Nitrate NO more

There are many examples of how nature breaks down small molecules effortlessly. However, such reactions pose a significant challenge to chemists and thus the enzymatic cleavage of strong bonds is a prominent source of inspiration for small-molecule chemistry. One class of reactions that has proven reluctant to translate into the flask is the reduction of oxyanions. In particular, nitrates (NO₃⁻) and perchlorates (ClO₄⁻) are difficult to reduce, demanding harsh reaction conditions to furnish NO and Cl⁻, respectively. These oxyanions are water-supply pollutants that are critical — but problematic — to remove in a practical manner. Now, a team from the University of Illinois at Urbana-Champaign led by Alison Fout has demonstrated a bio-inspired approach to carrying out these reductions catalytically (Science 354, 741; 2016). Fout and co-workers looked to natural systems for inspiration, noting that their active sites featured redox-active heme centres surrounded by extended hydrogen-bonding networks that work in tandem to stabilize the high-valent iron–oxo intermediates. Previously, Fout and co-workers had synthesized azafulveneamine (N(afa^Cy)₃) iron complexes and observed structural and electronic characteristics similar to those of metalloenzyme active sites, wherein ligand tautomerization triggers the formation of the hydrogen bonds necessary to carry out multi-electron redox in a stoichiometric fashion.

Now, the group has found that the triflate salt of these complexes, N(afa^Cy)₃FeOTf₂, is effective for transient coordination of nitrate and perchlorate to the metal, forming intermediates that readily react to form the desired iron nitrosyl and chloride complexes. Comparison to the redox-inert zinc analogues showed that the secondary coordination sphere was not involved in coordinating the oxyanions. It was, however, required for reduction, playing a significant role in stabilizing the metal–oxo centre. The team then demonstrated catalytic reductions, resulting in 3.5 turnovers for nitrate and 3 turnovers for perchlorate — while low, even a few turnovers for such challenging reactions is impressive. Although there is work to come in developing a more practical reduction process, these results show that well-designed small-molecule catalysts can handle even the toughest of redox reactions.