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 N2O, 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)}2­(μ-OOCR)]­(X)2 (1) can be readily converted into the corresponding
nitrosyl adducts ([L­{Fe­(NO)}2­(μ-OOCR)]­(X)2, 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)}7 (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 N2O. 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 = ClO4). 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)2}2]­(ClO4), 3a′) and a diacetato-bridged ferrous complex [LFe2(μ-OAc)2]­(ClO4) (3a″). Especially 3a′ parallels suggested products in the decay of nitrosylated
methane monooxygenase hydroxylase (MMOH), for which N2O
release is much less efficient than for FNORs.