Anomalous Reactivity of Ceric Nitrate in Ruthenium “Blue Dimer”-Catalyzed Water Oxidation

Abstract

At high concentrations, nitrate ion alters the dynamics of ruthenium "blue dimer"-catalyzed water oxidation by Ce(IV) such that the oxidation rate is enhanced and a unique reaction intermediate accumulates. This intermediate is characterized by distinct EPR, optical, and resonance Raman (RR) spectra, with the appearance in the latter of a new oxygen isotope-sensitive band. Both Ce(IV) and nitrate are required to generate this intermediate, which suggests ceric-nitrate complexes as the causative agents. Use of (18)O-labeled and (15)N-labeled materials has established that (1) the new RR band is not an O-O stretching mode (for example, as might be associated with a peroxo species) but involves the O atom coordinated to a Ru center, and (2) the O(2) product does not contain an O atom derived from nitrate, eliminating several plausible pathways involving O-atom transfer to oxidized dimer. Although these results are surprising, similar phenomena have been reported for water oxidation catalyzed by monomeric Ru complexes. The dramatic effects observed for the "blue dimer" make it an ideal candidate for further study.

Molecular Catalysts for Water Oxidation

“…In the unexpected twist, the nitrate counterion of the Ce(IV) oxidant, usually assumed to be innocent, was shown by UV−visible, resonance Raman, and Raman data to have an influence on the dynamics of the BD, although no O derived from nitrate appeared in the O 2 formed. 156 The central position of the BD is further emphasized by the many computational studies that have been carried out on it. For example, Bianco et al 157,158 looked at the O−O bondforming step by DFT with an explicit solvent treatment.…”

Mechanism of Catalytic Water Oxidation by the Ruthenium Blue Dimer Catalyst: Comparative Study in D2O versus H2O

“…The BD[5,5] intermediate is included in Figure 1 based on a previous literature report [9] as well as a recent manuscript by Stull et al [16] which summarizes conditions for observation of this intermediate in the BD catalytic cycle. The authors, however, do not explain the origin of the EPR signal assigned to BD[5,5] which has two interacting Ru d 3 centers and thus is not compatible with the spin S = 1/2 electronic configuration.…”

“…Resonance Raman experiments previously detected a 683 cm −1 band which underwent a 46 cm −1 shift upon 16 O/ 18 O substitution [11]. Assignment of this band to the O–O vibration coupled to the Ru-O-Ru bridge was premature [11], as an isotope labeling experiment in 50% H 2 16 O and 50% H 2 18 O did not reveal the 1:2:1 intensity pattern expected for the O–O fragment (see Figure S6, [12] and [16]). This still does not rule out the possibility that the detected vibration is a Ru–O stretch in the Ru–OOH peroxide fragment.…”

Synergistic oxygen atom transfer by ruthenium complexes with non-redox metal ions

“…Thus, collecting the valence and coordination information of compound 4, with the absorption peak at 650 nm, is essential to understand the mechanism of Lewis acid-accelerated-epoxidation. 92 The above results strongly suggest that our unexpected species, compound 4, which can be generated from the oxygen transfer reaction between high valent Ru(IV) species and olefin in the presence of Lewis acid, is an oxo-bridged Ru(III) dimer.…”