Novel iron(III) complexes with imidazole containing tripodal ligands as model systems for catechol dioxygenases

The iron (III) complexes [Fe(bpia)Cl2][FeCl4] (1), [Fe(bipa)Cl2](ClO4) (2), and [Fe2O(bpia)2Cl2]Cl2(4MeOH (3) (bpia: bis[(2-pyridyl)methyl][(1-methylimidazol-2-yl)methyl]amine, bipa: bis[(1-methylimidazol-2-yl)methyl][(2-pyridyl)methyl]amine) were synthesized. 1 and 2 are structural and functional models for catechol 1,2-dioxygenase. All compounds are characterized by spectroscopic methods and X-ray structure analysis. 3 was also investigated by EXAFS. The coordination environment around all Fe(III) cores is distorted octahedral by the tripodal ligand, chloride, and in the case of 3, additionally, oxygen. Chloride and oxygen are bonded labile in the functionally essential cis positions to the iron core. These are the binding sites of the catechol. The cation in 3 is formed as a dimerization product of 1 so that the oxo ligand acts as a linear bridge with a Fe⋯Fe separation of 3.5756(8) Å. These properties of 3 have been used to study the amplitude enhancement in the EXAFS signal due to multiple scattering. In situ prepared iron(III) complexes with the ligands bpia and bipa show significant intradiol catechol dioxygenase activity with respect to the cleavage of 3,5-di-tert-butylcatechol and pyrocatechol