Pollutant Shows Potential As Feedstock

Nov. 1, 2019

Work by researchers in isolating novel compounds of nitrous oxide points to a possible future strategy for dealing with nitrous oxide emissions by using the gas in value-added chemical processes.

Nitrous oxide often is called laughing gas but dealing with the compound is no laughing matter. The U.S. Environmental Protection Agency cautions that nitrous oxide has an atmospheric lifetime of 110 years and that one ton is equivalent to 298 tons of carbon dioxide. Nitrous oxide also is an ozone depleter.

Work by researchers at the University of Warwick, Coventry, U.K., in isolating novel compounds of nitrous oxide points to a possible future strategy for dealing with nitrous oxide emissions by using the gas in value-added chemical processes.

“As a chemical reagent its potential has yet to be fully harnessed and to do so in a sustainable manner is a formidable challenge for the scientific community,” explains project leader Adrian Chaplin, a research fellow in Warwick’s department of chemistry.

This challenge largely revolves around the robust triatomic structure of nitrous oxide that typically requires energy-intensive forcing conditions to enable reactions. Efforts to develop mild and selective alternatives have borne little fruit.

The Warwick team first looked at the application of homogenous transition-metal complexes to the gas. However, members of the team write in a recent article in Angewandte Chemie that while such a strategy is attractive, the inorganic chemistry underpinning such interactions is “conspicuously under-developed.” The researchers only identified four such discrete transition-metal complexes of nitrous oxide in the literature — all of which were poorly characterized largely because of the gas’s poor ligand properties.

Potential Raw Material

Figure 1. Well-defined transition metal complexes of nitrous oxide (color overlays) may foster its use in chemical reactions. Source: University of Warwick.

So, instead, the team built upon earlier work by chemists at Indiana University, Bloomington, Ind., and the University of North Carolina, Chapel Hill, N.C., — and its own in-house studies — which used cationic phosphine-based pincer complexes of rhodium as a platform for studying the coordination chemistry of nitrous oxide (Figure 1).

Rhodium is one of the most widely employed transition metals in organic synthesis; advanced analysis techniques enabled the team to identify and characterize its coordination with nitrous oxide in two separate pincer complexes.

“The compounds that we have prepared represent the starting point of our journey, but the associated experimental data seems to be guiding us in the right direction and we are looking forward to where it takes us,” notes Chaplin.

The next step is to explore the onward reactivity of such adducts in the context of catalytic applications.

The potential industrial application of the work will hinge on the chemical robustness of the catalyst. “In our case, we have used chelating pincer ligands, which are noted for conferring such stability, so that is promising,” says Chaplin.

Eventually, he believes, the work could generate new routes to fine chemical synthesis, along with bespoke oxidation reactions.