Kinetico-Mechanistic Study of the Chemical Redox Cycling of Cubic PBA {M⁺} ⊂ (Co^III₄/Fe^II₄) Structures

We report a kinetic and electrochemical study of the four-electron redox processes (Fe^II/Fe^III) in cyanido-bridged cubic structures {M⁺} ⊂ (Co^III₄/Fe^II₄), (M = {K⁺}, {Na(OH₂)⁺}). Oxidation with S₂O₈^2- is kinetically inhibited in neutral/borate media and requires >10³-fold oxidant; at pH = 0 the four Fe^II → Fe^III steps become resolvable. Time-resolved UV-vis reveal four sequential oxidations whose second-order rate constants decrease by 1-2 orders of magnitude per step; Eyring analysis shows modest ΔH^⧧ values and increasingly negative ΔS^⧧ with advancing oxidation steps. The {Na(OH₂)⁺}-encapsulated cube oxidizes over a narrow time window and, on full oxidation, ejects its encapsulated cation, whereas the symmetrical {K⁺} cube displays more widely spaced kinetics between steps and retention of the encapsulated cation. Partial reduction of the oxidized species at alkaline pH proceeds through three consecutive steps but is limited by the insolubility of fully oxidized material. Heterogeneous oxidation by cyclic voltammetry indicates that low external [Na⁺] also promotes expulsion of {Na(OH₂)⁺}, while the larger {K⁺} cation remains encapsulated even at high [Na⁺], as indicated by the lack of changes in the electrochemical reversibility.