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  • 1
    UID:
    edochu_18452_25367
    Format: 1 Online-Ressource (8 Seiten)
    Content: CuI/TEMPO-Katalysatorsysteme (TEMPO=2,2,6,6-Tetramethylpiperidinoxyl) sind vielseitige Katalysatoren für aerobe Alkoholoxidationsreaktionen zur selektiven Synthese von Aldehyden. Jedoch sind mehrere Aspekte des Reaktionsmechanismus noch nicht aufgeklärt, was hauptsächlich daran liegt, dass bisher keine reaktiven Intermediate identifiziert werden konnten. Wir zeigen hier die Synthese und Charakterisierung eines dinuklearen Komplexes [L12Cu2]2+ (1), der in Gegenwart von TEMPO die katalytische 4 H+/4 e− Reduktion von O2 zu Wasser an die Oxidation von benzylischen und aliphatischen Alkoholen koppeln kann. Die Mechanismen der katalytischen O2-Reduktion und der Alkoholoxidation wurden sowohl durch spektroskopische Detektion der reaktiven Intermediate in der Gas- und der kondensierten Phase als auch durch kinetische Studien an jedem Reaktionsschritt in den Katalysezyklen aufgeklärt. Die intermediären Bis(μ-oxido)dikupfer(III)- (2) und Bis(μ-hydroxido)dikupfer(II)-Spezies (3) wurden als wichtige Reaktanden in beiden Reaktionen identifiziert. Die vorliegende Studie ermöglicht tiefgehende mechanistische Einblicke in die aerobe Alkoholoxidation, die eine wertvolle Grundlage bieten um übergangsmetallkatalysierte Reaktionen mit redoxaktiven Cokatalysatoren besser zu verstehen.
    Content: Peer Reviewed
    In: Weinheim : Wiley-VCH, 133,42, Seiten 23201-23208
    Language: German
    URL: Volltext  (kostenfrei)
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  • 2
    UID:
    edochu_18452_19292
    Format: 1 Online-Ressource (7 Seiten)
    ISSN: 1477-9234 , 1477-9234
    Content: Two new heterobimetallic [LNiO2Cu(RPY2)]+ (RPY2 = N-substituted bis 2-pyridyl(ethylamine) ligands with R = indane, 3a or R = Me, 3b) complexes have been spectroscopically trapped at low temperatures. They were prepared by reacting the mononuclear side-on LNiII superoxo precursor bearing a β-diketiminate ligand (L = [HC-(CMeNC6H3(iPr)2)2]) with the Cu(I) complexes. In contrast to the oxo groups in known high-valent [M2(μ-O)2]n+ (M = Fe, Co, Ni, Cu) cores that display electrophilic reactivities, 3a and 3b display rather nucleophilic oxo cores active in aldehyde deformylation reactions. However, the spectroscopic and reactivity properties of 3a/3b are found to be distinct relative to that of the previously reported [LNiO2Cu(MeAN)]+ complex containing a more basic (nucleophilic) N,N,N′,N′,N′-pentamethyl-dipropylenetriamine (MeAN) ligand at the copper centre. The geometry and electronic properties of the copper ligands affect the electron density of the oxygen atoms of the heterodinuclear {Ni(μ-O)2} core and 3a/3b undergo slower nucleophilic and faster electrophilic reactions than the previously reported [LNiO2Cu(MeAN)]+ intermediate. The present study therefore demonstrates the tuning of the electrophilicity/nucleophilicity of the oxygen atoms of the heterobimetallic [Ni(μ-O)2Cu]2+ cores by controlling the electron donation from the ancillary ligands, and underlines the significance of subtle electronic changes in the physical and chemical properties of the biologically relevant heterobimetallic metal–dioxygen intermediates.
    Content: Peer Reviewed
    Note: Available open access thanks to the RSC Gold for Gold initiative. Shared according to the terms set out in the CC licence.
