Applications of the Marcus cross relation to inner sphere reduction of O₂: Implications in small-molecule activation

Application of the Marcus cross relation (MCR) to determine electron transfer rate constants is well documented. In addition to outer sphere reduction/oxidation of O₂/O₂^-, the MCR is also applicable for proton transfer (PT), proton-coupled electron transfer (PCET), and hydride transfer rate constants. A question this report aims to address is whether or not the MCR can predict rate constants for inner sphere electron transfer (ISET) reactions between iron complexes and O₂. By including the iron-superoxide bond dissociation energy (BDFE_Fe-O2) into the MCR equilibrium constant, close values to experimental inner sphere O₂ reductions are estimated. This insight of including the iron-superoxide binding energy as a design element for the development of synthetic oxygenases is considered.