Reductive Transformations of a Pyrazolate-Based Bioinspired Diiron-Dinitrosyl Complex.

Flavo-diiron nitric oxide reductases (FNORs) are a subclass of nonheme diiron proteins in pathogenic bacteria that reductively transform NO to NO, thereby abrogating the nitrosative stress exerted by macrophages as part of the immune response. Understanding the mechanism and intermediates in the NO detoxification process might be crucial for the development of a more efficient treatment against these bacteria. However, low molecular weight models are still rare, and only in a few cases have their reductive transformations been thoroughly investigated. Here, we report on the development of two complexes, based on a new dinucleating pyrazolate/triazacyclononane hybrid ligand L, which serve as model systems for nonheme diiron active sites. Their ferrous nitrile precursors [L{Fe(R'CN)}(μ-OOCR)](X) (1) can be readily converted into the corresponding nitrosyl adducts ([L{Fe(NO)}(μ-OOCR)](X), 2). Spectroscopic characterization shows close resemblance to nitrosylated nonheme diiron sites in proteins as well as previous low molecular weight analogues. Crystallographic characterization reveals an anti orientation of the two {Fe(NO)} (Enemark-Feltham notation) units. The nitrosyl adducts 2 can be (electro)chemically reduced by one electron, as shown by cyclic voltammetry and UV/vis spectroscopy, but without the formation of NO. Instead, various spectroscopic techniques including stopped-flow IR spectroscopy indicated the rapid formation, within few seconds, of two well-defined products upon reduction of 2a (R = Me, X = ClO). As shown by IR and Mössbauer spectroscopy as well as X-ray crystallographic characterization, the reduction products are a diiron tetranitrosyl complex ([L{Fe(NO)}](ClO), 3a') and a diacetato-bridged ferrous complex [LFe(μ-OAc)](ClO) (3a″). Especially 3a' parallels suggested products in the decay of nitrosylated methane monooxygenase hydroxylase (MMOH), for which NO release is much less efficient than for FNORs.