    In: Dalton Transactions, : The Royal Society of Chemistry, 45,2016,40, Seiten 15994-16000, 1477-9234
    Language: English
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  • 3
    UID:
    edochu_18452_19291
    Format: 1 Online-Ressource (6 Seiten)
    ISSN: 1477-9234 , 1477-9234
    Content: In addition to oxometal [Mn+[double bond, length as m-dash]O] and imidometal [Mn+[double bond, length as m-dash]NR] units, transient metal–iodosylarene [M(n−2)+–O[double bond, length as m-dash]IPh] and metal–iminoiodane [M(n−2)+–N(R)[double bond, length as m-dash]IPh] adducts are often invoked as a possible “second oxidant” responsible for the oxo and imido group transfer reactivity. Although a few metal–iodosylarene adducts have been recently isolated and/or spectroscopically characterized, metal–iminoiodane adducts have remained elusive. Herein, we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting the formation of a metal–iminoiodane complex 2 and its scandium adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the hydrogen-atom and nitrene transfer reactions, which confirm their potential as active oxidants in metal-catalyzed oxidative transformations. Oxidation of para-substituted 2,6-di-tert-butylphenols by 2 and 2-Sc can occur by both coupled and uncoupled proton and electron transfer mechanisms; the exact mechanism depends on the nature of the para substituent.
    Content: Peer Reviewed
    Note: Available open access thanks to the RSC Gold for Gold initiative. Shared according to the terms set out in the CC licence.
    In: Dalton Transactions, : The Royal Society of Chemistry, 45,2016,40, Seiten 15994-16000, 1477-9234
    Language: English
    URL: Volltext  (kostenfrei)
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  • 4
    UID:
    edochu_18452_23661
    Format: 1 Online-Ressource (6 Seiten)
    Content: The ferrocene titration method has been employed to determine the one‐electron reduction potentials of a series of oxido‐iron(IV), oxido‐cobalt(IV) and oxido/hydroxido‐nickel(III) complexes based on the same tetradentate TMG3tren ligand (TMG3tren=tris[2‐(N‐tetramethylguanidyl)ethyl]amine). The S=2 ground state of the [(TMG3tren)FeIV=O]2+ complex allows an exchange enhanced reactivity, which enables it to perform efficient oxygen atom transfer (OAT) and hydrogen atom abstraction (HAA) reactions with a low one‐electron reduction potential of 270 mV vs. SCE. In the absence of exchange enhanced reactivity, the OAT and HAA abilities of the S=3/2 [(TMG3tren)CoIV−O(Sc(OTf)3)]2+, S=1/2 [(TMG3tren)NiIII−O(H)]2+ and the previously reported S=1 [(TMC)(CH3CN)FeIV=O]2+ and [(N4Py)FeIV=O]2+ complexes can be directly correlated to their reduction potentials. Notably, [(N4Py)FeIV=O]2+ and [(TMG3tren)FeIV=O]2+ exhibit similar OAT and HAA reactivities although the reduction potential of [(N4Py)FeIV=O]2+ is 0.24 V more positive than that of [(TMG3tren)FeIV=O]2+. The present study therefore provides experimental evidence for exchange enhanced reactivity and rationalizes nature's choice for employing S=2 oxido‐iron(IV) cores to achieve difficult oxidation reactions at biologically viable potentials.
    Content: Peer Reviewed
    In: Israel Journal of Chemistry, Weinheim, ,2020
    Language: English
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  • 5
    UID:
    edochu_18452_27843
    Format: 1 Online-Ressource (6 Seiten)
    Content: Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates (Q in methane monooxygenase and X in ribonucleotide reductase) are debated since Fe−Fe distances of 2.5–3.4 Å were attributed to “open” or “closed” cores with bridging or terminal oxido groups. We report the crystallographic and spectroscopic characterization of a FeIII2(μ-O)2 complex (2) with tetrahedral (4C) centres and short Fe−Fe distance (2.52 Å), persisting in organic solutions. 2 shows a large Fe K-pre-edge intensity, which is caused by the pronounced asymmetry at the TD FeIII centres due to the short Fe−μ−O bonds. A ≈2.5 Å Fe−Fe distance is unlikely for six-coordinate sites in Q or X, but for a Fe2(μ-O)2 core containing four-coordinate (or by possible extension five-coordinate) iron centres there may be enough flexibility to accommodate a particularly short Fe−Fe separation with intense pre-edge transition. This finding may broaden the scope of models considered for the structure of high-valent diiron intermediates formed upon O2 activation in biology.
