An annular "bracelet" for the synthesis of complex nanostructures

A team of chemists succeeded in synthesizing an innovative metallic conjugate capable of performing conversions on carbon structures, a reaction that could, for the first time ever, pave the way for the construction of one-of-a-kind carbon nanostructures.

A team of chemists from Nagoya University in Japan succeeded in synthesizing an innovative metallic conjugate capable of performing conversions on carbon structures. The researchers prepared innovative chromium conjugates of the substance cycloparaphenylene (CPP) and demonstrated their ability to convert a single group in the carbon ring to obtain starting materials that could be used in the preparation of carbon nanotubes with unusual structures. Cycloparaphenylene consists of a chain of benzene rings and is the shortest segment of carbon nanotubes. Since their first preparation and isolation in 2008, cycloparaphenylenes have attracted much attention in the fields of materials science and supramolecular chemistry.

Applying their knowledge of chromium chemistry, the researchers were able to perform the first ever selective preparation of a monosubstituted group on cycloparaphenylene, a particularly challenging reaction due to the presence of many active arene rings on the overall structure. The researchers believe that through selective preparation and targeting of the functional groups on the ring it will be possible to obtain carbon nanotubes with new and interesting properties. The research findings were published in the scientific journal Journal of the American Chemical Society. The findings of this research are considered a significant step forward in the fields of arene chemistry and organometallic chemistry.

Arenes are known as substances that are able to give rise to couplings with transition metals while obtaining substances with different activity rates compared to the free ring. Cycloparaphenylenes, which consist of a chain of arenes, also react with the chromium metal to obtain the corresponding conjugates. Surprisingly and interestingly, the main product obtained contained a single chromium group attached to a single arenite ring on the outside, as verified by a variety of analysis methods (GDP, mass spectroscopic and X-ray diffraction). "The chemistry of arenes and chromium is a well-established field, so we decided to apply this organometallic method to the synthesis of our special conjugate," explains the lead researcher. "Since cycloparaphenylenes include several arene rings, we initially expected that the chromium group would attach to each of the rings," adds the researcher. "However, and to our surprise, we saw that the material reacts with the chromium metal in a 1:1 ratio under all the different conditions we applied. Computer simulations of the molecular structure hinted to us that the first chromium group attached to the structure causes its activity to decrease, thereby preventing the reaction with a second chromium group."

From this material, the researchers tried to make conversions in the Haren ring. "This was not an easy task since Arani bracelets with chrome are usually sensitive to air and light, and our bracelets were no different in this respect. But the researchers worked tenaciously to obtain a pure crystal of the first such conjugate," said the lead researcher. "Then we carried out the reaction in the tool itself, without any intermediate steps, to obtain the monoconverted ring together with the exit of the chromium group from the conjugate."

Selective monoconversion of cycloparaphenylenes, i.e. the introduction of a single functional group at a single position on the arene ring, is a challenging reaction since all the carbon-hydrogen bonds in the arene rings are chemically identical. Direct conversion of non-conjugated cycloparaphenylenes usually leads to multiple transformations on the arene ring indiscriminately. Although materials from this family (CPPs) are required components for the preparation of carbon nanotubes, to date there has not been any effective method for their direct preparation.

"We are very pleased with the ability to selectively insert a functional group at one position on the arene ring using chromium," notes the lead researcher. "As groups for conversion, we have successfully used silyl, boryl and ester groups, which in turn can be converted into other useful groups. We hope that this new approach will develop and become a useful method for the preparation of carbon nanotubes with unique structures and properties."

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One-pot selective monoconversion of cycloparaphenylene [Courtesy: ITbM, Nagoya University]