Modeling the 2-His-1-Carboxylate Facial Triad: Iron−Catecholato Complexes as Structural and Functional Models of the Extradiol Cleaving Dioxygenases

Mononuclear iron(II)− and iron(III)−catecholato complexes with three members of a new 3,3-bis(1-alkylimidazol-2-yl)propionate ligand family have been synthesized as models of the active sites of
the extradiol cleaving catechol dioxygenases. These enzymes are part of the superfamily of dioxygen-activating mononuclear non-heme iron enzymes that feature the so-called 2-His-1-carboxylate facial triad.
The tridentate, tripodal, and monoanionic ligands used in this study include the biologically relevant
carboxylate and imidazole donor groups. The structure of the mononuclear iron(III)−tetrachlorocatecholato
complex [Fe(L3)(tcc)(H2O)] was determined by single-crystal X-ray diffraction, which shows a facial N,N,O
capping mode of the ligand. For the first time, a mononuclear iron complex has been synthesized, which
is facially capped by a ligand offering a tridentate Nim,Nim,Ocarb donor set, identical to the endogenous
ligands of the 2-His-1-carboxylate facial triad. The iron complexes are five-coordinate in noncoordinating
media, and the vacant coordination site is accessible for Lewis bases, e.g., pyridine, or small molecules
such as dioxygen. The iron(II)−catecholato complexes react with dioxygen in two steps. In the first reaction
the iron(II)−catecholato complexes rapidly convert to the corresponding iron(III) complexes, which then, in
a second slow reaction, exhibit both oxidative cleavage and auto-oxidation of the substrate. Extradiol and
intradiol cleavage are observed in noncoordinating solvents. The addition of a proton donor results in an
increase in extradiol cleavage. The complexes add a new example to the small group of synthetic iron
complexes capable of eliciting extradiol-type cleavage and provide more insight into the factors determining
the regioselectivity of the enzymes.