An Electron-Deficient CpE Iridium(III) Catalyst: Synthesis, Characterization, and Application to Ether-Directed C-H Amidation.

The synthesis, characterization, and catalytic performance of an iridium(III) catalyst with an electron-deficient cyclopentadienyl ligand ([CpE IrI2 ]2 ) are reported. The [CpE IrI2 ]2 catalyst was synthesized by complexation of a precursor of the CpE ligand with [Ir(cod)OAc]2 , followed by oxidation, desilylation, and removal of the COD ligand. The electron-deficient [CpE IrI2 ]2 catalyst enabled C-H amidation reactions assisted by a weakly coordinating ether directing group. Experimental mechanistic studies and DFT calculations suggested that the high catalytic performance of [CpE IrI2 ]2 is due to its electron-deficient nature, which accelerates both C-H activation and IrV -nitrenoid formation.

Double-Helix Supramolecular Nanofibers Assembled from Negatively Curved Nanographenes.

The layered structures of graphite and related nanographene molecules play key roles in their physical and electronic functions. However, the stacking modes of negatively curved nanographenes remain unclear, owing to the lack of suitable nanographene molecules. Herein, we report the synthesis and one-dimensional supramolecular self-assembly of negatively curved nanographenes without any assembly-assisting substituents. This curved nanographene self-assembles in various organic solvents and acts as an efficient gelator. The formation of nanofibers was confirmed by microscopic measurements, and an unprecedented double-helix assembly by continuous π-π stacking was uncovered by three-dimensional electron crystallography. This work not only reports the discovery of an all-sp2-carbon supramolecular π-organogelator with negative curvature but also demonstrates the power of three-dimensional electron crystallography for the structural determination of submicrometer-sized molecular alignment.

Stepwise Generation of Mono-, Di-, and Triply-Reduced Warped Nanographenes: Charge-Dependent Aromaticity, Surface Nonequivalence, Swing Distortion, and Metal Binding Sites.

The stepwise chemical reduction of a molecular warped nanographene (WNG) having a negatively curved π-surface and defined C80 H30 composition with Cs metal used as the reducing and complexing agent allowed the isolation of three different reduced states with one, two, and three electrons added to its π-conjugated system. This provided a unique series of nanosized carbanions with increasing negative charge for in-depth structural analysis of consequences of controlled electron charging of non-planar nanographenes, using X-ray crystallographic and computational tools. The 3D molecular electrostatic potential (MEP) maps identified the negative charge localization at the central part of the WNG surface where selective coordination of Cs+ ions is confirmed crystallographically. In-depth theoretical investigation revealed a complex response of the WNG to the stepwise electron acquisition. The extended and contorted π-surface of the WNG undergoes subtle swinging distortions that are accompanied by notable changes in the electronic structure and site-dependent aromaticity of the resulting carbanions.

A Nonalternant Aromatic Belt: Methylene-Bridged [6]Cycloparaphenylene Synthesized from Pillar[6]arene.

The synthesis, structure, and properties of methylene-bridged [6]cycloparaphenylene ([6]CPP), a nonalternant aromatic belt, are described. This belt-shaped methylene-bridged [6]CPP, in which each phenylene unit is tethered to its neighbors by methylene bridges, was constructed through 6-fold intramolecular nickel-mediated aryl-aryl coupling of triflate-functionalized pillar[6]arene in 18% isolated yield. As compared to the analogous [6]CPP, the methylene bridges coplanarize neighboring paraphenylene units and enhance the degree of π-conjugation, which results in a significant decrease in energy gap. Moreover, the incorporation of small molecules in the defined pocket of methylene-bridged [6]CPP makes it an attractive supramolecular architecture. Methylene-bridged [6]CPP is characterized by high internal strain energy reaching 110.2 kcal mol-1, attributed to its restricted structure. This work not only exhibits an efficient strategy to construct a new family of aromatic belt, but also showcases their properties, which combine the merits of CPPs and pillararenes.

