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C&EN’s molecules of the year for 2020

Our editors highlight the coolest molecules unrelated to COVID-19 that were reported this year

by Celia Henry Arnaud
December 8, 2020 | A version of this story appeared in Volume 98, Issue 48



Poll: Our readers have voted for their favorite molecule of 2020. The winner is Porous ionic liquid

Molecule Votes Percentage
Porous ionic liquid 292 28.4%

Largest aromatic ring

Asymmetrical molecular knots 178 17.3%

2-D metallo-supramolecule


Boron quadruple bond


Threading rings on a polymer chain

Beryllium radical cation 27 2.6%

Molecule: Porous ionic liquid

Votes: 292

Percentage: 28.4%

Molecule: Largest aromatic ring

Votes: 181

Percentage: 17.6%

Molecule: Asymmetrical molecular knots

Votes: 178

Percentage: 17.3%

Molecule: 2-D metallo-supramolecule

Votes: 143

Percentage: 13.9%

Molecule: Boron quadruple bond

Votes: 126

Percentage: 12.3%

Molecule: Threading rings on a polymer chain

Votes: 80

Percentage: 7.8%

Molecule: Beryllium radical cation

Votes: 27

Percentage: 2.6%

Largest aromatic ring to date synthesized

Density plot showing the ring current induced by the aromaticity of a 12-porphyrin molecular wheel.
Credit: Harry L. Anderson
In its +6 oxidation state, this aromatic, 12-porphyrin nanoring produces a ring current that induces magnetic fields both opposing (blue) and aligning with (red) an external field.

University of Oxford researchers constructed a 16 nm wide molecular wheel, which contains 12 porphyrins and 162 π electrons. In the ring, zinc porphyrins are linked by alkynes and held in place by molecular spokes. The molecule is of interest for quantum computing (Nat. Chem. 2020, DOI: 10.1038/s41557-019-0398-3).

Huge 2-D metallo-supramolecule assembled

Model of a metallo-supramolecule overlaid on a scanning electron micrograph of the structure.
Credit: Xiaoping Li
A model of the supramolecule overlaid on a scanning tunneling microscope image

A multi-institution team made a 2-D metallo-supramolecule that measures nearly 20 nm across (Nat. Chem. 2020, DOI: 10.1038/s41557-020-0454-z). Ruthenium-complexing terpyridine ligands form hexagons that assemble into a larger grid by complexing iron(II). The team aims to use these and related supramolecules as single-molecule information storage devices.

Porous ionic liquid captures alcohols and CFCs

Porous ionic liquid

This new porous ionic liquid has cavities that are approximately 6.2 Å wide—big enough to capture alcohols and chlorofluorocarbons (CFCs). Previous porous liquids couldn’t grab anything larger than methane or carbon dioxide. Researchers at the University of Cambridge made the new liquid from tetrahedrons of zinc surrounded by coordinating ligands (Nat. Chem. 2020, DOI: 10.1038/s41557-020-0419-2).

First beryllium radical cation isolated

Crystal structure of a compound containing a beryllium radical cation.
Credit: J. Am. Chem. Soc.
A radical cation (left) and its accompanying anion

Chemists at the University of Virginia synthesized this radical that contains beryllium in the +1 oxidation state by oxidizing a previously reported compound containing Be(II) (J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.9b13777). Compounds with main-group elements with low oxidation states could participate in chemistry typically seen only in transition metals.

Threading multiple rings on a polymer chain

Credit: Stoddard Group/Northwestern University
A polyrotaxane synthesizer threads rings onto a chain two at a time using a cycle of reduction (purple) and oxidation (blue).

Chemists at Northwestern University designed a molecular machine that uses redox chemistry to thread rings two at a time onto a polymer chain (Science 2020, DOI: 10.1126/science.abb3962). The researchers would like to use this system to build polyrotaxanes for information storage by encoding data in the sequence of rings.

Boron makes quadruple bond

Scheme showing the molecular orbital of rhodium boride.
Credit: J. Phys. Chem. Lett.
These molecular orbitals (purple and silver) show the two σ bonds and two π bonds that make up the quadruple bond in rhodium boride (aquamarine = Rh; pink = B).

For more than a decade, chemists have thought that rhodium boride contains a triple bond. Using vibrational spectroscopy and theoretical calculations, researchers at Brown University discovered that the bond is likely a quadruple bond instead (J. Phys. Chem. Lett. 2020, DOI: 10.1021/acs.jpclett.9b03484).

Tying molecular knots

Credit: Stuart Jantzen, Biocinematics
Lutetium-coordinating (blue) and copper-coordinating (green) motifs direct the tying of a molecular knot.

Researchers at the University of Manchester used metal ions to tie molecular strings into two types of knots. Adding lutetium ions alone or in combination with copper ions leads to symmetrical trefoil knots or asymmetrical three-twist knots, respectively (Nature 2020, DOI: 10.1038/s41586-020-2614-0).



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