If suitably substituted, the oligomers are soluble and can be isolated and characterized. The products are exclusively cyclic. In the crystalline state, [6]COP displays an alternating crown-shaped conformation.

Ring-Expanding Rearrangement of Benzo-Fused Tris-Cycloheptenylenes towards Nonplanar Polycyclic Aromatic Hydrocarbons

A strongly twisted benzo-fused tris-cycloheptenylene, containing three dibenzosuberenone units fused to a common benzene ring, was subjected to Ramirez olefination and subsequent palladium-catalyzed Suzuki-Miyaura cross-coupling with 4-substituted phenylboronic acids. The high steric demand within the overcrowded, benzene-rich benzo-fused tris-cycloheptenylenes enforced an unprecedented 1,2-rearrangement upon π-extension during the Suzuki coupling reaction. According to crystal structure analysis, the resulting negatively curved polycyclic aromatic hydrocarbons consist of two heptagons and one octagon surrounding a central benzene ring as a result of strain release. In the solid state, the materials exhibit a blue to blue-green fluorescence with increased quantum yields and a hypsochromic shift of the emission maxima compared to their respective solutions.

Tridecacyclene Tetraimide: An Easily Reduced Cyclooctatetraene Derivative

Tridecacyclene tetraimide, TCTI, an electron-deficient non-benzenoid nanocarbon with a C56 N4 polycyclic framework was obtained in a concise synthesis. TCTI has a non-planar structure and forms π-stacked dimers in the solid state. In solution, it undergoes eight single-electron reductions, yielding a range of negatively charged states up to an octaanion. Except for the latter species, which has a remarkably large electronic gap, the anions feature extended near-infrared absorptions, with a particularly strong band at 1692 nm observed for the dianion. A computational analysis of the TCTI anions shows that their stability originates from the combined effects of electron-deficient imide groups and the local aromaticity of reduced acenaphthylene units. The properties of TCTI make it potentially useful in electrochromic and charge storage applications.

Nanographenes and Graphene Nanoribbons as Multitalents of Present and Future Materials Science

As cut-outs from a graphene sheet, nanographenes (NGs) and graphene nanoribbons (GNRs) are ideal cases with which to connect the world of molecules with that of bulk carbon materials. While various top-down approaches have been developed to produce such nanostructures in high yields, in the present perspective, precision structural control is emphasized for the length, width, and edge structures of NGs and GNRs achieved by modern solution and on-surface syntheses. Their structural possibilities have been further extended from "flatland" to the three-dimensional world, where chirality and handedness are the jewels in the crown. In addition to properties exhibited at the molecular level, self-assembly and thin-film structures cannot be neglected, which emphasizes the importance of processing techniques. With the rich toolkit of chemistry in hand, NGs and GNRs can be endowed with versatile properties and functions ranging from stimulated emission to spintronics and from bioimaging to energy storage, thus demonstrating their multitalents in present and future materials science.

Synthesis of octagon-containing molecular nanocarbons

Nanocarbons, such as fullerenes, carbon nanotubes, and graphenes, have long inspired the scientific community. In order to synthesize nanocarbon molecules in an atomically precise fashion, many synthetic reactions have been developed. The ultimate challenge for synthetic chemists in nanocarbon science is the creation of periodic three-dimensional (3D) carbon crystals. In 1991, Mackay and Terrones proposed periodic 3D carbon crystals with negative Gaussian curvatures that consist of six- and eight-membered rings (the so-called Mackay-Terrones crystals). The existence of the eight-membered rings causes a warped nanocarbon structure. The Mackay-Terrones crystals are considered a "dream material", and have been predicted to exhibit extraordinary mechanical, magnetic, and optoelectronic properties (harder than diamond, for example). To turn the dream of having this wonder material into reality, the development of methods enabling the creation of octagon-embedding polycyclic structures (or nanographenes) is of fundamental and practical importance. This review describes the most vibrant synthetic achievements that the scientific community has performed to obtain curved polycyclic nanocarbons with eight-membered rings, building blocks that could potentially give access as templates to larger nanographenes, and eventually to Mackay-Terrones crystals, by structural expansion strategies.

Palladium-catalyzed cross-coupling of 2-iodobiphenyls with ortho-chloroacetophenones through dual C–H arylation for the construction of tribenzo[a,c,f]cyclooctanones

A Pd-catalyzed cross-coupling reaction of 2-iodobiphenyls with ortho-chloroacetophenones has been developed through C–H activation. The reaction provides a straightforward method for the construction of tribenzo[a,c,f]cyclooctanones.