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| 2. |
- Duan, Lele, et al.
(författare)
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Isolated Seven-Coordinate Ru(IV) Dimer Complex with HOHOH (-) Bridging Ligand as an Intermediate for Catalytic Water Oxidation
- 2009
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 131:30, s. 10397-
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Tidskriftsartikel (refereegranskat)abstract
- With the inspiration from an oxygen evolving complex (OEC) in Photosystern II (PSII), a mononuclear Ru(II) complex with a tetradentate ligand containing two carboxylate groups has been synthesized and structurally characterized. This Ru(II) complex showed efficient catalytic properties toward water oxidation by the chemical oxidant cerium(IV) ammonium nitrate. During the process of catalytic water oxidation, Ru(III) and Ru(IV) species have been successfully isolated as intermediates. To our surprise, X-ray crystallography together with HR-MS revealed that the Ru(IV) species is a seven-coordinate Ru(IV) dimer complex containing a [HOHOH](-) bridging ligand. This bridging ligand has a short O center dot center dot center dot O distance and is hydrogen bonded to two water molecules. The discovery of this very uncommon seven-coordinate Ru(IV) dimer together with a hydrogen bonding network may contribute to a deeper understanding of the mechanism for catalytic water oxidation. It will also provide new possibilities for the design of more efficient catalysts for water oxidation, which is the key step for solar energy conversion into hydrogen by tight-driven water splitting, the ultimate challenge in artificial photosynthesis.
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| 3. |
- Ellis, W. Chadwick, et al.
(författare)
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Design of More Powerful Iron-TAML Peroxidase Enzyme Mimics
- 2009
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 131:50, s. 18052-
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Tidskriftsartikel (refereegranskat)abstract
- Environmentally useful, small molecule mimics of the peroxidase enzymes must exhibit very high reactivity in water near neutral pH. Here we describe the design and structural and kinetic characterization of a second generation of iron(III)-TAML activators with unprecedented peroxidase-mimicking abilities. Iterative design has been used to remove the fluorine that led to the best performers in first-generation iron-TAMLs. The result is a superior catalyst that meets a green chemistry objective by being comprised exclusively of biochemically common elements. The rate constants for bleaching at pH 7, 9, and 11 of the model substrate, Orange 11, shows that the new Fe-III-TAML has the fastest reactivity at pH's closer to neutral of any TAML activator to date. Under appropriate conditions, the new catalyst can decolorize Orange 11 without loss of activity for at least 10 half-lives, attesting to its exceptional properties as an oxidizing enzyme mimic.
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| 4. |
- Ellis, W. Chadwick, et al.
(författare)
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Designing Green Oxidation Catalysts for Purifying Environmental Waters
- 2010
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 132:28, s. 9774-9781
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Tidskriftsartikel (refereegranskat)abstract
- We describe the synthesis, characterization, aqueous behavior, and catalytic activity of a new generation of Fe-III-TAML (tetraamido macrocycle ligand) activators of peroxides (2), variants of [Fe{(OC)(2)(o,o'-NC6H4NCO)(2)CMe2}(OH2)(-)] (2d), which have been designed to be especially suitable for purifying water of recalcitrant oxidizable pollutants. Activation of H2O2 by 2 (k(I)) as a function of pH was analyzed via kinetic studies of Orange II bleaching. This was compared with the known behavior of the first generation of Fe-III-TAMLs (1). Novel reactivity features impact the potential for oxidant activation for water purification by 2d and its aromatic ring-substituted dinitro (2e) and tetrachloro (2f) derivatives. Thus, the maximum activity for 2e occurs at pH 9, the closest yet to the EPA guidelines for drinking water (6.5-8.5), allowing 2e to rapidly activate H2O2 at pH 7.7. In water, 2e has two axial water ligands with pK(a)'s of 8.4 and 10.0 (25 degrees C). The former is the lowest for all Fe-III-TAMLs developed to date and is key to 2e's exceptional catalytic activity in neutral and slightly basic solutions. Below pH 7, 2d was found to be quite sensitive to demetalation in phosphate buffers. This was overcome by iterative design to give 2e (hydrolysis rate 2d > 100 x 2e). Mechanistic studies highlight 2e's increased stability by establishing that to demetalate 2e at a comparable rate to which H2PO4- demetalates 2d, H3PO4 is required. A critical criterion for green catalysts for water purification is the avoidance of endocrine disruptors, which can impair aquatic life. Fe-III-TAMLs do not alter transcription mediated by mammalian thyroid, androgen, or estrogen hormone receptors, suggesting that 2 do not bind to the receptors and reducing concerns that the catalysts might have endocrine disrupting activity.
