Functionalised nanocarbons: Chrome-plated NMR spectroscopy

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  • Published: Feb 1, 2015
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Proton NMR (nuclear magnetic resonance) spectroscopy, high-resolution mass spectrometry and X-ray crystallography have been used to probe chromium-centred cycloparaphenylene organometallic rings. The results could allow chemists to define ways to readily functionalize nanocarbons, according to research published in the Journal of the American Chemical Society. c/o Nagoya

Proton NMR (nuclear magnetic resonance) spectroscopy, high-resolution mass spectrometry and X-ray crystallography have been used to probe chromium-centred cycloparaphenylene organometallic rings. The results could allow chemists to define ways to readily functionalize nanocarbons, according to research published in the Journal of the American Chemical Society.

Natsumi Kubota, Yasutomo Segawa and Kenichiro Itami of Nagoya University, Japan, have synthesized a new type of transition metal-complexed cycloparaphenylene (CPP), which allows them to selectively add functional groups to these ring-shaped structures. The work could lead to a novel way to construct nanocarbon entities, specifically these molecules might be used as building blocks for building functionalised carbon nanotubes.

CPPs consist essentially of a band of benzene rings and are in some sense the shortest possible segment of a carbon nanotube that might exist. They were first synthesised and isolated in 2008, and since then have been the focus of supramolecular chemists and materials scientists hoping to use them as host compounds for smaller molecules or ions or to construct more complex molecular devices, for instance.

Functional bands

Itami and colleagues have now used fundamental chromium arene chemistry to add selectively a functional group on CPP. Until this present work, selective functionalisation has been difficult because there are several reactive arene sites in each CPP ring. The team's skill in installing a functional group on a particular arene and to tune the functional groups points to the possibility of making new types of carbon nanotubes that have additional properties endowed on them by the specifics of the functional groups. This is the first one-pot synthesis, isolation and analysis of a CPP chromium complex, the team reports in JACS.

Arenes are well known for their ability to coordinate to transition metals ions. The resulting metal complexes, of course, exhibit different reactivities when compared to the free arene molecule. The ability to incorporate transition metal ions at specific sites on a CPP thus allows those ring molecules to be fine tuned chemically. The main product the team made was shown by proton NMR spectroscopy to have the chromium group complexed to one arene on the outer side of the CPP ring. High-resolution mass spectrometry and X-ray crystallography were also used to corroborate the results.

"Chromium arene chemistry is a well-established area and we decided to apply this organometallic method to synthesize the first CPP chromium complex," explains Itami, who is Director of the JST-ERATO project and the Institute of Transformative Bio-Molecules at Nagoya. The team had originally anticipated that the chromium would complex to all of the arene rings in the CPP band, but reaction occurs in a ratio of 1 to 1. The team's computer simulation of the molecular structure pointed to the chromium complexation effectively lowering the CPP reactivity below the threshold at which a subsequent arene would react.

Electrophilic choice

Itami explains that the task of complexing a CPP with chromium was not entirely straightforward as the organometallic complexes of this metal are commonly sensitive to both light and air. Natsumi persisted with the syntheses in order to obtain the first sufficiently pure crystal of a CPP chromium complex for a structure determination.

"We were pleased to see that a functional group could be selectively installed on one arene ring via chromium coordination of CPPs," explains team member Segawa. "We utilized silyl, boryl and ester groups as electrophiles, which act as handles that can be easily transformed to other useful functionalities."

Related Links

J Am Chem Soc,  2015, 137, 1356-1361: "η6‑Cycloparaphenylene Transition Metal Complexes: Synthesis, Structure, Photophysical Properties, and Application to the Selective Monofunctionalization of Cycloparaphenylenes"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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