Enzyme-like denitrification with an oxidorhenium(V) complex: reduction of nitrite to N$_2$O

Ever-growing levels of nitrate are threatening a constantly diminishing natural resource, namely drinking water. With the Haber-Bosch process ensuing industrial-scale production of nitrate for artificial fertilizers, mankind is significantly interfering with the global nitrogen cycle. While nature can solve the problem via a cascade of nitrate reducing enzymes, there is no comparable technology available, due to a fundamental lack of scientific knowledge. We have developed oxidorhenium(V) complex [ReOCl(L1)$_2$] (1), that not only catalytically reduces NO$_3$$^−$ to NO$_2$$^−$ and NO under ambient, aqueous conditions, but is the first example of a catalyst to also catalytically reduces NO further to N$_2$O, thereby mirroring the pathway of biological, enzymatic denitrification of NO$_3$$^−$. DFT calculations support a mechanism a reductive coupling of an NO molecule to a Re-nitrosyl complex to give an intermediate Re-(κ$^2$-O$_2$N$_2$$^2−$) hyponitrite complex.

The reduction of nitrite occurs via a single electron transfer, yielding NO and paramagnetic dioxdiorhenium(VI) complex [ReO$_2$(L1)$_2$] (2′). Neutral complex 2′ is not catalytically active anymore, but addition of [Cu(II)Cl$_2$] oxidizes complex 2′ back into the catalytically active cationic complex [ReO$_2$(L1)$_2$]+ (2), thereby closing the catalytic cycle. ... mehrIn addition, we could identify the decomposition products of 2′, namely complex [ReO(L1)$_2$][ReO$_4$] (3a) and of 2, namely complex (H$_2$L1)[ReO$_4$] (3b). The kinetics of formation of both 2′ and 2 as well as their respective decomposition products 3a and 3b were studied by UV–Vis spectroscopy and electrochemistry.