Topologically Unique Molecular Nanocarbons.

As new forms of carbon are unearthed, they invariably transform the scientific landscape. Numerous researchers have been inspired to discover the unique characteristics of these fascinating materials, consistently leading to the development of important technological innovations in materials science. Recently, studies on the preparation of molecular nanocarbons (small molecule analogues of larger carbon nanostructures) by precision organic synthesis have attracted much attention. Cycloparaphenylene (CPP), the substructure of carbon nanotubes (CNTs), is the oldest of such organic molecules, and since 2008 the successful synthesis of CPP dramatically advanced the synthetic chemistry of molecular nanocarbons. In fact, as pioneering research, we succeeded in producing carbon nanotubes using seed CPP molecules in 2013. This method represented an important landmark in the quest for controlling the diameter of CNTs via utilization of a well-defined small molecule as a template. Other avenues of research on graphene nanoribbons and partial structures of fullerenes such as corannulene and sumanene are also highly active at the current time. On the other hand, carbon forms with nontrivial topologies, i.e., topological nanocarbons, are virtually unexplored. In addition to the 3D network structures represented by the Mackay crystal, many topologically complex structures have been envisioned. To date, there is no rational approach toward the bottom-up synthesis of these carbon structures. As with the case of fullerenes and CNTs, access to these unique carbon structures should undoubtedly revolutionize a wide range of sciences. This Account highlights our efforts toward the synthesis of topologically unique molecular nanocarbons. Starting from CPP as the topologically simple subunit, we have successfully created novel molecular nanocarbons that have more complexed topologies. The first topic is carbon nanobelts, fully fused cylinder-shaped molecular nanocarbons representing the segment structure of armchair-type CNTs. The second topic is carbon nanocages, molecular nanocarbons having a "three-holed" topology representing the joint unit of branched CNTs. The third and fourth topics are all-benzene catenanes consisting of two CPP rings and an all-benzene trefoil knot topologically related to a carbon nanotorus. The world of nanocarbon molecules is only limited by our imagination and creativity. As history has proved, the synthesis of new forms of carbon and topologically complex molecules has always subsequently led to new fields and applications associated with their unforeseen properties and functions.

Synthetic Strategies of Carbon Nanobelts and Related Belt-Shaped Polycyclic Aromatic Hydrocarbons.

The development of carbon nanobelts and related belt-shaped polycyclic aromatic hydrocarbons has gained momentum in recent years. This Minireview focuses on the synthetic strategies used in constructing these aesthetically appealing molecular nanocarbons. Examples of carbon nanobelts and related belt-shaped polycyclic aromatic hydrocarbons reported in recent years as well as some representative synthetic attempts in earlier times are discussed.

Six-fold C-H borylation of hexa-peri-hexabenzocoronene.

Hexa-peri-hexabenzocoronene (HBC) is known to be a poorly soluble polycyclic aromatic hydrocarbon for which direct functionalization methods have been very limited. Herein, the synthesis of hexaborylated HBC from unsubstituted HBC is described. Iridium-catalyzed six-fold C-H borylation of HBC was successfully achieved by screening solvents. The crystal structure of hexaborylated HBC was confirmed via X-ray crystallography. Optoelectronic properties of the thus-obtained hexaborylated HBC were analyzed with the support of density functional theory calculations. The spectra revealed a bathochromic shift of absorption bands compared with unsubstituted HBC under the effect of the σ-donation of boryl groups.

Polycyclic Arene Synthesis by Annulative π-Extension.

Annulative π-extension (APEX) has emerged as a powerful and efficient synthetic method for the construction of polycyclic aromatic hydrocarbons, nanographenes, and π-extended fused heteroarenes. In contrast to classical multistep syntheses requiring substrate prefunctionalization, APEX reactions minimize the number of preparative steps by direct C-H activation of small aromatic templates. In this Perspective, we review recently reported APEX reactions to shed light on the utility, scope, and promise of this approach for next-generation syntheses of polycyclic arenes.

