Innovative synthetic technology unlocks new classes of planar organometallic compounds

Since the 1950s, organometallic chemistry has been a widely explored field, as there are six Nobel Prizes in the category. However, the discovery of a new class of organometallic compounds remains a rare occurrence.

A team of researchers from China and the US decided to change it by identifying a new class of organometallic compounds, the planar annulene framework for three new metal centres.

A ring is a cyclic hydrocarbon containing the largest number of alternating carbon-carbon single and double bonds possible, containing the even number of n or n cnhn + 1 general formula CNHN. The rings presented in this study consisted of 15 carbon atoms attached to one atom of the central transition metal osmium.

New compounds have been reported in nature.

In the mid-20th century, the rise of a new field of chemical called organic contrast, kickstarted with the discovery of ferrocene. In most organometallic compounds, the metal is π-tuned at locations above or below the flat annulene anion plane.

Over the years, many such out-of-plane complexes have been synthesized, which have had a major impact on binding theory, finding diverse applications in scientific and industrial processes thanks to their unique useful catalysts, electrochemical and magnetic properties.

However, many in-plane metal-annulene complexes have not been reported in which metal atoms are located inside the ring and form σ(Sigma) bonds with carbon atoms rather than π bonds.

Synthesizing such complexes has proven to be much more difficult due to several factors. It deviates from planarity as it requires restriction by small annulene anions that do not have a central cavity large enough to accommodate metal atoms, flexible structures and strong carbon-hydrogen bonds to be broken and replaced by carbon bonds.

This new study overcomes synthetic obstacles to in-plane metalloanne through targeted molecular design and clever synthetic pathways. Instead of trying to insert metal into annulene, the researchers built annulene framework around the metal center.

The synthesis was performed in multiple steps, starting with a molecular precursor containing a reactive osmium-carbon triple bond and assembled a carbon carbon bond around the metal atom via a cycloaddition reaction. This formed an in-plane (15) annulene metal complex containing five fusion rings bound via a central osmium bisphosphine moiety.

The most symmetrical molecule designed in this study was metalloannulene, composed of five connected, five-membered aromatic rings. Using this as a parent structure, the researchers derive in-plane metalloannes in iodinated, chlorinated, and nitrified versions. They also discover that phosphine ligands in the parent moiety can be exchanged, providing a versatile platform for new derivatives.

Researchers emphasize that the high stability of the planar annular shape of the metal center and the functionalized capabilities position them as promising building blocks in material science.

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