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| 5. |
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| 6. |
- Polshin, Victor, et al.
(författare)
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Attaining control by design over the hydrolytic stability of Fe-TAML oxidation catalysts
- 2008
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 130:13, s. 4497-4506
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Tidskriftsartikel (refereegranskat)abstract
- The iron(III) complexes of tetra amidato macrocyclic ligands (TAMLs) ([Fe{1-X-1-2-X2C6H2-4,5-(NCOCMe2NCO)(2)CR2}(OH2)](-), 1: X-1 = X-2 = H, R-2 = Me-2 (a), R-2 = (CH2)(2) (b); X-1 = X-2 = Cl, R-2 = F-2, (c), etc.), which the proton is known to demetalate at pH < 3, are also subject to catalyzed demetalation by Bronsted acid buffer components at pH 4-9 such as H2PO4-, HSO3-, and CH3- CO2H, HO2CCH2CO2-. Buffers based on pyridine (py) and tris(hydroxymethyl)aminomethane (TRIS) are catalytically inactive. Where reactions proceed, the products are demetalated TAMLs and iron species of variable composition. Pseudo-first-order rate constants for the demetalation (k(obs)) are linear functions of the acid concentrations, and the effective second-order rate constants k(1),(eff) have a hyperbolic dependence on [H+] (k(1),eff = a(1)[H+]/(b(1)+[H+]). The rate of demetalation of 1a in H2PO4-/HPO42- buffer is appreciable, but the k(obs) values for 1b and 1c are immeasurably low, showing that the rates are strongly affected by the CR2 or "tail" fragments, which are known to potently affect the TAML basicity. The reactivities of 1 depend insignificantly on the aromatic ring or "head" group of 1. The proposed mechanism involves precoordination of the acidic buffer species followed by hydrolysis. The demetalating abilities of buffer species depend on their structures and acidities. Thus, although pyridine-2-carboxylic (picolinic) acid catalyzes the demetalation, its 3- and 4-isomers (nicotinic and isonicotininc acids) are inactive. The difference is rationalized to result from the ability that only coordinated picolinic acid has to deliver a proton to an amidato nitrogen in an intramolecular manner. The reaction order in picolinic acid equals one for la and two for 1a. For 1b, "inactive" pyridine and nicotinic acid speed up the demetalation in the presence of picolinic acid, suggesting that the second order arises from the axial binding of two pyridine molecules, one of which must be picolinic acid. The binding of pyridine- and imidazole-type ligands was confirmed by UV/vis equilibrium measurements and X-ray crystallography. The implications of these mechanistic findings for designing superior Fe-TAML oxidation catalysts and catalyst formulations are discussed using the results of DFT calculations.
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| 7. |
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| 8. |
- Zalubovskis, Raivis, et al.
(författare)
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Self-adaptable Catalysts : Substrate-Dependent Ligand Configuration
- 2008
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Ingår i: Journal of the American Chemical Society. - Washington DC : American Chemical Society. - 0002-7863 .- 1520-5126. ; 130:6, s. 1845-1855
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Tidskriftsartikel (refereegranskat)abstract
- Pd(II) allyl and Pd(0) olefin complexes containing the configurationally labile ligand 1,2-bis-[4,5dihydro-3H-dibenzo[c-e]azepino]ethane were studied as models for intermediates in Pd-catalyzed allylic alkylations. According to NMR and DFT studies, the ligand prefers C-s conformation in both eta(3)-1,3-diphenylpropenyl and eta(3)-cyclohexenyl Pd(II) complexes, whereas in Pd(0) olefin complexes it adopts different conformations in complexes derived from the two types of allyl systems in both solution and, as verified by X-ray crystallography, in the solid state. These results demonstrate that the Pd complex is capable of adapting its structure to the reacting substrate. The different structural preferences also provide an explanation for the behavior of 1,3-diphenyl-2-propenyl acetate and 2-cyclohexenyl acetate in Pd-catalyzed allylic alkylations using pseudo-C-2 and pseudo-C-s symmetric ligands.
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