Innovative synthesis technique unlocks new class of planar organometallic compounds

With six Nobel Prizes in the category, organometallic chemistry has been a widely explored field since the 1950s. Yet, the discovery of new classes of organometallic compounds remains a rare occurrence.

A team of researchers from China and the U.S. decided to change that by identifying a new class of organometallic compounds—three new metal-centered planar annulene frameworks.

Annulenes are cyclic hydrocarbons that contain the maximum number of alternating carbon–carbon single and double bonds possible, with a general formula C_nH_n for even numbers of n or C_nH_{n + 1} for odd numbers of n. The annulenes presented in this study were made up of 15 carbon atoms bonded to one atom of the transition metal osmium, at the center.

The new compounds were reported in Nature.

The mid-20th century saw the rise of a new field of chemistry called organometallics, kickstarted by the discovery of ferrocene—a compound consisting of two five-carbon annulene rings sandwiching a central iron atom. In most organometallic compounds, a metal is π-coordinated where it sits above or below the plane of the flat annulene anions.

Over the years, many such out-of-plane complexes have been synthesized, significantly influencing bonding theory and finding diverse applications in scientific and industrial processes thanks to their unique useful catalytic, electrochemical, and magnetic properties.

However, not many in-plane metal-annulene complexes where the metal atom sits inside the ring and forms a σ (sigma) bond with the carbon atoms rather than π bonds, have been reported.

Synthesizing such complexes has proven far more challenging due to several factors: restrictions due to smaller annulene anions not having a large enough central cavity to accommodate a metal atom, larger annulenes deviating from planarity because of their flexible structures and the need to break strong carbon–hydrogen bonds and replacing them with carbon–metal bonds.

The new study overcomes the synthetic obstacles to in-plane metallo-annulenes through targeted molecular design and a clever synthetic route. Instead of trying to insert a metal into an annulene, the researchers built an annulene framework around a metal center.

The synthesis was carried out in multiple steps, starting with a molecular precursor containing a reactive osmium–carbon triple bond and then assembling carbon–carbon bonds around the metal atom via cycloaddition reaction. This led to the formation of in-plane [15]annulene metal complexes, containing five fused rings joined through the central osmium bis-phosphine moiety.

The most symmetrical molecule designed in the study was a metallo-annulene made up of five connected five-membered aromatic rings. Using this as the parent structure, the researchers derived iodinated, chlorinated and nitrated version of the in-plane metallo-annulenes. They also found that the phosphine ligands of the parent moiety can be swapped out, offering a versatile platform for newer derivatives.

The researchers highlight that the high stability of the metal-centered planar annulenes and their ability to be functionalized position them as promising building blocks for materials science.

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