Oxygen Isotope Effects as Structural and Mechanistic Probes in Inorganic Oxidation Chemistry (original) (raw)
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Probing metal-mediated O2 activation in chemical and biological systems
Journal of Molecular Catalysis A: Chemical, 2006
The use of oxygen ( 18 O) isotope fractionation as a mechanistic probe of chemical and biological oxidation reactions, particularly those which involve metal-O 2 adducts, is currently being explored. Summarized here are reactions of enzymes and inorganic compounds for which competitive isotope effect measurements have been performed using natural abundance molecular oxygen and isotope ratio mass spectrometry. The derived 18 O equilibrium isotope effects (EIEs) and kinetic isotope effects (KIEs) reflect the ground state and transition state structures, respectively, for reactions of 16 O-16 O and 18 O-16 O. Normal isotope effects (>1) characterize the binding of O 2 to transition metal centers. The magnitudes, which are primarily determined by the decrease in the O-O force constant accompanying formal electron transfer from the metal to O 2 , suggest that metal superoxo complexes can be distinguished from metal peroxo complexes. Because 18 O isotope effects can be measured during catalytic turnover, they complement existing approaches to elucidating the structures of activated oxygen intermediates based on low-temperature spectroscopy and crystallographic analysis of inorganic model compounds.
Structures of Transition States in Metal-Mediated O2-Activation Reactions
Angewandte Chemie International Edition, 2005
Enrichment of heavy oxygen ( 18 O) from its level at natural abundance is widely used to probe O 2 -dependent processes in the atmospheric and biological sciences. Here we describe the first application of this technique to O 2 -binding reactions of classic inorganic compounds. Competitive 18 O kinetic isotope effects (KIEs) reflecting the ratio of secondorder rate constants for the formation of h 2 -peroxide compounds from 16,16 O 2 and 18,16 O 2 (k 16,16 /k 18,16 ) have been determined (Scheme 1). Related measurements on enzymatic reactions have been interpreted on the basis of equilibrium isotope effects (EIEs) and simple models that neglect variations in the transition-state structure. The present studies suggest that the proposed enzyme mechanisms may need to be reevaluated in light of the sensitivity of O KIEs to changes in the height of the free-energy barrier for reactions involving the formation of a metal-O 2 bond. Oxygenation of solutions prepared from trans-[IrCl(CO)-(PPh 3 ) 2 ], [Ir(dppe) 2 ]Cl, [Pt(PPh 3 ) 4 ], [Pd(PPh 3 ) 4 ], [Ni(PPh 3 ) 4 ], and [Ni(CNtBu) 4 ] (PPh 3 = triphenylphosphine, dppe = 1,2bis(diphenylphosphino)ethane, CNtBu = tert-n-butylisocya-Scheme 1. Competitive 18 O isotope effect on the formation of h 2peroxo complexes.
2014
Competitive oxygen kinetic isotope effects ( 18 O KIEs) on water oxidation initiated by ruthenium oxo (Ru]O) complexes are examined here as a means to formulate mechanisms of O-O bond formation, which is a critical step in the production of "solar hydrogen". The kinetics of three structurally related catalysts are investigated to complement the measurement and computation of 18 O KIEs, derived from the analysis of O 2 relative to natural abundance H 2 O under single and multi-turnover conditions. The findings reported here support and extend mechanistic proposals from 18 O tracer studies conducted exclusively under non-catalytic conditions. It is shown how density functional theory calculations, when performed in tandem with experiments, can constrain mechanisms of catalytic water oxidation and help discriminate between them. † Electronic supplementary information (ESI) available: Full experimental characterization and computational details. See
Metal-oxo-mediated O-O bond formation reactions in chemistry and biology
BioInorganic Reaction Mechanisms, 2012
O-O bond formation is one of the key reactions that ensure life on earth. Dioxygen is produced in photosystem II, as well as in chlorite dismutase. The reaction mechanisms occurring in the enzyme active sites are controversially discussed -although their structures have been resolved with less unambiguity. Artificial molecular catalysts have been developed in the last years to obtain vital insights into the O-O bond formation step. This review put together the scarce literature on the topic that helped in understanding the key steps in the O-O bond formation reactions mediated by high-valent oxo complexes of the first-row transition metals.
