Electron spin resonance studies on neutral aromatic hydrocarbon radicals

Dalton, David R.; Liebman, Shirley A.

doi:10.1021/ja01033a027

Cited by 33 publications

(9 citation statements)

References 1 publication

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“…Hyperfine coupling was not included in the simulation due to the broad, unresolved nature of the spectrum. This spectrum is distinct from reported spectra of fluorenyl-based radicals, which reveal well-resolved hyperfine coupling to H and isotropic g values of 2.003. , The EPR signal for {Bo M Flu}Rh(η 4 -C 8 H 12 ) decays over 90 min at room temperature. A spectrum of a glassed sample at 20 K showed a sharp feature along with a broadened signal (Figure B).…”

Section: Resultscontrasting

confidence: 63%

Redox Chemistry of Bis(oxazolinyl)cyclopentadienyl and -fluorenyl Rhodium and Iridium Organometallic Compounds

Schmidt

Basemann

et al. 2018

Organometallics

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Rhodium and iridium compounds supported by 1-cyclopentadienyl-1,1-bis(4,4-dimethyl-2-oxazolinyl)ethane (MeC(Ox Me2 ) 2 (C 5 H 4 ); Bo M Cp) form the 18-electron piano-stool compounds {Bo M Cp}ML 2 (L = C 2 H 4 , C 8 H 12 , CO) containing two noncoordinated oxazolines. Bromination of {Bo M Cp}Rh(C 2 H 4 ) 2 gives {Bo M Cp}RhBr 2 . A single-crystal X-ray diffraction study reveals that only one oxazoline is coordinated in the solid-state structure of {Bo M Cp}RhBr 2 . The oxazolines exchange rapidly on the 1 H NMR time scale at room temperature but slowly at −10 °C. In contrast, the fluorenyl derivative MeC(Ox Me2 ) 2 (C 13 H 8 ) (Bo M Flu) forms 16-electron square-planar rhodium(I) and iridium(I) complexes containing bidentate {C,N-Bo M Flu}M coordination that features a M−C single bond (i.e., monohapto-fluorenyl bonding). {Bo M Flu}Rh(η 4 -C 8 H 12 ) undergoes two electrochemically and chemically reversible one-electron redox events with E 1/2 at −640 and 220 mV. One-electron chemical oxidation provides the long-lived rhodium(II) hydrocarbyl species [{Bo M Flu}Rh(C 8 H 12 )] + , which reacts to give the monovalent species [{Bo M Flu-H}Rh(η 4 -C 8 H 12 )] + . Alternatively, one-electron oxidation of {Bo M Flu}Ir(η 4 -C 8 H 12 ) provides a transient diamagnetic iridium hydride, detected by 1 H NMR spectroscopy, that ultimately rearranges into [{Bo M Flu-H}Ir(η 4 -C 8 H 12 )] + . This process can be prevented for both congeners by employing the allylic H-free dibenzo[a,e]cyclooctatetraene (C 16 H 12 ) ligand. Oxidation of {Bo M Flu}M(η 4 -C 16 H 12 ) (M = Rh, Ir) provides [{Bo M Flu}M(η 4 -C 16 H 12 )] + with lifetimes of greater than 1 day.

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Section: Resultscontrasting

confidence: 63%

Redox Chemistry of Bis(oxazolinyl)cyclopentadienyl and -fluorenyl Rhodium and Iridium Organometallic Compounds

Schmidt

Basemann

et al. 2018

Organometallics

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“…It is known that 3,3′-diphenyl-3,3′dibenzofuran-2,2′-dione 17 dissociates readily on heating in an inert solvent to form 2 radical. The ESR spectrum for 2 radical [a H ortho (2H) ) 2.51 G, a H meta (2H) ) 0.97 G, a H para -(1H) ) 2.74 G] suggests less interaction between the unpaired electron and the protons of the phenyl group than that reported for the diphenylmethyl radical 18 From a different perspective, carbon-centered radicals are known to have greatly reduced reactivity toward oxygen in cases of extensive resonance stabilization, such as triphenylmethyl and 9-phenylfluorenyl. In the former example, this reduced reactivity results in reversible reaction with oxygen; the corresponding equilibrium has been examined by Howard and Ingold.…”

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confidence: 65%

Lactone-Derived Carbon-Centered Radicals: Formation and Reactivity with Oxygen

2001

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[reaction: see text] Several lactones were examined to test the reactivity of carbon-centered radicals toward oxygen. Notably, the radical derived from 2-coumaranone (4) is unreactive toward oxygen, while 2-cuomaranone itself shows enhanced reactivity toward hydrogen abstraction by alkoxyl radicals. We propose that five parameters influence diminished reactivity toward oxygen, i.e., (a) benzylic resonance stabilization, (b) unpaired spin delocalization on oxygen, (c) favorable stereoelectronic effects, (d) electron-withdrawing effects, and (e) steric effects.

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“…The electron spin resonance (ESR) spectrum reveals the presence of diarylmethyl radicals with a large doublet splitting due to the interaction of the unpaired electron with the 31 P nucleus 23, 24. The spin densities of Ar 2 Ċ–P + Ph 3 of the unsubstituted radical are ζ

_{C}^{ortho}

= 0.100 (0.113), ζ

_{C}^{meta}

= −0.043 (−0.045), ζ

_{C}^{para}

= 0.110 (0.113), ζ

_{C}^{centre}

= 0.654 (0.602) with a 31 P doublet splitting of 74.55 MHz and for the p ‐OCH 3 substituent ζ

_{C}^{ortho}

= 0.092, ζ

_{C}^{meta}

= −0.027, ζ

_{C}^{centre}

= 0.609 with a doublet splitting of 69.36 MHz; the values in brackets are for Ph 2 ĊH 25. The results indicate the presence of the diphenylmethyl radical without delocalization of the unpaired electron on the triphenylphosphonium part.…”

Section: Resultsmentioning

confidence: 99%

Trigonal pyramidal carbon geometry as model for electrophilic addition–substitution and elimination reactions and its significance in enzymatic processes

Buck¹

2006

Int J of Quantum Chemistry

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ABSTRACT:Various examples are given in which compounds are characterized as products or intermediates in a (distorted) trigonal pyramidal (TP) geometry. These observations have taken place mainly in the field of carbocation chemistry. Special attention is given to carbenium ions formed by halogen addition to 1,1-diarylsubstituted ethylenes focused on the electronic effects of the C-halogen bond as axial bond in a TP geometry with regard to the -distribution in the rest of the molecular system. The experimental verification is accompanied by quantum chemical calculations. We also used the TP structure as a reactive model for specific enzymatic reactions. The relevance of this geometry is shown for the dehalogenation reaction of the nucleophilic displacement in dichloroethane catalyzed by haloalkane dehalogenase and for the decarboxylation of l-ornithine with ornithine decarboxylase under loss of carbon dioxide.

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Electron spin resonance studies on neutral aromatic hydrocarbon radicals

Cited by 33 publications

References 1 publication

Redox Chemistry of Bis(oxazolinyl)cyclopentadienyl and -fluorenyl Rhodium and Iridium Organometallic Compounds

Redox Chemistry of Bis(oxazolinyl)cyclopentadienyl and -fluorenyl Rhodium and Iridium Organometallic Compounds

Lactone-Derived Carbon-Centered Radicals: Formation and Reactivity with Oxygen

Trigonal pyramidal carbon geometry as model for electrophilic addition–substitution and elimination reactions and its significance in enzymatic processes

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