Isolating Fe-O(2) Intermediates in Dioxygen Activation by Iron Porphyrin Complexes

Dioxygen (O(2)) is an environmentally benign and abundant oxidant whose utilization is of great interest in the design of bioinspired synthetic catalytic oxidation systems to reduce energy consumption. However, it is unfortunate that utilization of O(2) is a significant challenge because of the thermodynamic stability of O(2) in its triplet ground state. Nevertheless, nature is able to overcome the spin state barrier using enzymes, which contain transition metals with unpaired d-electrons facilitating the activation of O(2) by metal coordination. This inspires bioinorganic chemists to synthesize biomimetic small-molecule iron porphyrin complexes to carry out the O(2) activation, wherein Fe-O(2) species have been implicated as the key reactive intermediates. In recent years, a number of Fe-O(2) intermediates have been synthesized by activating O(2) at iron centers supported on porphyrin ligands. In this review, we focus on a few examples of these advances with emphasis in each case on the particular design of iron porphyrin complexes and particular reaction environments to stabilize and isolate metal-O(2) intermediates in dioxygen activation, which will provide clues to elucidate structures of reactive intermediates and mechanistic insights in biological processes.