Recent developments in the chemistry of nitrogen fixation

Abstract

This discussion is concerned mainly with the biological fixation of nitrogen, and I am the only speaker to discuss its purely chemical aspects. I shall consider first the pure chemistry, and outline recent developments which may be relevant to the biological process. Until three years ago chemistry was concerned only with the mapping of possible routes from nitrogen to simple inorganic compounds, such as ammonia or nitric acid, under mild conditions. Intermediates such as di-imine, hydrazine, hydroxylamine, or hyponitrous acid were postulated. These routes were purely speculative with nothing more to commend them than that they were thermodynamically possible. None had been realized experimentally under mild aqueous conditions and no intermediate between nitrogen and ammonia, the primary product of the biological process, has been detected in the natural system. The nitrogen molecule is a very stable entity with a dissociation energy of 224.5 kcal and an ionization potential as high as 15.58 eV. Its unique electronic structure in relation to nitrogen fixation has already been outlined (Chatt & Leigh 1968). The high dissociation energy and low ionization potential precludes any routes involving either dissociation, or oxidation by stable molecules such as oxygen or even fluorine in their ground states at room temperature. Reduction is the third major reaction type; it depends upon the possibility of adding electrons to the nitrogen molecule, which for this reaction must have vacant orbitals of low energy to receive them. The energetically lowest vacant molecular orbitals are two degenerate antibonding π -orbitals at — 7 eV. These are too high in energy to be attacked by any but the strongest reducing agents, such as the more electropositive metals, and indeed very electropositive metals do attack nitrogen at room temperature. This is part of the explanation of the rapid nitriding of lithium wire observed during the formation of lithium reagents from lithium and alkyl halide in pentane under nitrogen. This reaction can be induced to yield considerable quantities of Li 3 N (J. Chatt & G. J. Leigh unpublished). Indeed a clean film of any metal to the left of Group VII in the long form of the Periodic Table, except the heavier alkali metals, is rapidly nitrided on the surface by gaseous nitrogen at room temperature. All of these are very electropositive metals and very strong reducing agents, which equally rapidly react with water or oxygen under the same conditions. It seems unlikely that nitriding in this manner could form the basis of the activation of nitrogen in the aqueous environment of the biological system, although such a possibility certainly cannot be ignored. I personally do not favour such a nitriding route. However, it is receiving attention at the present time, and I shall outline briefly the direction it is taking.