Oxidation of Mixed-Valence CoIII/FeII Complexes Reversed at High pH:  A Kinetico-Mechanistic Study of Water Oxidation

• Published in: Inorganic chemistry Vol. 43; no. 22; pp. 7187 - 7195
• Main Authors: Bernhardt, Paul V, Bozoglian, Fernando, Macpherson, Brendan P, Martínez, Manuel, Merbach, André E, González, Gabriel, Sienra, Beatriz
• Format: Journal Article
• Language: English
• Published: American Chemical Society 01-11-2004
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0493191

The outer-sphere oxidation of FeII in the mixed-valence complex trans-[L14SCoIIINCFeII(CN)6]-, being L14S an N3S2 macrocylic donor set on the cobalt(III) center, has been studied. The comparison with the known processes of N5 macrocycle complexes has been carried out in view of the important differences occurring on the redox potential of the cobalt center. The results indicate that the outer-sphere oxidation reactions with S2O8 2- and [Co(ox)3]3- involve a great amount of solvent-assisted hydrogen bonding that, as a consequence from the change from two amines to sulfur donors, are more restricted. This is shown by the more positive values found for ΔS ⧧ and ΔV ⧧. The X-ray structure of the oxidized complex has been determined, and it is clearly indicative of the above-mentioned solvent-assisted hydrogen bonding between nitrogen and cyanide donors on the cobalt and iron centers, respectively. trans-[L14SCoIIINCFeIII(CN)6], as well as the analogous N5 systems trans-[L14CoIIINCFeIII(CN)6], trans-[L15CoIIINCFeIII(CN)6], and cis-[L13CoIIINCFeIII(CN)6], oxidize water to hydrogen peroxide at pH > 10 with a rather simple stoichiometry, i.e., [L n CoIIINCFeIII(CN)5] + OH- → [L n CoIIINCFeII(CN)5]- + 1/2H2O2. In this way, the reversibility of the iron oxidation process is achieved. The determination of kinetic and thermal and pressure activation parameters for this water to hydrogen peroxide oxidation leads to the kinetic determination of a cyanide based OH- adduct of the complex. A second-order dependence on the base concentration is associated with deprotonation of this adduct to produce the final inner-sphere reduction process. The activation enthalpies are found to be extremely low (15 to 35 kJ mol-1) and responsible for the very fast reaction observed. The values of ΔS ⧧ and ΔV ⧧ (−76 to −113 J K-1 mol-1 and −5.5 to −8.9 cm3 mol-1, respectively) indicate a highly organized but not very compressed transition state in agreement with the inner-sphere one-electron transfer from O2- to FeIII.