New, man-made N2-fixing systems

The major inputs of fixed N into the global nitrogen cycle are assessed and compared as indicators of both the need for and the likely basis of new, complementary, manmade N $\_2$ -fixing processes. The development, since 1964, of the purely chemical, highly reactive systems for the reduction of N $\_2$ , including those driven electro- and photochemically, is traced, along with the parallel efforts to synthesize metal-N $\_2$ complexes (the first step in any likely fixation process) and subsequently protonate them to produce hydrazine or ammonia. These experimental approaches are convergent. Successful cycling or catalysing of some of these N $\_2$ -binding systems has been achieved. The advantages and limitations of the more successful systems are noted. Approaches to this problem via direct modelling of the nitrogenase active site are outlined, as is the one successful use of such complexes in achieving N $\_2$ reduction. This wealth of effort on the reductive approaches contrasts vividly with the almost complete absence of research on N $\_2$ oxidation. Currently, only a re-evaluation of the arc discharge process is continuing. Finally, the author's studies of the extruded molybdenum-containing prosthetic group of nitrogenase, the enzymic N $\_2$ -reducing site, are described in relation to future N $\_2$ -fixing systems.