Iron(III)−Catecholato Complexes as Structural and Functional Models of the Intradiol-Cleaving Catechol Dioxygenases

The structural and spectroscopic characterization of mononuclear iron(III)−catecholato complexes of ligand L4
(methyl bis(1-methylimidazol-2-yl)(2-hydroxyphenyl)methyl ether, HL4) are described, which closely mimic the enzyme−substrate complex of the intradiol-cleaving catechol dioxygenases. The tridentate, tripodal monoanionic ligand
framework of L4 incorporates one phenolato and two imidazole donor groups and thus well reproduces the His2Tyr
endogenous donor set. In fact, regarding the structural features of [FeIII(L4)(tcc)(H2O)] (5·H2O, tcc =
tetrachlorocatechol) in the solid state, the complex constitutes the closest structural model reported to date. The
iron(III)−catecholato complexes mimic both the structural features of the active site and its spectroscopic
characteristics. As part of its spectroscopic characterization, the electron paramagnetic resonance (EPR) spectra
were successfully simulated using a simple model that accounts for D strain. The simulation procedure showed
that the observed g = 4.3 line is an intrinsic part of the EPR envelope of the studied complexes and should not
necessarily be attributed to a highly rhombic impurity. [FeIII(L4)(dtbc)(H2O)] (dtbc = 3,5-di-tert-butylcatechol) was
studied with respect to its dioxygen reactivity, and oxidative cleavage of the substrate was observed. Intradiol- and
extradiol-type cleavage products were found in roughly equal amounts. This shows that an accurate structural
model of the first-coordination sphere of the active site is not sufficient for obtaining regioselectivity.