Synthesis
C&EN’s molecules of the year for 2023
C&EN editors picked these molecules as the coolest of the past year
by Celia Arnaud, special to C&EN
Metallocenes circle up
![A cyclocene made with strontium and cyclooctatetraene with bulky groups. A cyclocene made with strontium and cyclooctatetraene with bulky groups.](/content/dam/cen/101/41/WEB/10141-cover2-cyclob.jpg)
![A cyclocene made with europium and cyclooctatetraene with bulky groups. A cyclocene made with europium and cyclooctatetraene with bulky groups.](/content/dam/cen/101/41/WEB/10141-cover2-cycloc.jpg)
Chemists made a new type of supersize sandwich complex by coaxing 18 metallocene units to curve into a nanometer-scale ring called a cyclocene (Nature 2023, DOI: 10.1038/s41586-023-06192-4). They made these cyclocenes from metals—strontium, samarium, or europium—sandwiched between layers of cyclooctatetraenes that carry two bulky triisopropylsilyl groups. The bulky substituents force the metallocenes to bend as they stack, forming a ring.
Carbene breaks octet rule
![Crystal structure of a doubly oxidized carbene dication. Crystal structure of a doubly oxidized carbene dication.](/content/dam/cen/101/41/WEB/10141-cover2-carbene.jpg)
Some rules are made to be broken. Carbon usually has eight valence electrons, in accordance with the so-called octet rule. A new compound—a doubly oxidized carbene dication—features a carbon atom with only four valence electrons (Nature 2023, DOI: 10.1038/s41586-023-06539-x). Researchers created a carbene with bulky substituents, oxidized the carbene, and removed an oxide anion, leaving a carbene carbon with no nonbonding electrons.
Chain-link covalent organic frameworks
![Crystal structure of a covalent organic framework shows that each building block is interlocked with six neighbors. Crystal structure of a covalent organic framework shows that each building block is interlocked with six neighbors.](/content/dam/cen/101/41/WEB/10141-cover2-cof.jpg)
Researchers used catenanes—molecules that interlock like the links of a fence—to make a new type of covalent organic framework (COF) (Nat. Synth. 2023, DOI: 10.1038/s44160-022-00224-z). Each subunit in the COF is made by condensing precursors around copper(I) ions to form a network of three-ring polyhedrons. Removing the copper template ions enables the polyhedrons to move around without separating, making the resulting material soft and flexible. The COFs could be used for filtration membranes and soft robotics.
Chiral oxonium ion
![The chemical structure of a helically chiral triaryloxonium ion. The chemical structure of a helically chiral triaryloxonium ion.](/content/dam/cen/101/41/WEB/10141-cover2-oxonium.jpg)
![A 3D crystal structure of a chiral oxonium ion. When the compound is viewed with the three oxygen bonds pointing into the page, the carbon-based ring structure encircles the central oxygen with a slight twist. The oxygen has three bonds locked in a pyramidal geometry. A 3D crystal structure of a chiral oxonium ion. When the compound is viewed with the three oxygen bonds pointing into the page, the carbon-based ring structure encircles the central oxygen with a slight twist. The oxygen has three bonds locked in a pyramidal geometry.](/content/dam/cen/101/41/WEB/10141-cover2-3dstructure.jpg)
Think chirality, and carbon usually comes to mind. But other atoms can form chiral centers too. Chemists synthesized an oxonium ion—a compound with a positively charged oxygen bonded to three substituents—in which the oxygen atom is the only chiral center (Nature 2023, DOI: 10.1038/s41586-023-05719-z). The researchers locked the oxygen lone pair in place by bonding the oxygen atom to a fused triaryl ring system.
Solid-state diberyllium compound
![X-ray crystallography structure of diberyllocene. The two beryllium atoms sit at the bottom and top of pyramids that are pointing at each other. X-ray crystallography structure of diberyllocene. The two beryllium atoms sit at the bottom and top of pyramids that are pointing at each other.](/content/dam/cen/101/41/WEB/10141-cover2-diberyl.jpg)
Chemists created diberyllocene, the first solid-state compound to contain a beryllium-beryllium bond (Science 2023, DOI: 10.1126/science.adh4419). Each beryllium atom is bound to a cyclopentadienyl group. Beryllium, which usually exists in the +2 oxidation state, adopts the unusual +1 oxidation state in diberyllocene. The compound could be used to develop new catalysts.
Molecular motor powered by electricity
![A space-filling quantum mechanical representation of a molecular motor shows a large ring that has viologen followed by a moving ring, viologen, an isopropylphenylene group, a moving ring, and a 2,6-dimethylpyridinium group. A space-filling quantum mechanical representation of a molecular motor shows a large ring that has viologen followed by a moving ring, viologen, an isopropylphenylene group, a moving ring, and a 2,6-dimethylpyridinium group.](/content/dam/cen/101/41/WEB/10141-cover2-model.jpg)
![A schematic of the first step of the electrical motor’s four-step cycle. On the left-hand side is a circle with two smaller rings on opposing sides. The right-hand side of the schematic shows that after six electrons are added, the smaller rings have moved clockwise along the track and have been reduced. A schematic of the first step of the electrical motor’s four-step cycle. On the left-hand side is a circle with two smaller rings on opposing sides. The right-hand side of the schematic shows that after six electrons are added, the smaller rings have moved clockwise along the track and have been reduced.](/content/dam/cen/101/41/WEB/10141-cover2-scheme.jpg)
Researchers invented a molecular motor powered by electricity, not chemical fuels or light, which drive most other molecular motors (Nature 2023, DOI: 10.1038/s41586-022-04910-y). The new electric motor consists of two cyclobis (paraquat-p-phenylene) rings that travel around a larger loop in response to an oscillatory voltage that drives a series of oxidation and reduction reactions. Using electricity as a power source could make it easier to integrate molecular motors with other technologies.
POLL: OUR READERS HAVE VOTED strontium cyclocene AS THEIR FAVORITE MOLECULE OF 2023.
Molecule | Votes | Percentage |
Strontium cyclocene | 430 | 40% |
Chiral oxonium ion | 230 | 21% |
Electric molecular motor | 127 | 12% |
Diberyllocene | 119 | 11% |
Carbene dication | 112 | 10% |
Catenated covalent organic framework | 61 | 6% |
Molecule: Strontium cyclocene
Votes: 430
Percentage: 40%
Molecule: Chiral oxonium ion
Votes: 230
Percentage: 21%
Molecule: Electric molecular motor
Votes: 127
Percentage: 12%
Molecule: Diberyllocene
Votes: 119
Percentage: 11%
Molecule:Carbene dication
Votes: 112
Percentage: 10%
Molecule: Catenated covalent organic framework
Votes: 61
Percentage: 6%
Celia Henry Arnaud is a freelance writer based in College Park, Maryland.
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