Activation of nitrogen using alkaline earth metals

Dr Ryan Schwamm
Research Fellow 2018

Dr Ryan Schwamm
University of Bath

Dinitrogen (N2) is the most abundant molecule in the Earth’s atmosphere. However, the efficient utilisation of N2 remains one of the great challenges in synthetic chemistry. Current industrial processes for the transformation of N2 into ammonia (NH3) require harsh conditions (Haber-Bosch process, ~500 °C and >100 atm) and consume 1-2% of the world’s annual energy supply. The development of catalytic systems capable of reducing the energy requirements of such processes is of importance.

To date, investigations involving the activation and reduction of N2 have largely been limited to transition metal chemistry. Over the past 20 years the main group elements have received significant attention as potential alternatives to the transition metals for a variety of catalytic processes. This is especially true for the alkaline earth metals (Mg, Ca, Sr, Ba), compounds of which have been shown to display remarkable reactivity with a range of typically unreactive substrates and have been developed as economical and efficient catalysts. For example, molecular magnesium and calcium hydrides have been demonstrated to reduce carbon monoxide (CO) and, when performed in the presence of a hydride source, allow catalytic reduction and utilisation of CO as a source of carbon.

"The efficient utilisation of dinitrogen (N2) as an abundant and sustainable feedstock remains one of the great challenges in synthetic chemistry"

This research seeks to extend this reactivity towards the catalytic reduction and functionalisation of dinitrogen (N2) through a cooperative process involving a transition metal co-catalyst. While N2 and CO possess comparable bond strengths, N2 is generally considered inert and CO is amenable to activation by a range of metal complexes. The difficulty of alkaline earth metal centred reduction of dinitrogen lies primarily in the non-polarity and poor polarizability of the N2 molecule, which impede formation of the requisite highly polarised transition states.  Coordination of free N2 to an appropriate transition metal has been shown to induce strong polarisation of the N≡N bond, such that the metal-bound nitrogen centre bears a positive charge and the unbound nitrogen atom a negative charge

(i.e. M-d+N≡Nd-).  Well-defined transition metal co-catalysts will be used to deliver the highly polarised N2 molecule to the reactive alkaline earth complex in a similar manner to the established reaction with free CO.