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Bispidine ligands are extremely rigid, easy to synthesize and available in a large variety. They enforce coordination geometries derived from cis-octahedral, and the two vacant coordination sites with the tetradentate ligand systems are sterically and electronically distinct; with the isomeric pentadentate ligands, the site of the oxo group is enforced by the ligand (see Figure). Coligands coordinated trans to N3 generally have strong and short bonds, those trans to N7 are more labile. Reasons are thoroughly analyzed on the basis of computational work as well as experimental structural data, thermodynamics, spectroscopy and reactivities. Implications with respect to the mechanism of formation and the structure and spin state of high valent iron oxidants are analyzed, and possibilities to tune the spin state, structure and reactivity of high valent iron complexes are discussed.
Special emphasis will be on the question why different families of supporting ligands and the variation of the metal center (e.g. FeIV vs. MnIV) may lead to subtle changes in the reaction mechanism. The problem of the experimental determination and computation of accurate FeIV/III redox potentials will also be addressed, and a novel environmentally important reaction, the anaerobic formation of methane from methionine and from acetic acid will be discussed. |