    Content: Peer Reviewed
    In: Weinheim : Wiley-VCH, 62,10
    Language: English
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  • 6
    UID:
    edochu_18452_27842
    Format: 1 Online-Ressource (7 Seiten)
    Content: Dieisen-Kofaktoren in Enzymen vollziehen diverse anspruchsvolle Transformationen. Die Strukturen hoch-valenter Intermediate (Q in der löslichen Methanmonooxygenase und X in der Ribonukleotidreduktase) sind Gegenstand aktueller Diskussionen, seit Fe-Fe-Abstände von 2.1–3.4 Å eine Zuordnung zu geöffneten und geschlossen Kernen mit verbrückenden oder terminalen Oxido-Gruppen in den aktiven Zentren ermöglichen. In dieser Studie berichten wir die kristallografische und spektroskopische Charakterisierung eines FeIII2(μ-O)2-Komplexes (2) mit tetraedrischen (4C) Zentren sowie einem geringen Fe-Fe-Abstand (2.52 Å), der in Lösungsmitteln beständig ist. 2 zeigt eine große Fe-K-Vorkanten-Intensität. Diese resultiert aus der starken Asymmetrie an den TD-FeIII-Zentren bedingt durch die kurzen Fe-μ-O-Bindungen. Während ein Fe-Fe-Abstand von ≈2.5 Å für sechsfach-koordinierte Zentren in Q und X unwahrscheinlich ist, könnte ein Fe2(μ-O)2-Kern mit vierfacher (oder möglicherweise auch fünffacher) Koordination flexibel genug sein, um eine kurze Fe-Fe-Separierung mit großer Vorkanten-Intensität zu ermöglichen. Diese Erkenntnis kann dazu beitragen, dass weitere Modelle für die Strukturen der hoch-valenten Dieisen-Intermediate, die sich im Zuge der biologischen O2-Aktivierung bilden, in Betracht gezogen werden.
    Content: Peer Reviewed
    In: Weinheim : Wiley-VCH, 135,10
    Language: German
    URL: Volltext  (kostenfrei)
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  • 7
    UID:
    edochu_18452_22401
    Format: 1 Online-Ressource (10 Seiten)
    Content: In a previous communication we reported the site‐directed generation of a heterodinuclear FeIIICuII complex (1 ) by using an asymmetric dinucleating ligand FloH. The iron(III) ion was introduced first on the preferential metal‐binding site of the ligand that led to the formation of the thermodynamically favored five‐membered chelate ring upon metal‐binding. Copper(II) was introduced in the next step. The stepwise metalation strategy reported previously has now been extended to synthesize a series of heterodinuclear FeIIIMII [M = Mn (2 ), Fe (3 ), Co (4 ), and Ni (5 )] and FeIICuI (1a ) as well as the homodinuclear CuICuI (6 ) complexes. The complexes were characterized by X‐ray crystallography (except for 1a and 6 ), and by a limited number of spectroscopic methods. Complex 1 with a labile solvent binding site at FeIII reacted with H2O2 to form a transient intermediate that showed reactivity typical of metal peroxide complexes. The metal centers in the complexes 2 –5 are coordinatively saturated, and hence they showed no reactivity with H2O2. Complex 1a reacted with O2 via an intermolecular pathway to form a μ‐oxo bridged tetrameric complex 1b , which was structurally characterized. This is in contrast to the homodinuclear CuICuI and heme FeIICuI cores, which prefer an intramolecular pathway for O2 activation.
