The research, reported in the 20 April issue of Science, involves a class of carbenes, molecules with unusual, highly reactive carbon atoms containing six electrons rather than the usual eight.
These cyclic alkyl amino carbenes (CAACs) have been found to share certain characteristics with metals. Specifically, they can split hydrogen under mild conditions, a function usually associated with transition metals. This suggests potential utility in carbon-based systems for storing hydrogen that would offer a more cost-effective and environmentally friendly alternative to metals.
What transition metals cannot normally do is chemically activate ammonia. Transforming ammonia efficiently into useful amino acid compounds would be a boon both to pharmaceutical researchers and manufacturers of bulk industrial materials.
Lead researcher Guy Bertrand, professor of chemistry at the University of California, Riverside (UCR) in the US, described the process as "one of the top challenges for the 21st century."
The process could pave the way for transforming abundant and inexpensive ammonia into useful amino compounds used to make pharmaceuticals, he added.
In contrast to dihydrogen, Bertrand and his colleagues write in Science, ammonia (NH
3) "usually forms simple Lewis acid-base adducts with transition metal complexes … because of the presence of a lone pair of electrons at nitrogen. Consequently, examples of NH
3splitting are rare."
What enables the CAACs to activate ammonia is their nucleophilic propensity, the researchers explain. While the carbenes may mimic the chemical behaviour of metals in splitting hydrogen, their mode of action is quite different.
"However, in contrast to transition metals that act as electrophiles towards dihydrogen, these carbenes primarily behave as nucleophiles, creating a hybride-like hydrogen, which then attacks the positively polarised carbon centre," the UCR researchers note.
The nucleophilic activation of ammonia by CAACS under very mild conditions "offers an alternative paradigm in the continuing search for catalytic systems capable of transforming ammonia efficiently into useful amino compounds", they comment.
