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Volumn 47, Issue 6, 2008, Pages 1753-1764

Mechanisms of water oxidation catalyzed by ruthenium diimine complexes

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EID: 42149095410     PISSN: 00201669     EISSN: None     Source Type: Journal    
DOI: 10.1021/ic700724h     Document Type: Review
Times cited : (124)

References (66)
  • 15
    • 42149182719 scopus 로고    scopus 로고
    • Complexes formed from monomelic precursors whose bipyridine ligands bear weakly electron-donating substituents, such as 4,4′-dimethoxy-2,2′- bipyridine or 4,4′,5,5′-tetramethyl-2,2′-bipyridine, are prone to irreversible ligand oxidation and preferential formation of trimeric and higher oligomeric species, presumably via cis and trans geometrical isomerizations. Conversely, bipyridine ligands bearing moderately electron-withdrawing substituents, such as carboxy derivatives, form μ-oxo dimers only under forcing conditions; to date, we have been unable to synthesize dimeric analogues whose bipyridine ligands contain more strongly electron-withdrawing substituents, such as nitro or trifluoromethyl groups
    • Complexes formed from monomelic precursors whose bipyridine ligands bear weakly electron-donating substituents, such as 4,4′-dimethoxy-2,2′- bipyridine or 4,4′,5,5′-tetramethyl-2,2′-bipyridine, are prone to irreversible ligand oxidation and preferential formation of trimeric and higher oligomeric species, presumably via cis and trans geometrical isomerizations. Conversely, bipyridine ligands bearing moderately electron-withdrawing substituents, such as carboxy derivatives, form μ-oxo dimers only under forcing conditions; to date, we have been unable to synthesize dimeric analogues whose bipyridine ligands contain more strongly electron-withdrawing substituents, such as nitro or trifluoromethyl groups.
  • 16
    • 42149106579 scopus 로고    scopus 로고
    • The notation in parentheses is used only to specify the overall oxidation state of the binuclear ion and, unless otherwise specified, is not intended to imply discrete oxidation states on the individual metal ions. For example, 5,5} represents an ion that contains four fewer electrons than {3,3, Although {5,5} is formally written as, bpy 2RuV(O)]2O4, recent DFT calculations suggest that the electron distribution for this ion is more appropriately represented by the chemical formula, bpy)2Ru IV(O•)]2O4, i.e, a complex containing two terminal radicaloid oxo moieties.15
    • 15
  • 19
    • 42149105407 scopus 로고    scopus 로고
    • The catalysts are slowly irreversibly inactivated (by as-yet undetermined pathways) in solutions containing excess oxidant and can also undergo reversible anation at the cis-aqua positions to produce catalytically inactive complexes when appropriate ligands are present.9 Mechanistic investigations by Meyer's group suggest that this anation can also occur during water oxidation, leading to accumulation of inactive {3,4} forms of the catalyst.18-19 Under these conditions, O2 formation becomes rate-limited by (slow) aquation back to the diaqua form of the complex. Because the complex may undergo several cycles of catalysis during its passage through the carbon fiber electrode, particularly at more anodic applied potentials, it is worth considering whether the complexes have been chemically modified by this treatment. Anation is readily detected by cyclic voltammetric experiments, in which large cathodic shifts are observed in E 1
    • 10,20-22 These results constitute strong evidence that the electrochemically prepared solutions contain unmodified catalyst.
  • 33
    • 42149108246 scopus 로고    scopus 로고
    • This interpretation is valid only if water exchange in the higher oxidation states is slow relative to catalyst turnover. The rates of water exchange cited above were directly measured for the {3,3} and {3,4} ions by time-dependent determination of their isotopic composition following incubation of 18OH2-enriched complexes in water of normal isotopic composition.28 This was determined from RR spectra obtained following chemical oxidation to {5,5} by comparing the intensities of the Ru=16O and Ru=18O modes. It was also evident from these analyses that water exchange on {5,5} was slower than loss of the isotope label from the coordination sphere by catalytic O 2 generation. Water exchange on {4,4} has not been directly measured, but kinetic simulations clearly indicate that the time course of evolution of the various isotopes (Figure 4) cannot be reproduced by mechanisms that assume rapid
    • 2 generation. Water exchange on {4,4} has not been directly measured, but kinetic simulations clearly indicate that the time course of evolution of the various isotopes (Figure 4) cannot be reproduced by mechanisms that assume rapid exchange of water from any of the higher oxidation states. In contrast, the reaction traces are accurately modeled by the proposed mechanism using independently determined kinetic parameters.


* 이 정보는 Elsevier사의 SCOPUS DB에서 KISTI가 분석하여 추출한 것입니다.