Tandem Activation and Reduction of N2

Recent research has described the catalytic reduction and utilisation of the small oxygenates CO2 and CO as C1 synthons. We have reported the first homogeneous reduction of these commodity chemicals to methanol, methane and homologated products reminiscent of those produced by Fischer-Tropsch catalysis and have shown that the superficially simple calcium hydride not only enables the synthesis of long sought n-alkyl derivatives but also facilitates the unprecedented nucleophilic alkylation of benzene.
One of the remaining grand challenges in synthetic chemistry is the efficient utilization of molecular dinitrogen in the generation of complex nitrogen-containing products. The inertness of dinitrogen, however, is not just due to its strong triple bond. Most pertinently, the isoelectronic carbon monoxide has an even greater bond dissociation enthalpy undergoes a wide variety of chemical reactions, including our recent report of calcium-centred homologation and reduction cited above. The major challenge of group 2 centred N2 reduction lies partly in its large HOMO-LUMO gap which disfavours both electron transfer and Lewis acid-base reactions, but primarily its non-polarity and polarisability which precludes the requisite highly polarised transition states.
To circumvent these limitations this project will target strategies to overcome the kinetic barriers to N2 activation at typical s-block centres. In addition to more direct approaches, we will develop hybrid systems whereby the group 2 centre is administered in conjunction with coordination of N2 to an appropriate transition metal centre. The binding of free N2 to a single metal centre has long been known to induce a strong polarisation of the NN bond whereupon the metal-bound nitrogen centre bears a positive charge and the non-bound N atom a negative charge. The use of well precedented species which bind N2 without reduction will, thus, enable the delivery of N2 to a reducing equivalent of, for example, a calcium hydride in a similar manner our established reactivity with free CO and metal carbonyl complexes. By way of illustration, Scheme 2 shows the use of an iridium(I) complex to deliver the desymmetrised N2 molecule to a calcium hydride. Ir compounds are typically synthesised under reductive condition with borohydrides so should be amenable to use under the hydroboration conditions my group have already used for CO and CO2 reduction. In the first instance we will target sequential N2 reduction to diazenes, hydrazines and ultimately boryl amines/ammonia, and will study the stoichiometric reactivity of each of these classes of compound with our suite of available group 2 hydrides. Once established we will seek to utilise these species in further onward reactivity and to combine this chemistry with our established insertion and hydroamination reactivity to produce amine derivatives.