Probing the Mechanism for 2,4′-Dihydroxyacetophenone Dioxygenase Using Biomimetic Iron Complexes

In this study, we report the synthesis and characterization of [Fe(T1Et4iPrIP)(2-OH-AP)(OTf)](OTf) (2), [Fe(T1Et4iPrIP)(2-O-AP)](OTf) (3), and [Fe(T1Et4iPrIP)(DMF)3](OTf)3(4) (T1Et4iPrIP = tris(1-ethyl-4-isopropyl-imidazolyl)phosphine; 2-OH-AP = 2-hydroxyacetophenone, and 2-O-AP–= monodeprotonated 2-hydroxyacetophenone). Both 2and 3serve as model complexes for the enzyme–substrate adduct for the nonheme enzyme 2,4′-dihydroacetophenone (DHAP) dioxygenase or DAD, while 4serves as a model for the ferric form of DAD. Complexes 2–4have been characterized by X-ray crystallography which reveals T1Et4iPrIP to bind iron in a tridentate fashion. Complex 2additionally contains a bidentate 2-OH-AP ligand and a monodentate triflate ligand yielding distorted octahedral geometry, while 3possesses a bidentate 2-O-AP–ligand and exhibits distorted trigonal bipyramidal geometry (τ = 0.56). Complex 4displays distorted octahedral geometry with 3 DMF ligands completing the ligand set. The UV–vis spectrum of 2matches more closely to the DAD-substrate spectrum than 3, and therefore, it is believed that the substrate for DAD is bound in the protonated form. TD-DFT studies indicate that visible absorption bands for 2and 3are due to MLCT bands. Complexes 2and 3are capable of oxidizing the coordinated substrate mimics in a stoichiometric and catalytic fashion in the presence of O2. Complex 4does not convert 2-OH-AP to products under the same catalytic conditions; however, it becomes anaerobically reduced in the presence of 2 equiv 2-OH-AP to 2.