Electronic structure and substitution and redox reactivity of imidazolate-bridged complexes of pentacyanoferrate and pentaammineruthenium

The mixed-valence compound [(NC)(5)Fe-II-Im-Ru-III(NH3)(5)](-),M(i), was prepared in solution and as a solid sodium salt from [Fe(CN)(5)H2O](3-) and [Ru(NH3)(5)Im](2+). The binuclear complex shows two bands at 366 nm (epsilon = 3350 M(-1) cm(-1)) and 576 nm (epsilon = 1025 M(-1) cm(-1)), assigned as LMCT transitions, as well as a near-IR band at 979 nm (epsilon = 962 M(-1) cm(-1)) associated with an intervalence transition. By calculation of the Hush model parameters alpha(2) and H-ab (delocalization and electronic coupling factors, respectively), the complex is defined as a valence-trapped Fe-II-Ru-III system; this is confirmed by the measured redox potentials at -0.20 V and 0.30 V, associated with redox processes at the ruthenium and iron center, respectively. The formation stability constant of the mixed-valence ion was obtained through independent measurements of k(f) and k(d), the formation and dissociation specific rate constants, respectively. The stabilization of M(i) with respect to disproportionation into the isovalent states, as well as toward the formation of the electronic isomer, Fe-III-Im-Ru-II, was also estimated. The fully reduced (R(i)) and fully oxidized (O-i) binuclear complexes were prepared in solution and characterized by UV-vis spectroscopy. The kinetics of the reactions of R(i) and M(i) with peroxydisulfate were measured and a mechanistic analysis was performed, showing the relevance of electronic isomerization in completing the full conversion to O-i, through the assistance of the Ru-II(NH3)(5)(2+) center in the oxidation of the neighboring Fe-II(CN)(5)(3-) moiety. The latter results are compared with those obtained with related complexes comprising different X(5)M-L moieties bound to Ru-II(NH3)(5)(2+). A linear correlation is displayed by plotting ln k(et) against E degrees(Ru), associated with the intramolecular oxidation rate constant of Ru-II in the ion pair (binuclear species + peroxydisulfate) and the reduction potential of the corresponding Ru-III,Ru-II couple in the ion pair.