Thermodynamics of Oxygen Activation by Macrocyclic Complexes of Rhodium

The oxidation of ABTS^2- [2,2‘-azino-bis(3-ethylbenzothiazoline-6-sulfonate)] with a superoxorhodium(III) complex, L²(H₂O)RhOO²⁺ (L² = meso-hexamethylcyclam) is characterized by an acid-dependent equilibrium constant, log(K_e/[H⁺]) = 4.91 ± 0.10 in the pH range of 4.89−6.49. This equilibrium constant was used to calculate the reduction potential for the L²(H₂O)RhOO²⁺/L²(H₂O)RhOOH²⁺ couple, E⁰ = 0.97 V vs NHE. The pH dependence of the kinetics of the L²(H₂O)RhOOH²⁺/I^- reaction yielded the acid dissociation constant for the coordinated water in L²(H₂O)RhOOH²⁺, pK_a = 6.9. Spectrophotometric pH titrations provided pK_a = 6.6 for the superoxo complex, L²(H₂O)RhOO²⁺. The combination of the two pK_a values with the reduction potential measured in acidic solutions yielded the reduction potential E⁰ = 0.95 V for the L²(HO)RhOO⁺/L²(HO)RhOOH⁺ couple. Thermochemical calculations yielded the bond-dissociation free energy of the L²(H₂O)RhOO−H²⁺ bond as 315 kJ/mol at 298 K.