Resonance Raman Spectroscopic Study of Phenoxyl Radical Complexes

Schnepf, Robert; Sokolowski, Achim; Müller, Jochen; Bachler, Vinzenz; Wieghardt, Karl; Hildebrandt, Peter

doi:10.1021/ja972269x

Cited by 89 publications

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“…This discrepancy is due to the failure of the quantum chemical methods to describe the excess electron density adequately. [8,37,38] Based on the present unambiguous identification of the band at 1489 ] are now reassigned to this mode, thereby revising any previous suggestions. [8] For this mode in the various complexes, we note that the frequencies are lower for those complexes with a tert-butyl group at the ortho position compared with the methoxy-substituted complexes.…”

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confidence: 86%

“…[8] The 4 cm À1 shift to lower energy, however, is smaller than that of the adjacent band at 1499 cm À1 , which is found at 1482 cm À1 in the 18 O-labeled compound. This latter band is attributed to the mode n 19a , which, according to normal mode analyses, should also exhibit a contribution from the CÀO stretch.…”

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confidence: 88%

“…In particular, it may be that the band at 614 cm À1 in [FeL but ] is a largely pure FeÀO stretching as suggested previously. [8] Phenoxyl radical complexes of iron(i ii ii i): Figure 6 shows the RR spectra of the radical complex [FeL dimet ]…”

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confidence: 99%

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The Molecular and Electronic Structure of Octahedral Tris(phenolato)iron(III) Complexes and Their Phenoxyl Radical Analogues: A Mössbauer and Resonance Raman Spectroscopic Study

et al. 1999

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Hexadentate macrocyclic ligands containing a 1,4,7-triazacyclononane backbone and three N-bound pendent-arm phenolates form extremely stable neutral complexes with Fe III Cl 3 . The octahedral complexes [Fe III L] undergo three reversible one-electron oxidation processes to yield the mono-and dications, [FeL] and [FeL] 2 , which are stable in solution for hours, whereas the trications, [FeL] 3 , are only stable in solution on the time scale of a cyclic voltammetric experiment. These oxidations are shown to be ligand-rather than metal-centered. Three coordinated phenoxyl radicals are formed successively as shown conclusively by Mössbauer spectroscopy. The neutral, mono-, di-, and tricationic species each contain an octahedral, high-spin ferric ion (S Fe 5 2), which is intramolecularly, antiferromagnetically coupled to the spin (S 1 2) of the bound phenoxyl ligands to yield an S t 2 ground state for the monocation, and an S t 3 2 ground state for the dications as shown by EPR spectroscopy. The vibrations of the coordinated phenolate are observed by resonance Raman (RR) spectroscopy by excitation in resonance with the phenolate-to-iron charge-transfer (CT) transition above 500 nm or, alternatively, of the coordinated phenoxyl by excitation in resonance with the intraligand p 3p* transition at about 410 nm. Use of 18 O isotopomers selectively labeled at the phenolic oxygen allowed the identification of the C À O stretching and Fe À O stretching and bending modes. It is shown that the substitution pattern of phenolates and phenoxyls in their respective ortho and para positions and the charge of the complexes have a pronounced influence on the vibrational modes observed.

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The Molecular and Electronic Structure of Octahedral Tris(phenolato)iron(III) Complexes and Their Phenoxyl Radical Analogues: A Mössbauer and Resonance Raman Spectroscopic Study

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“…Since the O-bound phenoxyl radical complexes are generally generated from the respective metal phenolate complexes by electrochemical or chemical oxidation, diagnostic tools for identifying the oxidation product as a coordinated phenoxyl radical complex needed to be developed. In addition to UV/Vis and ESR spectroscopy, [11,12] Resonance Raman [13] and XANES spectroscopy [1] have been proven to be very useful in this process. The study of the phenoxyl radical complexes provides new insights into the structural, electronic, and spectroscopic aspects of the coordination chemistry of phenoxyl radicals.…”

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What Can We Learn from Nature about the Reactivity of Coordinated Phenoxyl Radicals?—A Bioinorganic Success Story

Krüger

1999

Angew. Chem. Int. Ed.