Negatively Curved Warped Nanographene Self-Assembled on Metal Surfaces.

We report the investigation of a conjugated polycyclic hydrocarbon containing multiple nonbenzenoid rings and exhibiting negative curvature-the warped nanographene C80H30-adsorbed on several noble metal surfaces in an ultrahigh vacuum environment. From a detailed analysis of the molecular self-assembly at different molecular coverages via scanning tunneling microscopy and spectroscopy measurements in combination with theoretical modeling, the nature of the intermolecular interactions is unraveled. For high molecular coverages on Cu(111), the formation of homochiral porous networks is observed, which is rationalized by (i) intermolecular π-π interactions between neighboring C80H30 molecules that promote the formation of molecular dimers and (ii) enantioselective intermolecular CH···π interactions between the dimers. Such interactions are also observed after deposition of C80H30 molecules on Au(111) and Ag(111) substrates. Our results provide perspectives for the on-surface study of negatively curved nanographenes which open new avenues to the design of novel and functional chiral structures with potential use in the field of organic optoelectronics.

Ni-Catalyzed α-Selective C-H Borylations of Naphthalene-Based Aromatic Compounds.

The ability of α-borylated naphthalene-based aromatic compounds is important because it provides ready access to interesting novel extended π-systems. In this report, we disclose the Ni-catalyzed α-selective C-H borylations of naphthalene-based aromatic compounds. The reaction proceeds with peri-xanthenoxanthene and other aromatic compounds to regioselectively afford the α-borylated products without directing groups. The selectivities of these transformations are different from those of Ir-catalyzed C-H borylation reactions and can be used in a complementary manner.

Synthesis and Size-Dependent Properties of [12], [16], and [24]Carbon Nanobelts.

The synthesis and X-ray crystal structure of the first member of the carbon nanobelt family is reported. [12]Carbon nanobelt ([12]CNB) was originally obtained from a nickel-mediated reductive coupling reaction of a dodecabrominated macrocyclic precursor, albeit only in 1% yield. The present article reports on the development of this synthetic strategy and its extension to the preparation of the [16] and [24]CNB analogues. In particular, our extensive investigations on the final belt-forming, nickel-mediated reaction led to the development of a new ligand system that provides [12]CNB in up to 7% yield, contributing to the commercialization of [12]CNB. The belt structures of [12], [16], and [24]CNB were characterized by NMR, UV-vis, and Raman spectroscopy as well as mass spectrometry and X-ray crystallography. The fluorescence of the CNBs in solution displayed a remarkable dependence on the ring size, ranging from a broad red emission ([12]CNB) to a narrow-band blue emission ([24]CNB), while both features are observed for [16]CNB.

A Quintuple [6]Helicene with a Corannulene Core as a C5 -Symmetric Propeller-Shaped π-System.

The synthesis and structural analysis of a quintuple [6]helicene with a corannulene core is reported. The compound was synthesized from corannulene in three steps including a five-fold intramolecular direct arylation. X-ray crystallographic analysis revealed a C5 -symmetric propeller-shaped structure and one-dimensional alignment in the solid state. The enantiomers of the quintuple [6]helicene were successfully separated by HPLC, and the chirality of the two fractions was identified by CD spectroscopy. A kinetic study yielded a racemization barrier of 34.2 kcal mol-1 , which is slightly lower than that of pristine [6]helicene. DFT calculations indicate a rapid bowl-to-bowl inversion of the corannulene moiety and a step-by-step chiral inversion pathway for the five [6]helicene moieties.

A Water-Soluble Warped Nanographene: Synthesis and Applications for Photoinduced Cell Death.

