Complete Series of {FeNO}8, {FeNO}7, and {FeNO}6 Complexes Stabilized by a Tetracarbene Macrocycle
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文摘
Use of a macrocyclic tetracarbene ligand, which is topologically reminiscent of tetrapyrrole macrocycles though electronically distinct, has allowed for the isolation, X-ray crystallographic characterization and comprehensive spectroscopic investigation of a complete set of {FeNO}x complexes (x = 6, 7, 8). Electrochemical reduction, or chemical reduction with CoCp2, of the {FeNO}7 complex 1 leads to the organometallic {FeNO}8 species 2. Its crystallographic structure determination is the first for a nonheme iron nitroxyl {FeNO}8 and has allowed to identify structural trends among the series of {FeNO}x complexes. Combined experimental data including 57Fe Mössbauer, IR, UV–vis–NIR, NMR and Kβ X-ray emission spectroscopies in concert with DFT calculations suggest a largely metal centered reduction of 1 to form the low spin (S = 0) {FeNO}8 species 2. The very strong σ-donor character of the tetracarbene ligand imparts unusual properties and spectroscopic signatures such as low 57Fe Mössbauer isomer shifts and linear Fe–N–O units with high IR stretching frequencies for the NO ligand. The observed metal-centered reduction leads to distinct reactivity patterns of the {FeNO}8 species. In contrast to literature reported {FeNO}8 complexes, 2 does not undergo NO protonation under strictly anaerobic conditions. Only in the presence of both dioxygen and protons is rapid and clean oxidation to the {FeNO}7 complex 1 observed. While 1 is stable toward dioxygen, its reaction with dioxygen under NO atmosphere forms the {FeNO}6(ONO) complex 3 that features an unusual O-nitrito ligand trans to the NO. 3 is a rare example of a nonheme octahedral {FeNO}6 complex. Its electrochemical or chemical reduction triggers dissociation of the O-nitrito ligand and sequential formation of the {FeNO}7 and {FeNO}8 compounds 1 and 2. A consistent electronic structure picture has been derived for these unique organometallic variants of the key bioinorganic {FeNO}x functional units.

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