In various non-heme iron oxygenases the Fe(II) center is coordinated by 2 N and 1 O atoms of the 2-His-2-carboxylate facial triad; however, most artificial model complexes bear only N-based ligands. In an effort to closely mimic the coordination environment in α-ketoglutarate dependent oxygenases, we have now employed the Me2tacnO ligand (4,7-dimethyl-1-oxa-4,7-diazacyclononane) in the synthesis of the complexes [(Me2tacnO)FeCl2]2 (1-NNO), [(Me2tacnO)FeCl3] (1 b-NNO) and [(Me2tacnO)Fe(BF)Cl] (2-NNO; BF=benzoylformate). The weaker donation of the O atom in the ligand was found to result in stronger binding of the ligand in trans-position to the O-atom of the ancillary ligand as compared to the corresponding complexes involving the Me3tacn (1,4,7-trimethyl-1,4,7-triazacyclononane) ligand. Furthermore, by stopped-flow techniques we could detect an intermediate (3-NNO) in the reaction of 2-NNO with O2. The spectroscopic features of 3-NNO agree with the involvement of an Fe(IV)-oxo intermediate and hence this study represents the first detection of such an intermediate in the O2 activation of artificial α-ketoglutarate Fe(II) complexes.
