Haruka Omachi

Selective Synthesis of [12]Cycloparaphenylene

Shape-persistent, nanosized macrocycles that are composed of only spand sp-hydridized carbon atoms on their perimeter (conjugated molecular loops and belts) have attracted significant attention because of their potential applications in materials science and supramolecular chemistry. 2] Particularly interesting and challenging among these are aromatic belts and rings as exemplified by the V gtle belts, cyclophenacenes, 6] cycloparaphenylenes, and cyclacenes [10, 11] . Adding to their sheer aesthetic appeal, these unusual hydrocarbons represent structural models for carbon nanotube segments, and can be envisioned as potential precursors in the preparation of structurally uniform armchair or zigzag carbon nanotubes (Scheme 1). However, despite extensive trials the synthesis of these molecules remains a formidable challenge. In 2005, as a part of our research program exploring new synthetic methods and properties of oligoarenes and nanocarbons, we initiated a synthetic study of these aromatic belts/rings that aims at contributing to a bottom-up organic synthesis of structurally uniform single-walled carbon nanotubes. We selected cycloparaphenylene as a first target in view of a comparatively straightforward approach to their synthesis through aryl–aryl bond formation. Despite its structural simplicity, no successful synthesis had been reported at the inception of our work. Very recently, Bertozzi and co-workers have accomplished the elegant first synthesis of [9]-, [12]-, and [18]cycloparaphenylenes and coined them as “carbon nanohoops”. Herein, we report a selective synthesis of [12]cycloparaphenylene (1) through stepwise palladiumcatalyzed coupling reactions.

Synthesis of Cycloparaphenylenes and Related Carbon Nanorings: A Step toward the Controlled Synthesis of Carbon Nanotubes

Since their discovery in 1991, carbon nanotubes (CNTs) have attracted significant attention because of their remarkable mechanical, electronic, and optical properties. Structural uniformity of the CNT is critically important because the sidewall structures (armchair, zigzag, and chiral) determine many of the significant properties of CNTs. Ideally researchers would synthesize CNTs with a defined target sidewall structure and diameter, but the current synthetic methods, such as arc discharge and chemical vapor deposition, only provide CNTs as the mixtures of various structures. Purification of these mixtures does not allow researchers to isolate a structurally uniform CNT, which is the bottleneck for fundamental studies and advanced applications of these materials. Therefore, the selective and predictable synthesis of structurally uniform CNTs would represent a critical advance in both nanocarbon science and synthetic chemistry. This Account highlights our efforts toward the bottom-up synthesis of structurally uniform carbon nanotubes (CNTs). We envisioned a bottom-up synthesis of structurally uniform CNTs through a controlled growth process from a short carbon nanoring (template) that corresponds to the target structure of CNTs. Our simple retrosynthetic analysis led to the identification of cycloparaphenylenes (CPPs), acene-inserted CPPs, and cyclacenes as the shortest sidewall segments of armchair, chiral, and zigzag CNTs, respectively. With this overall picture in mind, we initiated our synthetic studies of aromatic rings/belts as an initial step toward structurally uniform CNTs in 2005. This research has led to (i) a general strategy for the synthesis of CPPs and related carbon nanorings using cyclohexane derivatives as a benzene-convertible L-shaped unit, (ii) a modular, size-selective, and scalable synthesis of [n]CPPs (a shortest segment of armchair CNTs), (iii) the X-ray crystal structure analysis of CPPs, (iv) the design and synthesis of acene-inserted CPPs as the shortest segment of chiral CNTs, and (v) the first synthesis of cyclo-1,4-naphthylene, a π-extended CPP. We believe this work will serve as important initial steps toward a controlled synthesis of CNTs.

Concise Synthesis and Crystal Structure of [12]Cycloparaphenylene

bottom-up chemical synthesis of this simple molecular entity had been recognized as a Holy Grail in synthetic chemistry, three groups including our own have recently succeeded in synthesizing some [n]CPPs. Although these studies from the three research groups established the synthetic viability of the long-awaited CPPs, important issues remain unresolved (Scheme 1). For example, any synthetic route must be more concise, cost-effective, and scalable to provide CPP in useful quantities and to ensure that this interesting molecular entity is studied further. In addition, the molecular structure of CPP must be concretely validated by X-ray crystallographic analysis. We herein report a concise nickel-based synthesis of [12]CPP and the first X-ray crystal structure of [12]CPP. Some of the key features of the previous methods of making CPPs are summarized in Scheme 2. Both the group of Bertozzi and ours utilized the palladium-catalyzed Suzuki–Miyaura coupling of terphenyl-convertible bent

A Modular and Size‐Selective Synthesis of [n]Cycloparaphenylenes: A Step toward the Selective Synthesis of [n,n] Single‐Walled Carbon Nanotubes

the groups led by Bertozzi, Itami, 7] and Yamago have finally accomplished the bottom-up organic synthesis of some [n]CPPs (n = 8, 9, 12, 18). Although several possible routes toward CPP structures have been identified in these studies, a uniform strategy allowing the flexible and size-selective synthesis of a range of [n]CPPs has yet to be developed. Devising such a strategy is important in view of the expectation that CPP could serve as a precursor or seed in the preparation of structurally uniform armchair single-walled carbon nanotubes (SWNTs) (Scheme 1). 9] As proposed and demonstrated by others, the amplification growth strategy using a relatively short hydrocarbon template such as CPP holds the promise of a long-awaited selective synthesis of SWNTs. In such a strategy, a modular and size-selective synthesis of [n]CPPs would be critically important in providing [n,n]SWNTs in a controlled and selective fashion. We now describe a modular and size-selective synthetic approach to [n]CPPs (n 14) and report the synthesis of [14]-, [15]-, and [16]CPP as a proof-of-principle of our new strategy. Our CPP synthesis is very flexible, assembling bent and linear building blocks in a controlled and programmable manner. In essence, we previously synthesized [12]CPP in a 3+3+3+3 mode using a terphenyl-equivalent L-shaped diphenylcyclohexane unit; Pd-catalyzed stepwise formation of a “box” (3+3+3 and then 9+3), followed by aromatization (Scheme 2). By strategically varying the number of bent

Synthesis and racemization process of chiral carbon nanorings: a step toward the chemical synthesis of chiral carbon nanotubes.

