Molecules of the year 2024

Hot MOF for capturing carbon

A portion of the structure of ZnH-MFU-4l as determined by X-ray crystallography

Credit: Science

This highly porous, crystalline metal-organic framework (MOF) can reversibly capture carbon dioxide between 200 and 300 °C, raising the possibility of capturing gas from industrial flues without first having to go through a costly cooling step (Science 2024, DOI: 10.1126/science.adk5697). The material uses zinc hydride as the carbon-capturing base instead of the more commonly used amines to minimize the entropy change associated with the reaction. Scientists first discovered the MOF a decade ago, but its carbon-capturing abilities were underexplored until now.

Single-electron carbon-carbon bond

This is the first example of a molecule with a single-electron carbon-carbon bond that is stable enough to isolate and study experimentally.

More than 90 years after Linus Pauling first proposed a single-electron carbon-carbon bond, chemists have experimentally confirmed its existence (Nature 2024, DOI: 10.1038/s41586-024-07965-1). Researchers created the unusual compound by oxidizing a strained hexaphenylethane derivative. They characterized the stable crystals using crystallography, spectroscopy, and theoretical analysis.

First soluble promethium complex

A water-soluble coordination complex made from promethium(III) and bispyrrolidine diglycolamide ligands

Credit: Alex Ivanov/ORNL/US Department of Energy

Promethium's radioactivity makes the element notoriously difficult to study. But this year, scientists managed to wrangle it into a water-soluble organometallic complex for the first time, making chemical analysis more manageable (Nature 2024, DOI: 10.1038/s41586-024-07267-6). They used X-ray spectroscopy to measure promethium's bonds to oxygen and compared them to metal-oxygen bonds in similar complexes. The comparison enabled researchers to establish trends in lanthanide elements' chemical properties.

Rule-bending strained alkene

This chiral molecule owes its stereochemistry entirely to the twistedness of its double bond.

Credit: Neil Garg/QRChem

Bridged organic molecules with a double bond at the bridgehead position (shown here as two lines in red) have been regarded for 100 years as too strained to make, but that hasn't stopped chemists from trying. And this year, they finally succeeded (Science 2024, DOI: 10.1126/science.adq3519). The structures are too unstable to isolate, but the researchers used them as intermediates in cycloaddition reactions to create complex 3D structures.

Molecular shuttle in a box

A coronene guest molecule (center) shuttles back and forth inside a nanosize box built from polycyclic aromatic molecules (teal, burgundy, and gold hexagons) and iridium complexes (beige balls).

Credit: Angew. Chem. Int. Ed.

This molecular machine thinks inside the box—literally. The long sides are polyaromatic molecules with iridium complexes on either end, and the short sides are pyrazine molecules (Angew. Chem., Int. Ed. 2024, DOI: 10.1002/anie.202318829). A guest molecule—coronene—fits inside, held within the outer box by π-electron interactions as the flat molecule zips from end to end. This type of shuttle could be used as the basis for future molecular switches.

Mirror-image cyclodextrin

This α-ʟ-cyclodextrin is made up of six sugar units. Researchers also made seven- and eight-sugar versions.

Chemists created three different-size cyclodextrins using ʟ⁠-⁠glucose, whose molecular geometry is the mirror image of that of naturally occurring ᴅ⁠-⁠glucose (Nat. Synth. 2024, DOI: 10.1038/s44160-024-00495-8). The switched-around stereochemistry of these ʟ⁠-⁠cyclodextrins could come in handy for several applications, such as drug delivery and enantioselective synthesis.