PUB - Publikationen an der Universität Bielefeld

The dinucleating bis(tetradentate) ligand susan [= 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10-tetraazadecane] reacts with [Fe(OH2)(6)](BF4)(2) to the complex [(susan){(FeF)-F-III(mu-O)(FeF)-F-III}](BF4)(2). In contrast, the reaction of the ligand derivative susan(6-Me) with 6-methylpyridyl instead of pyridyl donors with [Fe(OH2)(6)](BF4)(2) results in the complexes [(susan(Me)){Fe-II(mu-F)(2)Fe-II}](BF4)(2) and [(susan(Me)){(FeF)-F-II(mu-F)(FeF)-F-III}](BF4)(2). The synthesis of the mu-oxo diferric complex [(susan(6-Me)){(FeF)-F-III(mu-O)(FeF)-F-III}](2+) requires the addition of H2O2 as oxidant. The best synthesis is the H2O2 oxidation of the new diferrous precursor [(susan(6-Me)){Fe-II(mu-F)(2)Fe-II}](ClO4)(2). The introduction of the 6-methylpyridine donors results in an elongation of the Fe-N-py bonds. The resulting lower electron donation is not compensated by the mu-oxo bond but slightly by the Fe-N-amine bonds. These structural changes are correlated with the nu(as)(FeOFe) stretching mode, a smaller Mossbauer quadrupole splitting, a smaller ligand field splitting, a weaker antiferromagnetic exchange, and an anodic shift of the redox potentials. This destabilization of the ferric state correlates with the different reactivity of the diferrous precursors to the mu-oxo diferric complexes: air for susan and H2O2 for for susan(6-Me). Structural analysis indicates a steric repulsion of the 6-methyl group with the fluorido ligand in cis-position as origin for the longer Fe-N-py bonds, which seems to be a general phenomenon.