Quantum chemical modeling of the oxidation of dihydroanthracene by the biomimetic nonheme iron catalyst [(TMC)FeIV(O)]2+

Hybrid density functional theory has been employed to model the oxidation of dihydroanthracene (DHA = C14H12) to anthracene (C14H10) by the biomimetic iron complex [(TMC)Fe-IV(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 (CF3CO2) 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 (Delta G) 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 Fe-IV(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 Fe-II quintet state, releasing a water molecule and the anthracene product. Because of the formation of the aromatic anthracene molecule, the reaction is very exothermic.