The Irony of Iron – Biogenic Iron Oxides as an Iron Source to the Ocean

The reduction of N2 to NH3 is a requisite transformation for life. 1 While it is widely appreciated that the iron-rich cofactors of nitrogenase enzymes facilitate this transformation, 2-5 how they do so remains poorly understood. A central element of debate has been the exact site(s) of nitrogen coordination and reduction. 6,7 The synthetic inorganic community placed an early emphasis on Mo 8 , because Mo was thought to be an essential element of nitrogenases 3 and because pioneering work by Chatt and coworkers established that well-defined Mo model complexes could mediate the stoichiometric conversion of N2 to NH3. 9 This chemical transformation can be performed in a catalytic fashion by two well-defined molecular systems that feature Mo centres. 10,11 However, it is now thought that Fe is the only transition metal essential to all nitrogenases, 3 and recent biochemical and spectroscopic data has implicated Fe instead of Mo as the site of N2 binding in the FeMo-cofactor. 12 In this work, we describe a tris(phosphine)borane-supported Fe complex that catalyzes the reduction of N2 to NH3 under mild conditions, wherein >40% of the H+/e- equivalents are delivered to N2. Our results indicate that a single Fe site may be capable of stabilizing the various NxHy intermediates generated en route to catalytic NH3 formation. Geometric tunability at Fe imparted by a flexible Fe-B interaction in our model system appears to be important for efficient catalysis. 13-15 We propose that the interstitial light C-atom recently assigned in the nitrogenase cofactor may play a similar role, 16,17 perhaps by enabling a single Fe site to mediate the enzymatic catalysis via a flexible Fe-C interaction. 18

In the 1830s, iron bacteria were among the first groups of microbes to be recognized for carrying out a fundamental geological process, namely the oxidation of iron. Due to lingering questions about their metabolism, coupled with difficulties in culturing important community members, studies of Fe-oxidizing bacteria (FeOB) have lagged behind those of other important microbial lithotrophic metabolisms. Recently, research on lithotrophic, oxygen-dependent FeOB that grow at circumneutral pH has accelerated. This work is driven by several factors including the recognition by both microbiologists and geoscientists of the role FeOB play in the biogeochemistry of iron and other elements. The isolation of new strains of obligate FeOB allowed a better understanding of their physiology and phylogeny and the realization that FeOB are abundant at certain deep-sea hydrothermal vents. These ancient microorganisms offer new opportunities to learn about fundamental biological processes that can be of practical importance.