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How does nature work on the molecular level? With respect to the function of metal ions in biology, the answer to this important question is intensively pursued by those researchers engaged in the highly interdisciplinary field of bioinorganic chemistry. Inorganic chemists can contribute considerably to the understanding of the structural, electronic, and mechanistic aspects of metal sites in metalloproteins by producing small coordination compounds which mimic the specific properties of those metal sites. Most model complexes concentrate on reproducing specific structural or electronic features; only a few models succeed in displaying stoichiometric or even catalytic reactivity similar to that of the enzymes. But very rarely are both structural and functional modeling characters found in a single complex. The latest success story in the development of biomimetic model complexes that mimic structural as well as catalytic aspects of the active sites in metalloproteins was written by Stack et al. with a model for the copper site in the enzyme galactose oxidase.[1] Independently, a different catalytically functioning system was devised by the group of Wieghardt and Chaudhuri, based on the mechanistic principles of the same enzymatic reaction. [2] Galactose oxidase (GO) [3] is a fungal enzyme that catalyzes the oxidation of galactose and a number of other primary alcohols to the corresponding aldehyde; a reaction in which dioxygen is reduced to hydrogen peroxide [Eq. (1)]. The active site consists of a mononuclear copper ion in a squarepyramidal coordination geometry (Scheme 1).[4] At a pH of 7.0, the copper ion is coordinated to two histidine residues (His 496, His 581), a tyrosinate residue (Tyr 272), and a water Scheme 1. Schematic view of the active site of galactose oxidase in the inactive state at pH 7.0. [4] molecule in the equatorial plane and to a further tyrosinate residue (Tyr 495) in the apical position. A unique feature of this active site embodies the modification of the tyrosinate residue located in the equatorial plane by a covalent linkage to the sulfur atom of a nearby cysteine residue.The mononuclear copper site can be present in three redox forms (Scheme 2); among those, only the fully oxidized and fully reduced redox states, GO ox and GO red , were identified as Scheme 2. Redox states of galactose oxidase. HIGHLIGHTS

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“…Da O-gebundene Phenoxylradikal-Komplexe im allgemeinen aus den entsprechenden Metallphenolat-Komplexen durch elektrochemische oder chemische Oxidation hergestellt werden, muûten diagnostische Methoden zur eindeutigen Identifizierung des Oxidationsproduktes als ein Komplex mit einem koordinierten Phenoxylradikal entwickelt werden. Hierbei haben sich neben der UV/Vis-und der ESR-Spektroskopie [11,12] insbesondere die Resonanz-Raman- [13] und die XANES-Spektroskopie [1] Die in der ersten Untersuchung [1,15] verfolgte Strategie, um mit kleinen synthetischen Analogverbindungen eine enzymähnliche Reaktivität zu erhalten, bestand darin, die strukturellen und elektronischen Eigenschaften des aktiven Zentrums so weit wie möglich zu reproduzieren. Denn falls die Proteinmatrix selbst an den entscheidenden Reaktionsschritten der Enzymkatalyse nicht beteiligt ist und/oder [16] haben Vor noch kürzerer Zeit haben Wieghardt, Chaudhuri und Mitarbeiter ein weiteres katalytisches System mit dem Liganden 2,2'-Thiobis(2,4-di-tert-butylphenol) beschrieben.…”

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Was können wir von der Natur über die Reaktivität koordinierter Phenoxylradikale lernen? – Eine bioanorganische Glanzleistung

Krüger

1999

Angewandte Chemie

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Ein‐ und zweikernige Kupfer(II)‐Komplexe mit koordinierten Phenoxylgruppen (z. B. das von Wieghardt, Chaudhuri et al. kürzlich vorgestellte Komplexkation 1) ergaben hohe katalytische Aktivitäten bei der enzymähnlichen Oxidation von Alkoholen zu Aldehyden. Diese Komplexe dienen damit als hervorragende funktionale Modellkomplexe für die Galactose‐Oxidase und ermöglichen Untersuchungen zur Reaktivität der koordinierten Phenoxylradikale.

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Resonance Raman Spectroscopic Study of Phenoxyl Radical Complexes

Cited by 89 publications

References 54 publications

The Molecular and Electronic Structure of Octahedral Tris(phenolato)iron(III) Complexes and Their Phenoxyl Radical Analogues: A Mössbauer and Resonance Raman Spectroscopic Study

The Molecular and Electronic Structure of Octahedral Tris(phenolato)iron(III) Complexes and Their Phenoxyl Radical Analogues: A Mössbauer and Resonance Raman Spectroscopic Study

What Can We Learn from Nature about the Reactivity of Coordinated Phenoxyl Radicals?—A Bioinorganic Success Story

Was können wir von der Natur über die Reaktivität koordinierter Phenoxylradikale lernen? – Eine bioanorganische Glanzleistung

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