The synthesis of ammonia under milder condition, using less energy and fewer resources, has moved a step closer. Scientists in Japan have created a trinuclear titanium polyhydride complex that can cleave the dinitrogen bond and form nitrogen–hydrogen bonds at ambient temperature and pressure without additional reducing agents or proton sources.1
Nitrogen is the most abundant gas in our atmosphere, essential to life, yet largely inert. Some microbes generate bioavailable nitrogen by reducing nitrogen to ammonia. Industrially, ammonia is produced via the Haber–Bosch process, which is so energy intensive that it consumes 1% of the energy generated globally. The process combines nitrogen and hydrogen over activated iron surfaces to generate ammonia for use as a fertiliser or as a chemical feedstock.
The intrinsic inertness of nitrogen has made it challenging to discover metal complexes that can both bind and activate it. ‘By experimental and computational studies, we determined that the dinitrogen reduction by a trinuclear titanium hydride complex proceeds sequentially through scission of a nitrogen molecule bonded to three titanium atoms in an end-on-side-on fashion, followed by N–H bond formation,’ says study author Zhaomin Hou, of the RIKEN Center for Sustainable Resource Science, Japan. ‘The hydride ligands serve as the source of both electron and proton.’
Cleaving the N–N bond and forming N–H bonds directly from a hydride complex has been seen only rarely, with some of the most influential work coming from Michael Fryzuk at the University of British Columbia, Canada, who has championed the ‘hydride route’ to dinitrogen complexes.2 ‘The active sites of both major N2 reduction catalysts – nitrogenases and the Haber–Bosch process – have hydride species as their resting states, but in neither case is the detailed mechanism of hydrogen loss and nitrogen cleavage known,’ says Patrick Holland of the University of Rochester, US. The authors, he adds, ‘conclusively determined the structures of many of the intermediates along the pathway, giving insight into possible structures and pathways of intermediates on the catalysts’.
Fryzuk, who wrote an accompanying perspective,3 says the paper adds important fundamental knowledge about potential elementary reactions such as cleaving N–N triple bonds and forming N–H bonds, which are relevant to the Haber–Bosch process. He predicts it ‘will change the way people think about N2 activation so that in the future perhaps a soluble, suitably designed multi-metallic hydride complex will be able to both activate and functionalise molecular nitrogen productively to form ammonia or some other higher-value nitrogen containing material’.
However, there still challenges to overcome to make this process practically useful, Hou says. But if successful the low temperature, low pressure synthesis of ammonia in smaller reactors is on the cards.