Mechanistic Aspects of Water Oxidation Catalyzed by Organometallic Iridium Complexes

The reactions of three iridium water-oxidation catalysts {[Cp*(IrLLL3)-L-1-L-2]X-n; 1: L-1, L-2 = 2,2-bipyridine (bpy), L-3 = Cl, n = 1, X = C]; 2: L-1, L-2 = 2-benzoylpyridine (bzpy), L-3 = NO3; 3: L-1 = L-2 = L-3 = H2O, n = 2, X = NO3; Cp* = pentamethylcyclopentadienyl} with cerium ammonium nitrate (CAN) and NaIO4 (sacrificial oxidants, SOs) have been studied by Clark electrode measurements (both in solution and in the gas phase), on-line mass spectrometry, manometry and UV/Vis spectroscopy. Furthermore, cyclic voltammetry has been applied to evaluate the relative tendency of 1 and 2 to be oxidized. The turnover frequency (TOF) increases as the ratio (R) between the concentration of the SO and that of the catalyst increases. O-2 production with CAN is observed in experiments with R = 20 for 1 and 3, whereas O-2 becomes detectable with 2 only when R = 40. Catalyst 2 has the highest tendency to be oxidized to Ir-IV and forms a blue intermediate I characterized by a UV/Vis band at 574 nm. The formation of I occurs with the same velocity as that of the production of O-2, which indicates that I is a species directly involved in the catalytic cycle. The disappearance of I, when O-2 evolution is finished, is a second-order process more than one order of magnitude slower than O-2 production and is strongly accelerated by the presence of benzyl alcohol. This suggests that I is a molecular species that slowly undergoes disproportion when catalysis is over. Experiments in which multiple aliquots of SO (CAN) were added (R = 20 and 40) indicate that catalysts 1-3 can reinitiate the catalytic cycle once they have been kept in a dormant state for 0-9 min; the TOFs of the second and third additions are approximately equal and higher than that of the first addition. By combining manometry and on-line mass spectrometry measurements, it was found that O-2 evolution is parallel to the production of a small amount of CO2 owing to catalyst degradation. The TOFs of the experiments performed with NaIO4 as the SO are about 2-3 times lower than those with CAN, but the same reactivity order is found 3 > 2 > 1. The activation parameters were evaluated with NaIO4 for all catalysts and with CAN for 2 at 10-45 degrees C. Delta G(#) is practically the same in all situations (25-26 kcalmol(-1)), whereas Delta H-# is appreciably lower for 2 (13.1 kcalmol(-1) with CAN and 13.3 kcalmol(-1) with NaIO4) than for 1 (16 kcalmol(-1)) and 3 (16.9 kcalmol(-1)). The lowest enthalpic cost with 2 is balanced by the highest entropic cost (-41 calmol(-1)K(-1)) that approaches that typical for an associative bimolecular process.