Mechanistic and Energetic Aspects of the Thermal and Photochemical Redox Chemistry of the Octanuclear Cubane Complexes, FeIII8(μ4-O4)(μ-pyrazolate)12X4(X = Cl or Br)

Abstract

The mechanisms of the thermal and photochemical redox reactions of clusters [Fe(III)(8)(mu(4)-O(4))(mu-Pz)(12)X(4)] (Pz = pyrazolate anion, X = Cl or Br) were investigated in this work. Reactions of the complexes with e(-)(sol), C(*)H(2)OH, and several powerful reducing transition metal complexes were investigated using the pulse radiolysis technique. Reaction rates of the outer-sphere electron transfer reactions with transition metal complexes had to be rationalized by invoking the formation of a [Fe(III)(7)Fe(II) '(mu(4)-O(4))(mu-Pz)(12)X(4)](-) intermediate or excited state. A transient species observed in the reaction of the e(-)(sol) with the cubanes can be either an excited state or a reaction intermediate mediating the formation of the stable product, [Fe(III)(7)Fe(II)(mu(4)-O(4))(mu-Pz)(12)X(4)](-). Photoredox reactions, characteristic of the ligand X(-) to Fe(III) charge transfer excited sates, were observed in the 350 nm steady state and 351 nm laser flash irradiations of the cubanes. Quantum yields are limited by the rapid recombination of the photofragments. The charge transfer spectroscopy of the products was rationalized on the basis of parameters derived from the thermal electron transfer reactions.