The mechanism of water oxidation catalysis promoted by [tpyRu(IV)=O]2L3+: a computational study

The resting state of the recently reported water oxidation catalyst [tpyRu(II)-OH(2)](2)L(3+) (tpy = terpyridine; L = bipyridylpyrazolylic anion) ([2,2](3+)) must be activated by a series of proton-coupled oxidations in which four protons and four electrons are removed overall to afford the catalytically competent species [tpyRu(IV)O](2)L(3+) ([4,4](3+)). We have examined all of the plausible redox intermediates utilizing density functional theory coupled to a continuum solvation model. Our calculations reproduce well the first three redox potentials under pH = 1 conditions, and a reasonable correlation between theory and experiment is found for the fourth irreversible redox process that accompanies O(2) generation. The computed oxidation potentials to access [5,4](4+) and [5,5](5+), 1.875 and 2.032 V vs NHE, respectively, exclude the otherwise plausible possibilities of the catalytically active species having a higher oxidation state. [4,4](3+) has an antiferromagnetically coupled ground state in which one ruthenium has two unpaired electrons antiparallel to those of the other ruthenium. As we found in our previous work, two radicaloid terminal oxygen moieties with different spin orientations that are induced by spin polarization from the electron-deficient Ru(IV) centers are found. Two mechanistic scenarios are relevant and interesting for the key O-O bond formation event: intramolecular oxo-oxo coupling and coupling between one terminal oxo and the oxygen atom of the incoming water substrate. The intramolecular oxo-oxo coupling is facile, with a low barrier of 13.9 kcal mol(-1), yielding a peroxo intermediate. The necessary subsequent addition of water in an associative substitution mechanism to cleave one of the Ru-peroxo bonds, however, is found to be impractical at room temperature, with a barrier of DeltaG(double dagger) = 30.9 kcal mol(-1). Thus, while plausible, the intramolecular oxo-oxo coupling is unproductive for generating molecular dioxygen. The intermolecular O-O coupling is associated with a high barrier (DeltaG(double dagger) = 40.2 kcal mol(-1)) and requires the assistance of an additional proton, which lowers the barrier dramatically to 24.5 kcal mol(-1).