Chemistry of N₂H₂ and Other Nitrogen Hydrides Related to N₂ Fixation in the Coordination Sphere of Metal Sulfur Complexes

Abstract

The X-ray structural model of the FeMo cofactor (FeMoco) reported by Kim and Rees [1992] yielded the long awaited structural basis, it reconfirmed the importance of sulfur ligands, it told that molybdenum is still a potential binding site of N₂, however, it of course could not specify where the N₂ molecule binds and in that respect shattered the dreams of those who thought all one had to know is the structure of the active center of nitrogenase in order to develop a good catalyst for making cheap fertilizers. This challenge remains and it must be kept in mind that 1) the isolated native FeMoco without its protein does not convert N₂, 2) the isolated FeMoco is a relatively short-lived species in aqueous media (K. B. Burgess 1990) and 3) thermodynamics state that the reaction N₂ + 6 H⁺ + 6 e_- → 2 N₃ is exoenergetic needing a potential of only -280 mV at pH=7 which is less than is needed to reduce protons to dihydrogen (-410 mV at pH=7). In particular this “mild” redox potential eliminates all those model systems as potentially competitive catalysts which need at some stage strongly reducing reagents such as alkali metals in order to bind and to reduce N₂.