Nanographene, a small piece of graphene, has attracted unprecedented interest across diverse scientific disciplines particularly in organic electronics. The biological applications of nanographenes, such as bioimaging, cancer therapies and drug delivery, provide significant opportunities for breakthroughs in the field. However, the intrinsic aggregation behavior and low solubility of nanographenes, which stem from their flat structures, hamper their development for bioapplications. Herein, we report a water-soluble warped nanographene (WNG) that can be easily synthesized by sequential regioselective C-H borylation and cross-coupling reactions of the saddle-shaped WNG core structure. The saddle-shaped structure and hydrophilic tetraethylene glycol chains impart high water solubility to the WNG. The water-soluble WNG possesses a range of promising properties including good photostability and low cytotoxicity. Moreover, the water-soluble WNG was successfully internalized into HeLa cells and promoted photoinduced cell death.

Laterally π-Extended Dithia[6]helicenes with Heptagons: Saddle-Helix Hybrid Molecules.

A laterally π-extended dithia[6]helicene 1, representing an interesting saddle-helix hybrid molecule containing an unusual heptagon, has been synthesized by MoCl5-mediated oxidative stitching of tetrakis(thienylphenyl)naphthalene precursor 2 involving reactive-site capping by chlorination and subsequent Pd-mediated dechlorination of tetrachlorinated intermediate 1-Cl4. Highly distorted, wide helical structures of dithia[6]helicenes (1 and 1-Cl4) were clarified by single-crystal X-ray diffraction analyses where heterochiral slipped π-π stacking was displayed in a one-dimensional fashion. Notably, theoretical studies on the thermodynamic behavior of 1 predicted an extraordinarily high isomerization barrier of 49.7 kcal·mol-1, which enabled optical resolution and chiroptical measurements. Electronic structures of these huge helicenes were also examined by photophysical and electrochemical measurements.

Synthesis, properties, and crystal structures of π-extended double [6]helicenes: contorted multi-dimensional stacking lattice.

The synthesis and properties of a new π-extended double [6]helicene 2 and a dithia[6]helicene 3 are described. Compared to the previously reported parent double-helicene molecule 1, the introduction of n-butyl groups successfully improved the solubility, which allowed an experimental investigation into the electronic structure of 2 and 3 by photophysical measurements and cyclic voltammetry. The characteristic two-blade propeller structures of 2 and 3 were unambiguously determined by single-crystal X-ray diffraction analysis. The crystal packing structure of 2 exhibited a contorted two-dimensional stacking, whereby molecules of n-pentane were incorporated in the stacks. Despite the presence of n-butyl groups, 3 formed a unique three-dimensional stacking lattice in the crystal. Time-resolved microwave conductivity measurements revealed that the double helicenes (1-3) exhibited transient conductivities. An organic field-effect transistor fabricated using 3 was found to function as a p-type transistor.

Synthesis and Structural Features of Quadruple Helicenes: Highly Distorted π Systems Enabled by Accumulation of Helical Repulsions.

Quadruple helicenes, bearing dithia[6]helicene and [5]helicene substructures, were prepared by a well-controlled Scholl reaction. The 4-fold helicity provides 9 stereoisomers including 4 pairs of enantiomers and 1 meso isomer. Among them, differently distorted structures of a propeller-shaped isomer (QH-A) and a saddle-shaped isomer (QH-B) were unambiguously determined by X-ray crystallography. Especially in the latter isomer, a proper accumulation of repulsions on the helical substructures twisted the naphthalene core to the limit (69.5°), the highest degree of twisting deformation per benzene unit (35.3° at the most). Photophysical and electrochemical studies showed a broadened HOMO-LUMO gap and a HOMO of QH-B lying lower compared to those of QH-A. These results together with the density functional theory (DFT) calculations have clearly demonstrated the electronic state dependency on the molecular geometry. Additionally, kinetic studies of the isomerization between these isomers using (1)H NMR, circular dichroism, and DFT calculations shed light on the interconversion pathways among the stereoisomers. The height of barriers in the inversion of a certain helical substructure may be affected by the neighboring helical substructures.

Thiophene-Fused π-Systems from Diarylacetylenes and Elemental Sulfur.

