What factors tune the chemical equilibrium between metal-iodosylarene oxidants and high-valent metal-oxo ones?†
Metal-iodosylarene complexes (1) and high-valent metal-oxo complexes (2) are two key reactive intermediates in oxygenation reactions. Extensive experimental efforts have been carried out to explore the structure–function relatioship of these two elusive oxidants, however, controversial proposals based on these experimental results and the missing mechanistic details call for the interplay of a theoretical approach with these existing experimental ones, especially for the factors that tune the chemical equilibrium between the two oxidants 1 and 2. Herein, density functional calculations have been performed and the results demonstrated that the effect of triflate counterions (OTf) is not the well known axial ligand effect. Instead, it works via a novel halogen bond interaction. This halogen bond interaction not only increases the reaction rate of reversible reactions, but it also makes the equilibrium point shift to the direction of the metal-iodosylarene oxidant, compared with the non-halogen bond case. An experimentally observed species with a signal of S = 5/2 was identified as an OTf–halogen-bonding iron(III)-iodosylbenzene species. The substituent effects of iodoarenes were also studied and the results show that the more fluorine substitution, the higher the reaction barrier and the smaller the amount of metal-iodosylarene oxidant in the oxidation system. Our theoretical study will help researchers in the biomimetic oxidation field have a more profound knowledge on metal-iodosylarene chemistry and design more rational catalysts.