C&EN’s molecules of the year for 2019

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

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

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

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

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

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