    Content: Peer Reviewed
    In: Weinheim : Wiley-VCH, 646,13, Seiten 1010-1019
    Language: English
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  • 8
    UID:
    edochu_18452_27183
    Format: 1 Online-Ressource (8 Seiten)
    Content: In vielen Metallenzymen sind Schwefel-enthaltende Liganden an Elektronen-Transfer-Reaktionen beteiligt. In dem hier diskutierten biomimetischen Ansatz wird der Einfluss einer Schwefelkoordination auf eine Kobalt-katalysierte Sauerstoff-Reduktionsreaktion (ORR) demonstriert. Ein Vergleich des ORR-Vermögens eines vierfach Stickstoff-koordinierten [Co(Cyclam]2+-Komplexes (1; Cyclam=1,5,8,11-Tetraaza-cyclotetradecan) und dessen Schwefel-Analogons [Co(S2N2-Cyclam)]2+ (2; S2N2-Cyclam=1,8-Dithia-5,11-diazacyclotetradecan) zeigt verbesserte katalytische Eigenschaften mit dem in die Ligandensphäre am Kobalt eingeführten Chalkogen. Isolierung und Charakterisierung der Intermediate, die sich im Zuge der Sauerstoffaktivierung an den Kobalt(II)-Zentren von 1 und 2 bilden, identifizieren eine Beteiligung des Schwefels am O2-Reduktionsprozess als entscheidenden Faktor für die verbesserten Eigenschaften von 2 bei der katalytischen ORR.
    Content: Peer Reviewed
    In: Weinheim : Wiley-VCH, 135,6
    Language: German
    URL: Volltext  (kostenfrei)
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  • 9
    UID:
    edochu_18452_23755
    Format: 1 Online-Ressource (5 Seiten)
    ISSN: 1433-7851 , 1433-7851
    Content: S=2 oxoiron(IV) species act as reactive intermediates in the catalytic cycle of nonheme iron oxygenases. The few available synthetic S=2 FeIV=O complexes known to date are often limited to trigonal bipyramidal and very rarely to octahedral geometries. Herein we describe the generation and characterization of an S=2 pseudotetrahedral FeIV=O complex 2 supported by the sterically demanding 1,4,7-tri-tert-butyl-1,4,7-triazacyclononane ligand. Complex 2 is a very potent oxidant in hydrogen atom abstraction (HAA) reactions with large non-classical deuterium kinetic isotope effects, suggesting hydrogen tunneling contributions. For sterically encumbered substrates, direct HAA is impeded and an alternative oxidative asynchronous proton-coupled electron transfer mechanism prevails, which is unique within the nonheme oxoiron community. The high reactivity and the similar spectroscopic parameters make 2 one of the best electronic and functional models for a biological oxoiron(IV) intermediate of taurine dioxygenase (TauD-J).
    Content: Peer Reviewed
    In: Weinheim : Wiley-VCH, 60,12, Seiten 6752-6756, 1433-7851
    Language: English
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  • 10
    UID:
    edochu_18452_27192
    Format: 1 Online-Ressource (7 Seiten)
    Content: In Class Ib Ribonukleotidreduktasen (RNRs) aktiviert ein Dimangan(II)-Cluster Superoxid (O2⋅−) anstelle von molekularem Sauerstoff (O2), um eine hochvalente MnIII-O2-MnIV-Spezies zu bilden, die für die Oxidation von Tyrosin zum Tyrosyl-Radikal verantwortlich ist. Im vorliegenden biomimetischen Ansatz wird ein Thiolat-gebundener Dimangan-Komplex [MnII2(BPMT)(OAc)2](ClO)4 (BPMT=(2,6-Bis{[bis(2-pyridylmethyl)amino]methyl}-4-methylthiophenolat) (1) synthetisiert und dessen Reaktion mit O2⋅− zur Bildung des [(BPMT)MnO2Mn]2+-Komplexes 2 gezeigt. Resonanz-Raman-Untersuchungen zeigen das Vorliegen einer O-O-Bindung in 2, während die ESR-Analyse ein Signal bei g=2 mit 16 Linien für St=urn:x-wiley:00448249:media:ange202217076:ange202217076-math-0001 aufzeigt, das typischerweise mit einem MnIIIMnIV-Kern assoziiert wird, wie es auch in Class Ib RNR detektiert wurde. Anders als zuvor publizierte Mn-O2-Mn-Komplexe, die durch O2⋅−-Aktivierung an Mn2-Zentren gebildet wurden, ist 2 ein geeignetes elektrophiles Oxidationsmittel für Aldehyd-Deformylierungsreaktionen und Phenol-Oxidationsreaktionen, sodass es eines der besten strukturellen und funktionalen Modelle der Class Ib RNRs darstellt.
    Content: Peer Reviewed
    In: Weinheim : Wiley-VCH, 135,12
    Language: German
    URL: Volltext  (kostenfrei)
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