For such a seemingly simple ion, cyanoformate (NCCO2−) has been tough to catch. But scientists have now trapped the fleeting anion. The achievement could guide the design of agents that capture the greenhouse gas carbon dioxide.
Jason A. C. Clyburne of Saint Mary’s University, in Halifax, Nova Scotia, and coworkers captured cyanoformate as a tetraphenylphosphonium salt and obtained its crystal structure (Science 2014, DOI: 10.1126/science.1250808 ).
The structure shows that the anion is a Lewis acid base adduct in which the carbon from cyanide donates a pair of electrons to the carbon in CO2. “It’s a donor-acceptor complex between a base and an acid,” Clyburne says. “But it forms only a very weak carbon-carbon bond,” adds Heikki M. Tuononen of the University of Jyväskylä, in Finland, whose research group examined the electronic structure of the anion with theoretical methods.
Cyanoformate “is as reactive and as unstable and as sensitive a molecule as I’ve ever isolated,” Clyburne says. “Cyanide is a stable entity, and CO2 is a stable entity. Yet, under the right conditions we can pair them, isolate the complex, and study its decay.”
Clyburne and coworkers also stabilized another salt of the anion in an ionic liquid. By using different solvents, they were able to study the anion’s degradation as a function of the dielectric constant of the solution. “The chemistry of CO2 is significantly different in low-dielectric media,” Clyburne says.
In the anion, “cyanide is only barely able to hold onto the CO2,” says Philip G. Jessop, a professor at Queen’s University, in Kingston, Ontario, and technical director of GreenCentre Canada. That weak binding means the work could inspire new strategies for capturing CO2 from power plant emissions.
“If or when society decides to do something about global warming, work like that of Clyburne will be crucial to the design of the most energy-efficient CO2 scrubbing agent,” Jessop says. “In that process, one needs something to bind CO2 but not bind it too strongly, so that release of the CO2 is easy when the binding agent needs to be regenerated.”