Quantum Chemical Modeling of the Oxidation of Dihydroanthracene by the Biomimetic Nonheme Iron Catalyst [(TMC)FeIV(O)]2+

Abstract

Hybrid density functional theory has been employed to model the oxidation of dihydroanthracene (DHA = C14H12) to anthracene (C14H10) by the biomimetic iron complex [(TMC)FeIV(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). Experimentally, the reaction has been studied in a solution of the reactants and the counterion trifluoroacetate (CF3C ) in the solvent acetonitrile (CH3CN). Depending on the concentration of trifluoroacetate, different coordination situations have been observed by NMR spectroscopy. The complexity of the chemical environment offers a challenging modeling problem, and five different models were initially considered. The effects of the coordination of either a counterion or a solvent molecule were found to be rather small. The reaction was found to be a two-step process in which the first step is rate-limiting. The free energy of activation (ΔG⧧) for the first H-abstraction was found to be between 14.5 and 16.9 kcal mol-1 depending on the model, in reasonable agreement with experimental data. The second step has a much lower free-energy barrier, found to be completely entropic in origin. In all models, the system is found to have a triplet ground state in the FeIV(O) reactant. A spin-crossing of the triplet and quintet potential energy surfaces occurs before the first transition state, and the system is found to end up in the FeII quintet state, releasing a water molecule and the anthracene product. Because of the formation of the aromatic anthracene molecule, the reaction is very exothermic.