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Nature Chemistry | Article

Efficient room-temperature synthesis of a highly strained carbon nanohoop fragment of buckminsterfullerene

Journal name:
Nature Chemistry
Volume:
6,
Pages:
404–408
Year published:
DOI:
doi:10.1038/nchem.1888
Received
Accepted
Published online

Abstract

Warped carbon-rich molecules have captured the imagination of scientists across many disciplines. Owing to their promising materials properties and challenging synthesis, strained hydrocarbons are attractive targets that push the limits of synthetic methods and molecular design. Herein we report the synthesis and characterization of [5]cycloparaphenylene ([5]CPP), a carbon nanohoop that can be envisaged as an open tubular fragment of C60, the equator of C70 fullerene and the unit cycle of a [5,5] armchair carbon nanotube. Given its calculated 119 kcal mol−1 strain energy and severely distorted benzene rings, this synthesis, which employs a room-temperature macrocyclization of a diboronate precursor, single-electron reduction and elimination, is remarkably mild and high yielding (27% over three steps). Single-crystal X-ray diffraction data were obtained to confirm its geometry and previously disputed benzenoid character. First and second pseudoreversible oxidation and reduction events were observed via cyclic voltammetry. The ease of synthesis, high solubility and narrowest optical HOMO/LUMO gap of any para-polyphenylene synthesized make [5]CPP a desirable new material for organic electronics.

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  1. Prototypical distressed hydrocarbons that have succumbed to rational organic synthesis.
    Figure 1: Prototypical distressed hydrocarbons that have succumbed to rational organic synthesis.

    a, Classic examples of strained hydrocarbons that highlight the key techniques used to overcome the kinetic barrier during synthesis. b, The recently conquered small CPPs and trends in physical properties thereof.

  2. A rapid synthetic route to [5]CPP.
    Figure 2: A rapid synthetic route to [5]CPP.

    Our synthesis proceeds from the previously reported diboronate 1, which is macrocyclized to give 2 (shown here as a DFT structure with relevant π interactions highlighted) via an intramolecular boronate homocoupling open to air. On treatment with sodium naphthalenide, 2 forms a dianion that does not convert into [5]CPP at −78 °C, but can be protonated by adding methanol to yield the reduced macrocycle 3 (shown as a refined crystal structure that confirms the regioselectivity of reduction). Reaction with LDA at room temperature effects a twofold E1cB elimination to give 195 mg (69%) [5]CPP, shown as a refined crystal structure.

  3. Analysis of [5]CPP by X-ray diffraction crystallography.
    Figure 3: Analysis of [5]CPP by X-ray diffraction crystallography.

    a, Solid-state bond lengths (ORTEP ellipsoids displayed at 50% probability) confirm the benzenoid nature of the phenyl rings in [5]CPP, despite the extreme bending out of plane (b). b, [5]CPP packs in a herringbone fashion, unlike [6]CPP but similar to all other CPPs for which crystal data have been obtained.

  4. Ultraviolet–visible absorbance data for [5]CPP.
    Figure 4: Ultraviolet–visible absorbance data for [5]CPP.

    [5]CPP exhibits a major ultraviolet absorbance similar to that of other CPPs (1 µg ml−1, λmax = 335 nm). Inset: As apparent by its red colour, a minor green absorbance from a HOMO–LUMO transition is also present (0.1 mg ml−1, λmax = 502 nm). a.u., arbitrary units.

  5. Cyclic voltammetry of [5]CPP in THF.
    Figure 5: Cyclic voltammetry of [5]CPP in THF.

    a,b, Two oxidation events (a) and two reduction events (b) were observed for [5]CPP. All these appeared to be pseudoreversible. [5]CPP has a lower oxidation potential and less-negative reduction potential than those of any CPP.

Compounds

4 compounds View all compounds
  1. 2,2'-((1'S,1'''s,4's,4'''S)-1',1''',4',4'''-Tetramethoxy-1',1''',4',4'''-tetrahydro-[1,1':4',1'':4'',1''':4''',1''''-quinquephenyl]-4,4''''-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)
    Compound 1 2,2'-((1'S,1'''s,4's,4'''S)-1',1''',4',4'''-Tetramethoxy-1',1''',4',4'''-tetrahydro-[1,1':4',1'':4'',1''':4''',1''''-quinquephenyl]-4,4''''-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)
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  2. (31S,34s,51S,54s)-31,34,51,54-Tetramethoxy-1,2,4(1,4)-tribenzena-3,5(1,4)-dicyclohexanacyclopentaphane-32,35,52,55-tetraene
    Compound 2 (31S,34s,51S,54s)-31,34,51,54-Tetramethoxy-1,2,4(1,4)-tribenzena-3,5(1,4)-dicyclohexanacyclopentaphane-32,35,52,55-tetraene
    View:
  3. (31R,34r,51R,54r)-34,54-Dimethoxy-1,2,4(1,4)-tribenzena-3,5(1,4)-dicyclohexanacyclopentaphane-32,35,52,55-tetraene
    Compound 3 (31R,34r,51R,54r)-34,54-Dimethoxy-1,2,4(1,4)-tribenzena-3,5(1,4)-dicyclohexanacyclopentaphane-32,35,52,55-tetraene
    View:
  4. [5]Cycloparaphenylene
    Compound [5]CPP [5]Cycloparaphenylene
    View:

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Author information

  1. These authors contributed equally

    • Paul J. Evans &
    • Evan R. Darzi

Affiliations

  1. Department of Chemistry and Division of Materials Science and Engineering, Boston University, Boston, Massachusetts 02215, USA

    • Paul J. Evans,
    • Evan R. Darzi &
    • Ramesh Jasti

Contributions

E.R.D. and P.J.E. conceived the project, designed and carried out the experiments, performed the calculations, analysed the data and wrote the manuscript with equal contribution. As such, E.R.D. and P.J.E. are credited as co-first authors. R.J. played a critical role in discussion of the experimental design, project direction, experiments and results, and preparation of the manuscript.

Competing financial interests

The authors declare no competing financial interests.

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Supplementary information

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    Supplementary information

Crystallographic information files

  1. Supplementary information (1,176 KB)

    Crystallographic data for compound 3

  2. Supplementary information (133 KB)

    Crystallographic data for compound [5]CPP

Additional data