Investigations of Synthetic Models of Mononuclear Nonheme Iron Dioxygenases

Ring cleaving dioxygenases, such as o-aminophenol dioxygenase (APDO) and extradiol catechol dioxygenases (CatD), play an important role in human metabolism and the degradation of aromatic pollutants, yet questions still remain concerning the enzymatic mechanisms. One area of the catalytic cycle that remains controversial is the geometric and electronic structure of the intermediate formed after O2 binding to the Fe(II) centers. Synthetic model systems can be useful for studying enzyme active sites, as they are easier to modify and characterize than the enzymes themselves. However, synthetic models of APDOs have been relatively rare thus far. We prepared several monoiron(II) complexes that faithfully model the enzyme-substrate intermediates of o-aminophenol dioxygenases (APDO) and catechol dioxygenases. The complexes use either the Ph2Tp (Ph2Tp = hydrotris(3,5-diphenylpyrazol-1-yl)borate) or Ph2TIP (Ph2TIP = tris(4,5-diphenyl-1-methylimidazole)phosphine) supporting ligands and one of three bidentate, redox-active ligands: 4-tert-butylcatecholate, 4,6-di-tert-butyl-2-aminophenolate, or 4-tert-butyl-1,2-phenylenediamine. These complexes have been extensively characterized with crystallographic, spectroscopic, and electrochemical techniques, in conjunction with computational methods (e.g, density functional theory). Each complex is reactive towards O2, and the geometric and electronic structures of the resulting species were examined with various methods to determine whether the oxidation is iron-based, ligand-based, or a combination of both. Treatment of the Ph2Tp Ph2TIP monoiron(II) aminophenolate complex with a phenoxyl radical results in formation of a complex containing an iron(II) center coordinated to an iminobenzosemiquinonate radical, that to the best of our knowledge has no synthetic precedence. Further oxidation leads to a complex best described as a ferric center bound to the iminiobenzosemiquinate radical. The electronic structures of these complexes were determined with the aid of spectroscopic and computational methods. Several monoiron(II) complexes were also prepared to model the active-site structure of β-diketone dioxygenase (Dke1). For this purpose, we employed the Ph2Tp supporting ligand and acacX substrate ligands, where acacX represents the anion of dialkyl malonate. Upon exposure to O2 in toluene it was found that the complexes exhibited reactivity similar to Dke1, although at a much slower rate than the native enzyme