Dioxygen activation routes in Mars-van Krevelen redox cycles catalyzed by metal oxides
Catalytic redox cycles involve dioxygen activation via peroxo (OO ⁄) or H 2 O 2 species, denoted as inner-sphere and outer-sphere routes respectively, for metal-oxo catalysts solvated by liquids. On solid oxides, O 2 activation is typically more facile than the reduction part of redox cycles, making kinetic inquiries difficult at steady-state. These steps are examined here for oxidative alkanol dehydrogenation (ODH) by scavenging OO ⁄ species with C 3 H 6 to form epoxides and by energies and barriers from density functional theory. Alkanols react with O-atoms (O ⁄) in oxides to form vicinal OH pairs that eliminate H 2 O to form OO ⁄ at O-vacancies formed or react with O 2 to give H 2 O 2. OO ⁄ reacts with alkanols to reform O ⁄ via steps favored over OO ⁄ migrations, otherwise required to oxidize non-vicinal vacancies. C 3 H 6 epoxidizes by reaction with OO ⁄ with rates that increase with C 3 H 6 pressure, but reach constant values as all OO ⁄ species react with C 3 H 6 at high C 3 H 6 /alkanol ratios. Asymptotic epoxidation/ODH rate ratios are smaller than unity, because outer-sphere routes that shuttle O-atoms via H 2 O 2 (g) are favored over endoergic vacancy formation required for inner-sphere routes. The relative contributions of these two routes are influenced by H 2 O, because vacancies, required to form OO ⁄ , react with H 2 O to form OH pairs and H 2 O 2. OO ⁄-mediated routes and epoxidation become favored at low coverages of reduced centers, prevalent for less reactive alkanols and lower alkanol/O 2 ratios, because H 2 O 2 then reacts preferentially with O ⁄ (forming OO ⁄), instead of vacancies (forming O ⁄ /H 2 O). Such kinetic shunts between two routes compensate for lower barriers required to form H 2 O 2 than OO ⁄. These re-oxidation routes prefer molecular donor (H 2 O 2) or acceptor (alkanol) to perform stepwise two-electron oxidations by dioxygen, instead of kinetically demanding O-atom migrations. The quantitative descriptions, derived from theory and experiment on Mo-based polyoxometalate clusters with known structures, bring together the dioxygen chemistry in liquid-phase oxidations, including electro-catalysis and monooxygenase enzymes, and oxide surfaces into a common framework, while suggesting a practical process for epoxidation by kinetically coupling with ODH reaction.
Reactivity and selectivity descriptors of dioxygen activation routes on metal oxides
Journal of Catalysis, 2019
The activation of dioxygen at typically isolated two-electron reduced centers can lead to the formation of electrophilic superoxo or peroxo species, providing an essential route to form reactive O 2-derived species in biological, organometallic, and heterogeneous catalysts. Alternatively, O 2 activation can proceed via outer sphere routes, circumventing the formation of bound peroxo (OO*) species during oxidation catalysis by forming H 2 O 2 (g), which can react with another reduced center to form H 2 O. The electronic and binding properties of metal oxides that determine the relative rates of these activation routes are assessed here by systematic theoretical treatments using density functional theory (DFT). These methods are combined with conceptual frameworks based on thermochemical cycles and crossing potential models to assess the most appropriate descriptors for the activation barriers for each route using Keggin polyoxometalates as illustrative examples. In doing so, we show that inner sphere routes, which form OO* species via O 2 activation on the O-vacancies (*) formed in the reduction part of redox cycles, are mediated by early transition states that only weakly sense the oxide binding properties. Outer sphere routes form H 2 O 2 (g) via O 2 activation on OH pairs (H/OH*) formed by dissociation of H 2 O on O-vacancies; their rates and activation barriers reflect the rates of the first H-atom transfer from H/OH* to O 2. The activation barriers for this H-transfer step depend on the binding energy of more weakly-bound H-atom in H/OH* pairs (HAE 2) and on the Å OOH-surface interaction energy at its product state (E int 0). The E int 0 values are similar among oxides unless a large charge-balancing cation is present and interacts with Å OOH; consequently, HAE 2 acts as an appropriate descriptor of the outer sphere dynamics. HAE 2 also determines the thermodynamics of H 2 O dissociation on O-vacancies, which influence the inner and outer sphere rates by setting the relative coverage of * and H/OH*. These results, in turn, show that HAE 2 is a complete descriptor of the reactivity and selectivity of oxides for O 2 activation; the O-atoms in more reducible oxides (more negative HAE 2) exhibit a greater preference for the inner sphere routes and for the formation of electrophilic OO* intermediates that mediate epoxidation and O-insertion reactions during catalytic redox cycles. Large charge-balancing cations locally modify E int 0 values that determine the outer sphere rates and thus can be used to alter the preference of O-atoms to either inner or outer sphere routes.
Evaluating the catalytic activity of transition metal dimers for the oxygen reduction reaction
Journal of Colloid and Interface Science, 2020
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