CO₂-To-CO Route

CATALYSIS

Bacteria and plants have long known how to convert carbon dioxide into more useful compounds, but humans are still striving to meet this fundamental challenge in practical ways. A new step in this direction has now been taken with the development of a catalytic system that reduces CO₂ to CO in solution, with high turnover numbers and frequencies (J. Am. Chem. Soc. 2005, 127, 17196).

The system, reported by Joseph P. Sadighi, assistant professor of chemistry at Massachusetts Institute of Technology, and coworkers David S. Laitar and Peter Mller, is based on a copper(I) boryl complex with an N-heterocyclic carbene ligand. This complex quickly abstracts an oxygen atom from CO₂, yielding CO and a borate complex. The latter can then be reacted with bis(pinacolato)diboron to regenerate the boryl complex. The purloined oxygen is retained in a diboroxane by-product.

The chemists achieved the highest turnover numbers-1,000 per copper-at elevated temperatures. Using a less bulky carbene ligand led to a less stable boryl complex but one that worked faster before it decomposed. This faster complex achieved 100 turnovers within one hour at ambient temperature and below.

Bis(pinacolato)diboron's acceptance of the oxygen atom derived from CO₂ is essentially irreversible. That's a deal-breaker in terms of practicality, Sadighi says. If one had an oxygen acceptor that could later liberate the oxygen photochemically, we would have achieved both halves of an energy conversion cycle that consumes CO₂, he tells C&EN. This facile catalytic reduction represents, potentially, the first half.

This chemistry won't solve the world's CO₂ problem, Sadighi says. But because the oxygen abstraction and catalyst turnover involve well-defined reactants and products, unlike some other CO₂-to-CO reduction systems, he thinks further studies could point the way to more practical systems.