Bioorganometallic chemistry and super-reduced biological metal centers

Bioinorganic centers such as cobalamin (B12), hydrogenases (H2-ase) or nitrogenases (N2-ase) achieve reactions unusual for biochemistry, involving unusually reduced metal oxidation or ligation states (e.g., Fe(I), Co(I) and/or unusual (for biology) metal complexation – with -carbon and metal-hydrogen bonds. In this, they overlap with the relatively narrow field of bioorganometallic chemistry – i.e., organometallic chemistry in biological environments and/or with biological relevance. Their mechanistic details are still a matter of exploration. Examples of challenging questions in these fields include (1) the nature/properties of metal-hydride complexes known to be relevant for N2-ase and H2-ase, but also suggested in B12; in the case of N2-ase and to some degree H2-ase this also entails isomerism insofar as the location of the hydride, but also possibly redox isomerism; (2) the role of low-valent states (e.g., Fe(I)) in such bioinorganic centers; (3) the mechanisms and reasons whereby such low-valent centers become available; for instance, in the case of N2-ase (especially the Fe-only N2-ase) the coordination sphere is dominantly sulfur-based similarly to ferredoxins; (4) to what extend does the choice of a given transition metal for these bioinorganic centers rely on environmental/evolutionary availability, and to what extent on other factors (e.g., a comparison of ferribalamin vs. B12, or a comparison of other transition metals instead of iron in N2-ase, or instead of Fe/Ni in H2-ase)?