Chemists at the Nagoya University in Japan have synthesized novel cycloparaphenylene (CPP) chromium complexes and have shown the possibility of using them for obtaining monofunctionalized CPPs, which could help construct nanocarbons with unprecedented structures.

This image shows a one-pot selective monofunctionalization of CPP via a chromium complex. Credit: ITbM, Nagoya University The team comprising Professor Kenichiro Itami, Natsumi Kubota and Yasutomo Segawa have synthesized the novel CPPs. The researchers are from the JST-ERATO Itami Molecular Nanocarbon Project and the Institute of Transformative Bio-Molecules (ITbM) at Nagoya University.

This project enables selective monofunctionalization of CPPs, which is the first time that this has been achieved. This could help lead to the construction of carbon nanotubes with unprecedented structures. CPPs are the shortest segment of carbon nanotubes and they are made up of a chain of benzene rings. CPPs had been synthesized and isolated in 2008, and since then they have created significant interest in the fields of supramolecular chemistry and materials science.

Itami, along with his coworkers, applied the fundamental concepts of chromium arene chemistry, and conducted the first specific installation of a functional group on CPP. Prior to this, it had been very difficult to achieve as the CPP ring had multiple reactive arene sites. It is being considered that in the future, carbon nanotubes having new properties could be constructed using this method, by selectively installing and tuning the CPP’s functional groups.

The first synthesis, analysis and isolation of a CPP chromium complex have been demonstrated by this study. A one-pot access to monofunctionalized CPPs has been enabled by this achievement, and the outcome obtained is considered to be a significant advance in organometallic chemistry and CPP chemistry. Arenes coordinate to transition metals. The corresponding metal complexes that form demonstrate different reactivities when considered with reference to the free arene. CPPs are made up of a chain of arenes, and when they react with chromium carbonyl, CPPs first chromium complex was generated. An interesting part of the reaction was a CPP as a main product with one chromium moiety that was complexed to one arene on the rings outer side. This was confirmed using X-ray crystallography, high-resolution
mass spectrometry and 1H NMR (nuclear magnetic resonance) spectroscopy.

When the researchers found that a monometallic CPP complex could be realized, Itamis team analyzed the possibility of acquiring monofunctionalized CPPs from this CPP complex. The steps involved in achieving the monofunctionalized CPPs were described by Itami and Segawa.

Selective monofunctionalizations of CPPs are difficult to achieve due to the fact that all the carbon-hydrogen bonds that are on the arene rings are equivalent, chemically. Selective monofunctionalization is the process of installing a single functional group at a specific position on the arene ring. Usually, when direct functionalization of metal-free CPPs is done, they lead to uncontrolled multiple substitutions on the arene rings. CPPs are attractive for use as components in carbon nanotubes. However, till now there has not been any efficient method for obtaining directly functionalized CPPs.

"We were pleased to see that a functional group could be selectively installed on one arene ring via chromium coordination of CPPs," says Segawa. "As electrophiles, we utilized silyl, boryl and ester groups, which act as handles that can be easily transformed to other useful functionalities," he continues. Itami says, "We hope that this new approach evolves to become a valuable method to construct carbon nanotubes with unique structures and properties."