Complete Series of {FeNO}~8, {FeNO}~7, and {FeNO}~6 Complexes Stabilized by a Tetracarbene Macrocycle

摘要

Use of a macrocydic tetracarbene ligand, which is topologically reminiscent of tetrapyrrole macrocydes though electronically distinct, has allowed for the isolation, X-ray crystallo-graphic characterization and comprehensive spectroscopic investigation of a complete set of {FeNO}~x complexes (x = 6, 7, 8). Electrochemical reduction, or chemical reduction with CoCp_2, 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 ~(57)Fe Moessbauer, 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 ~(57)Fe Mossbauer 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 spedes. 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.