From pollution to solution – ‘Green’ catalysts transform carbon dioxide gas into valuable materials

Chemists are helping to reduce heat-trapping carbon dioxide (CO2)
emissions, which are a global concern. For example, they are devising
new catalytic systems that would enable waste CO2 to be recycled as
a non-toxic and practically free source of carbon for organic synthetic
reactions. However, current CO2 conversion techniques require expensive
metal catalysts or drawn-out procedures.
Now, Zhaomin Hou and colleagues from the RIKEN Advanced Science
Institute in Wako have found a way to insert CO2 directly into the framework
of aromatic molecules, turning them into carboxylic acid derivatives that
are widely used as pharmaceuticals, agrichemicals, and dyes1. Importantly,
this transformation can be achieved economically and with negligible
environmental impact, thanks to a low cost copper complex bearing an
organic ligand.
N-heterocyclic carbenes (NHCs) are molecules with near metal-like reactivity
because of an electron-deficient carbon center. For the past two decades,
scientists have used NHCs as organic replacements for metal catalysts
and as ‘spectator’ ligands that attach to metal centers and influence their
catalytic behavior. Hou and colleagues recently discovered that adding NHCs
to copper, one of the most abundant metals in nature, created a complex
that catalyzed CO2 addition to boron esters2—a trick the team hoped to
repeat with aromatic hydrocarbons.
The most efficient way to incorporate CO2 into benzene-like molecules is by
replacing one of the carbon–hydrogen (C–H) bonds on the outer ring; unfortunately,
these bonds are notoriously unreactive. To overcome this problem, the researchers
turned to benzoxazole: this double-ringed aromatic compound has a C–H bond
situated between nitrogen and oxygen atoms, making it easier to chemically
activate this position.
With just a pinch of the NHC–copper catalyst complex, the team found they could
convert a mixture of CO2 and several different benzoxazole-based molecules into
solid carboxylic acids and esters in excellent yields. Carefully characterizing
the crystal structures of several intermediate compounds revealed that CO2 inserted
in between a copper–carbon bond formed at the benzoxazole C–H site, followed
by a dissociation step that regenerated the catalyst.
According to Hou, the NHC ligand was essential in enabling CO2 capture. “The
electron-donating ability of NHC could make the C–H activation and CO2 insertion
steps easier, while its steric bulk brings stability to the active catalyst species,”
he notes. The researchers now hope to extend this technique to less reactive C–H
bonds by fine-tuning the catalyst complex and optimizing reaction conditions.