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C&EN’s molecules of the year for 2019

Chemistry news outlet highlights the coolest compounds reported this year

by Celia Henry Arnaud
December 3, 2019 | A version of this story appeared in Volume 97, Issue 48



Poll: What was the standout molecule of 2019?

Readers online voted on which of these molecules stood out the most this year. The winner is antiaromatic nanocage!

Answer Votes Percentage
Antiaromatic nanocage 316 29%
Cyclo C18 218 20%
C60 methane trap 210 19%
First hexagonal planar crystal structure 139 13%
Highly intertwined hydrocarbons 81 7%
Twisty tetracene 81 7%
Incredible chloride cage 60 5%

Answer: Antiaromatic nanocage

Votes: 316

Percentage: 29%

Answer: Cyclo C18

Votes: 218

Percentage: 20%

Answer: C60 methane trap

Votes: 210

Percentage: 19%

Answer: First hexagonal planar crystal structure

Votes: 139

Percentage: 13%

Answer: Highly intertwined hydrocarbons

Votes: 81

Percentage: 7%

Answer: Twisty tetracene

Votes: 81

Percentage: 7%

Answer: Incredible chloride cage

Votes: 60

Percentage: 5%

Structure of dodecaphenyltetracene.

Longest, most twisted perphenylacene synthesized

Researchers at Tulane University synthesized dodecaphenyltetracene—four fused benzene rings surrounded by 12 pendant phenyl rings—in three steps. The molecule is of interest for organic electronics and photovoltaics.

Angew. Chem., Int. Ed. 2019, DOI: 10.1002/anie.201812418



Stick structure of fullerene with methane trapped inside.
Credit: R. J. Whitby et al.

Methane was caged inside C60

This year, researchers trapped methane inside C60. Methane is the first organic molecule and the largest of any kind to be trapped this way. The University of Southampton researchers made a fullerene cage with a sulfur-containing 17-membered ring, forced methane inside at high pressure, and closed the cage by oxidizing the sulfur and ejecting sulfur monoxide.

Angew. Chem., Int. Ed. 2019, DOI: 10.1002/anie.201900983



Cryptand cage had record-breaking affinity for chloride

This molecule captured and bound chloride ions using only C–H bonds, which are typically considered weak hydrogen-bond donors. The cage’s affinity for chloride is so high that the Indiana University Bloomington team that synthesized the molecule wasn’t able to isolate it without chloride.

Science 2019, DOI: 10.1126/science.aaw5145

Ball-and-stick structure of cryptand cage with sequestered chloride.
Credit: Science
A crystal structure of the chloride-sequestering cryptand cage. C = gray, H = white, N = blue, O = red, Cl = green, and Na = yellow.



Intertwined structures contained only benzene rings

Chemists in Japan made a trefoil knot and interlocked rings known as catenanes with nothing but benzene rings. Molecules like these typically contain heteroatoms like nitrogen. The researchers achieved this feat by adjoining nanoring fragments with silicon templates, which they cleaved off after cyclizing the fragments.

Science 2019, DOI: 10.1126/science.aav5021

Trefoil knot and catenanes formed from only benzene rings.
Credit: Science



New allotrope of carbon achieved

Researchers made cyclo[18]­carbon, an 18-membered carbon ring with alternating single and triple bonds, by using voltage pulses from the needle tip of a scanning tunneling microscope to pick off carbon monoxide molecules from a multiring precursor.

Science 2019, DOI: 10.1126/science.aay1914

Four structures and accompanying atomic force microscope images showing the formation of cyclo[18]carbon.
Credit: IBM Research
Chemists started with a multiring precursor (from left) and used voltage pulses to pick off carbon monoxide molecules to form intermediates on the way to making cyclo[18]carbon (right). Atomic force microscope images (bottom) indicated that the carbon allotrope has ninefold symmetry.



First hexagonal planar crystal structure captured

The possibility of transition-metal complexes adopting hexagonal planar geometry was proposed more than 100 years ago, but the shape had never been seen in a crystal structure until this year. This rare structure consists of a palladium atom surrounded by three hydride and three magnesium-diisopropylphenyl ligands.

Nature 2019, DOI: 10.1038/s41586-019-1616-2

Ball-and-stick structure of transition-metal complex with hexagonal planar structure.
Credit: Imperial College London



Antiaromatic nanocage created

Researchers made this nanocage with antiaromatic Ni(II) norcorrole building blocks, to which they added substituents and iron ions to adjust the conditions so the molecule could self-assemble into a tetrahedral shape. Molecules trapped in the cage have their nuclear magnetic resonance signals shifted downfield thanks to the cage’s unusual magnetic properties.

Nature 2019, DOI: 10.1038/s41586-019-1661-x

Structure of antiaromatic nanocage.
Credit: Nature
Molecules that land inside this nanocage have their nuclear magnetic resonance signals shifted downfield, depending on their location, 3 (yellow) to 9 ppm (red). Blue sticks represent antiaromatic walls; gray represents substituents on walls. Ni = green; Fe = red.


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