A simple and realistic model for the shortest sidewall segments of chiral single-walled carbon nanotubes (SWNTs) has been designed, and one of the chiral carbon nanorings, cyclo[13]paraphenylene-2,6-naphthylene ([13]CPPN, 1) has been successfully synthesized. DFT calculations reveal that the racemization energy of 1 is 8.4 kcal·mol(-1). In addition, some important energetic values, such as racemization barriers and strain energies, of other chiral carbon nanorings have been systematically estimated for future molecular design.

Size-selective synthesis of [9]–[11] and [13]cycloparaphenylenes

The first size-selective synthesis of [9]–[11] and [13]cycloparaphenylenes (CPP) has been achieved by strategically utilizing cis-1,4-diphenylcyclohexane-1,4-diyl as the key terphenyl-convertible L-shaped unit. To access the designed triangular or rectangular macrocyclic precursors, we utilized palladium-catalysed C–B/C–Br cross-coupling (Suzuki–Miyaura coupling) and/or nickel-mediated C–Br/C–Br coupling. We also established step-economical routes to [14] and [16]CPP using nickel-mediated C–Br/C–Br coupling. The final aromatization steps toward CPPs were accomplished with NaHSO4. Thus, combined with our previous size-selective synthesis of [12] and [14]–[16]CPP, we completed our size-selective synthesis of [9]–[16]CPP. The successful size-selective syntheses of [n]CPPs speak well for the flexibility and reliability of our strategy using a cyclohexane ring.

Size-selective complexation and extraction of endohedral metallofullerenes with cycloparaphenylene.

A new strategy for the non-chromatographic extraction of metallofullerenes from solutions of arc-processed raw soot is based on the size-selective complexation with cycloparaphenylene (CPP). [11]CPP has a high affinity for Mx @C82 (x=1, 2); for example, Gd@C82 can be selectively extracted from a fullerene mixture by the addition of [11]CPP. This approach should open new opportunities in metallofullerene chemistry, including for the bulk extraction of metallofullerenes.

Isolation and Structure Determination of a Missing Endohedral Fullerene La@C70 through In Situ Trifluoromethylation

D5h-symmetric fullerene C70 (D5h-C70) is one of the most abundant members of the fullerene family. One longstanding mystery in the field of fullerene chemistry is whether D5h-C70 is capable of accommodating a rare-earth metal atom to form an endohedral metallofullerene M@D5h-C70, which would be expected to show novel electronic properties. The molecular structure of La@C70 remains unresolved since its discovery three decades ago because of its extremely high instability under ambient conditions and insolubility in organic solvents. Herein, we report the single-crystal X-ray structure of La@C70(CF3)3, which was obtained through in situ exohedral functionalization by means of trifluoromethylation. The X-ray crystallographic study reveals that La@C70(CF3)3 is the first example of an endohedral rare-earth fullerene based on D5h-C70. The dramatically enhanced stability of La@C70(CF3)3 compared to La@C70 can be ascribed to trifluoromethylation-induced bandgap enlargement.

Fabrication and Optical Probing of Highly Extended, Ultrathin Graphene Nanoribbons in Carbon Nanotubes

Nanotemplated growth of graphene nanoribbons (GNRs) inside carbon nanotubes is a promising mean to fabricate ultrathin ribbons with desired side edge configuration. We report the optical properties of the GNRs formed in single-wall carbon nanotubes. When coronene is used as the precursor, extended GNRs are grown via a high-temperature annealing at 700 °C. Their optical responses are probed through the diazonium-based side-wall functionalization, which effectively suppresses the excitonic absorption peaks of the nanotubes without damaging the inner GNRs. Differential absorption spectra clearly show two distinct peaks around 1.5 and 3.4 eV. These peaks are assigned to the optical transitions between the van Hove singularities in the density of state of the GNRs in qualitative agreement with the first-principles calculations. Resonance Raman spectra and transmission electron microscope observations also support the formation of long GNRs.

Template Synthesis of Linear-Chain Nanodiamonds Inside Carbon Nanotubes from Bridgehead-Halogenated Diamantane Precursors.

A simple method for the synthesis of linear-chain diamond-like nanomaterials, so-called diamantane polymers, is described. This synthetic approach is primarily based on a template reaction of dihalogen-substituted diamantane precursors in the hollow cavities of carbon nanotubes. Under high vacuum and in the presence of Fe nanocatalyst particles, the dehalogenated radical intermediates spontaneously form linear polymer chains within the carbon nanotubes. Transmission electron microscopy reveals the formation of well-aligned linear polymers. We expect that the present template-based approach will enable the synthesis of a diverse range of linear-chain polymers by choosing various precursor molecules. The present technique may offer a new strategy for the design and synthesis of one-dimensional nanomaterials.