We recently reported on our studies of the acidity dependence of the iron-catalyzed bromate-malonic acid Belousov-Zhabotinsky oscillating reaction (J. Phys. Chem. A 2018, 122, 6183-6195). Compelling agreement between the experiments and the simulations was achieved only with the inclusion of second-order proton-catalysis of the oxidation of bromomalonic acid (BrMA) by the [Fe(phen)3]3+ species. To further explore this mechanistic proposal, we performed potential energy surface analyses of species likely involved in the proton-catalysis in the electron transfer reaction of the ferriin species. The aggregation of a cationic transition metal complex with anionic or neutral outer sphere ligands provides stabilization due to electrostatic interactions, hydrogen-bonding, and van der Waals interactions. The stabilization is expected to increase with the charge of the transition metal complex, with the charge of the ligand, and with the number of ligands. As a consequence, it is reasonable to expect that the ionization energy decreases with increasing aggregation. As a corollary, one might expect a lower driving force for the reduction of the higher-oxidation state transition metal complex with increasing aggregation with outer sphere ligands. Here we are reporting the results of ab initio studies of the oxidation of ferroin [Fe(phen)3]2+ to ferriin [Fe(phen)3]3+ depending on the aggregation with varying numbers (n = 0-3) of sulfates (SO42-), bisulfates (HSO4-), and sulfuric acids (H2SO4). All structures were determined at the SMD(APFD/6-311G*) level. We will report structures and ionization energies as well as proton affinities and ligand binding energies. We will also discuss molecular orbital and spin densities to demonstrate that the radical density is associated with the α-HOMO-9 and consider implications. We found the expected dependence of the ionization energies of the aggregates with varying degrees of aggregation and protonation. Yet the effects on ionization energies are small and the pH-dependence of the exchange reaction of a sulfuric acid ligand by the organic reductant also may affect the rate of the ferriin reduction.

DFT investigation into the role of outer-sphere coordination in the pH dependence of the ferriin reduction in the iron-catalyzed Belousov-Zhabotinsky reaction

ACS Fall 2021

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