Osmium ensnared in a hydrocarbon ring

By constructing an aromatic hydrocarbon framework around an osmium ion, chemists have created a new class of organometallic compound. The in-plane metallo-annulenes are essentially carbon clusters embedded with transition metals. The molecules could have applications in high-performance electronics or could be used to make graphene doped with metal (Nature 2025, DOI: 10.1038/s41586-025-08841-2).

Organometallic compounds tend to marry metals with aromatic hydrocarbons so that the metal sits above or below the carbon-based group’s flat plane and interacts with the hydrocarbon’s p orbitals. The archetypal example is ferrocene, in which two cyclopentadiene ions make a sandwich with an iron ion in the center. The new, in-plane metallo-annulenes differ in that the metal sits within the plane of the aromatic hydrocarbon, and it participates in the molecule’s π conjugation and aromaticity. The molecules are reminiscent of porphyrins—flat, aromatic rings containing nitrogens that bind metal ions, such as iron in heme.

Southern University of Science and Technology’s Haiping Xia, who led the project, has been pursuing these molecules for a decade. He says in an email that “all current experimental and theoretical evidence demonstrates that the aesthetically elegant metal-centered planar annulene exhibits stronger aromaticity than benzene.”

Xia and colleagues synthesized three in-plane metallo-annulenes via a four-step synthesis, starting from a precursor with a reactive osmium-carbon triple bond. The most symmetrical compound that the chemists made contains 15 carbon atoms in five fused five-membered rings surrounding an osmium bis(phosphine) core. Xia’s team was able to convert this parent molecule to the corresponding chlorinated, iodinated, and nitrated derivatives. Xia says his group is working to make in-plane metallo-annulenes of varying sizes and with other metals.

Marina A. Petrukhina, who studies novel aromatic systems at the University at Albany, calls the new molecules a groundbreaking discovery that will provide new building blocks for organometallic and materials chemistry.

Michael M. Haley, an expert in conjugated aromatic molecules at the University of Oregon, says the work is a spectacular accomplishment. “The fact that this system can undergo electrophilic aromatic substitution cleanly and in very good yield is just ‘icing on the cake,’ ” he says in an email. “This work redefines metalla-aromaticity and will likely end up in textbooks.”