A simple yet effective method for the formation of thiophene-fused π-systems is reported. When arylethynyl-substituted polycyclic arenes were heated in DMF in the presence of elemental sulfur, the corresponding thiophene-fused polycyclic arenes were obtained via cleavage of the ortho-C-H bond. Thus, arylethynylated naphthalenes, fluoranthenes, pyrenes, corannulenes, chrysenes, and benzo[c]naphtho[2,1-p]chrysenes were effectively converted into the corresponding thiophene-fused π-systems. Apart from polycyclic hydrocarbons, thiophene derivatives are also susceptible to this reaction. The practical utility of this reaction is demonstrated by preparations on the decagram scale, one-pot two-step reaction sequences, and multiple thiophene annulations.

Design and Synthesis of Carbon Nanotube Segments.

The selective and predictable synthesis of structurally uniform carbon nanotubes (CNTs) represents a long-standing goal in both nanocarbon science and synthetic organic chemistry. This Review focuses on synthetic studies toward the controlled synthesis of CNTs with single chirality through the organic synthesis of CNT segments and the organic template assisted growth of CNTs.

Synthesis, Structures, and Properties of π-Extended Double Helicene: A Combination of Planar and Nonplanar π-Systems.

The synthesis, structures, and properties of a π-extended double helicene 1 are described. This double helicene 1 was synthesized by a four-fold oxidative C-H biphenylation of naphthalene followed by the Scholl reaction or via five steps including the Suzuki-Miyaura cross-coupling reaction and the Scholl reaction. Due to the two helical substructures, 1 has three isomers, i.e., two enantiomers in a twisted form [(P,P) and (M,M)] and one diastereoisomer in a meso form. X-ray crystallographic analysis of the twisted isomers (twisted-1) revealed a tightly offset packing pattern of (P,P)- and (M,M)-twisted isomers, affording a three-dimensional lamellar stacking structure. A high isomerization barrier (43.5 kcal mol(-1)) and the relative thermal stability of twisted-1 isomer over meso-1 by 0.9 kcal mol(-1) were estimated by DFT calculations. The three isomers were successfully separated by chiral HPLC and characterized by circular dichroism spectroscopy as well as by TD-DFT studies. Electronic state variation resulting from the molecular geometry difference between the two diastereoisomers (twisted-1 and meso-1) was observed by UV-vis absorption and fluorescence spectra.

η6-Cycloparaphenylene transition metal complexes: synthesis, structure, photophysical properties, and application to the selective monofunctionalization of cycloparaphenylenes.

The synthesis, structure, photophysical properties, and reactivity of cycloparaphenylenes (CPPs) coordinated to group 6 transition metal fragments are described. The η(6)-coordination of [9]CPP or [12]CPP with M(CO)6 (M = Cr, Mo, W) afforded the corresponding [n]CPP-M(CO)3 complexes (n = 9, 12; M = Cr, Mo, W). In the (1)H NMR spectra of these complexes, characteristic upfield-shifted singlet signals corresponding to the four hydrogen atoms attached to the coordinated C6H4 ring of the CPPs were observed at 5.4-5.9 ppm. The complex [9]CPP-Cr(CO)3 could be successfully isolated in spite of its instability. X-ray crystallographic analysis and computational studies of [9]CPP-Cr(CO)3 revealed that chromium-CPP coordination occurs at the convex surface of [9]CPP both in the solid state and in solution. TD-DFT calculations suggested that the emerging high-wavenumber absorption peak upon coordination of [9]CPP to Cr(CO)3 should be assigned to a weak HOMO-LUMO transition. Moreover, by using the complex [9]CPP-Cr(CO)3, a rapid and highly monoselective CPP functionalization has been achieved. The established one-pot method, consisting of complexation, deprotonation, nucleophilic substitution, and decomplexation steps, yielded silyl-, boryl-, and methoxycarbonyl-substituted CPPs in up to 93% yield relative to reacted starting material.