The chemical mechanism of nitrogenase: hydrogen tunneling and further aspects of the intramolecular mechanism for hydrogenation of η²-N₂ on FeMo-co to NH₃

The preceding paper (Dalton Trans., 2008, DOI: 10.1039/b806100a) describes the logical development of a chemical mechanism for the catalysis of hydrogenation of N₂ to 2NH₃ that occurs at the Fe₇MoS₉N^c(homocitrate) cofactor (FeMo-co) of the enzyme nitrogenase. The mechanism uses a single replenishable path for serial supply of protons which become H atoms on FeMo-co, migrating to become S–H and Fe–H donors to N₂ and to the intermediates that follow. This chemical catalysis at FeMo-co is distinctly intramolecular: transition states and reaction profiles for the preferred 21 step pathway were presented. This paper describes a number of alternative intermediates and pathways that were considered in developing the mechanism. These results reveal further relevant principles of the reactivity of hydrogenated FeMo-co, and the reasons why these pathways are less likely to be part of the mechanism. The intramolecular character of the mechanism, and the relatively small distances over which H atoms transfer, lead to expectations of extensive quantum mechanical hydrogen tunneling as part of the catalytic rate enhancement. This possibility is supported by comparisons of reaction profiles with those for enzyme reactions for which tunneling is established.