cis,cis-[(bpy)₂Ru^VO]₂O⁴⁺ Catalyzes Water Oxidation Formally via in Situ Generation of Radicaloid Ru^IV−O•

Abstract

The mechanism of the catalytic oxidation of water by cis,cis-[(bpy)(2)Ru(OH(2))](2)O(4+) to give molecular dioxygen was investigated using Density Functional Theory (DFT). A series of four oxidation and four deprotonation events generate the catalytically competent species cis,cis-[(bpy)(2)Ru(V)O](2)O(4+), which breaks the H-OH bond homolytically at the rate determining transition state to give a hydroperoxo intermediate. Our calculations predict a rate determining activation barrier of 25.9 kcal/mol in solution phase, which is in reasonable agreement with the previously reported experimental estimate of 18.7-23.3 kcal/mol. A number of plausible coupling schemes of the two metal sites including strong coupling, weak ferromagnetic and weak antiferromagnetic coupling have been considered. In addition, both high-spin and low-spin states at each of the Ru(V)-d(3) centers were explored and we found that the high-spin states play an important mechanistic role. Our calculations suggest that cis,cis-[(bpy)(2)Ru(V)O](2)O(4+) performs formally an intramolecular ligand-to-metal charge transfer when reacting with water to formally give a cis,cis-[(bpy)(2)Ru(IV)O*](2)O(4+) complex. We propose that the key characteristic of the diruthenium catalyst that allows it to accomplish the most difficult first two oxidations of the overall four-electron redox reaction is directly associated with this in situ generation of two radicaloid oxo moieties that promote the water splitting reaction. A proton coupled metal-to-metal charge transfer follows to yield a Ru(V)/Ru(III) peroxo/aqua mixed valence complex, which performs the third redox reaction to give the superoxo/aqua complex. Finally, intersystem crossing to a ferromagnetically coupled Ru(IV)/Ru(III) superoxo/aqua species is predicted, which will then promote the last redox event to release triplet dioxygen as the final product. A number of key features of the computed mechanism are explored in detail to derive a conceptual understanding of the catalytic mechanism.

Functional models for the oxygen-evolving complex of photosystem II

“…The most recent mechanistic proposal to include O-O bond formation has used DFT calculations to propose a mechanism involving a peroxo species by way of a ruthenium oxo radical that is very similar to mechanism C in Figure 6 [71]. This calculated mechanism is in agreement with the more recent mixed-label O 2 studies discussed above, as well as being analogous to the nucleophilic water mechanism for the OEC discussed above in section 2.2.3.…”

Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium–oxo molecular complex

“…Note that on the basis of the charge and spin analysis, it is not possible to unambiguously associate the Ru-O moiety to a Ru(VI)-oxo or to a Ru(V)-oxyl radical (Table S2). The oxyl radical, however, has been proposed to be a key intermediate in catalytic water splitting by ruthenium complexes as in the "blue dimer" (24,25) and also for the oxygen evolving complex of PSII (26). Similarly to cycle A, also the calculated free energy of cycle B (4.21 eV) is well below the thermodynamic limit for splitting water (Fig.…”

Similarities of artificial photosystems by ruthenium oxo complexes and native water splitting systems

“…Therefore the O─O bond formation between OH and Mn═O is essentially ionic (essentially acid-base type) in nature in accord with the mechanism (ii). Recent DFT computations for blue dimmer (22)(23)(24)28) also supported the acid-base mechanism under the hydrophilic condition. Fig.…”

“…Formation. Accumulated experimental and theoretical studies (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29) on artificial photosynthesis systems have elucidated dual possible mechanisms of the water splitting reaction: (i) the radical coupling (RC) mechanism and (ii) the acid-base mechanism. Tanaka and coworkers have proposed the former mechanism for the water splitting reaction by their Ru-quinone complexes as illustrated in Fig.…”

“…In the past decades, a number of experimental and theoretical studies (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29) have been performed to design artificial photosynthetic systems that mimic native PSII systems. Many binuclear transition-metal catalysts such as LðH 2 OÞM-O-MðOH 2 ÞL or LðH 2 OÞM-BL-MðOH 2 Þ (where L and BL are nonbridging and bridging organic ligands, respectively) were prepared and characterized for their catalytic properties toward water oxidation (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21).…”