The Substitution of Bisulfite by Water on the Hydrated Iron(III) Center: A Mechanistic Study

The formation of sulfate, a major component of fine particles posing significant threats to human health, is closely linked to the speciation of Fe(III) complexes with S(IV) at the air-water interface, a crucial yet underexplored aspect of atmospheric sulfur cycling. The speciation of Fe(III)-sulfito complexes at the interface remains poorly understood and thus hinders the understanding of the oxidizing properties of transition metals in atmospheric aerosol. Here, we reveal the coordination preference of the active reaction center (Fe(III)-sulfito complex) and the ligand substitution mechanism between Fe(III)-sulfito complexes and H₂O at the air-water interface. Our results demonstrate that the Fe(III) complexes coordinated by HSO₃⁻ undergo a rapid and spontaneous transformation from S-coordination to O-coordination within picoseconds. Furthermore, the substitution reaction of HSO₃⁻ in [Fe(H₂O)₄(OH)(HSO₃)]⁺/[Fe(H₂O)₃(OH)₂(HSO₃)] by the H₂O nucleophile proceeds via both a kinetically and thermodynamically favorable pathway, characterized by free energy barriers of ∼6.39/3.81 kcal/mol. These findings further demonstrate that HSO₃⁻ preferentially occupies the second hydration shell of hydrolyzed Fe(III) complexes. The dynamic equilibrium between competing coordination forms provides molecular-scale insights into interfacial metal-sulfur chemistry, where distinct Fe(III) complexes govern the catalytic behavior of the system, thereby bridging coordination chemistry and atmospheric science.