π- and σ-Diazo Radical Cations:  Electronic and Molecular Structure of a Chemical Chameleon

Bally, Thomas; Carra, Claudio; Matzinger, Stephan; Truttmann, Leo; Gerson, Fabian; Schmidlin, Reto; Platz, Matthew S.; Admasu, Atnaf

doi:10.1021/ja9903328

Cited by 12 publications

(17 citation statements)

References 33 publications

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“…These calculations assign 97.2% of the spin density to the diazo moiety (70.8% on the terminal nitrogen atom, 17.9% on the central nitrogen atom and 8.5% on the carbon atom). The calculations indicate that the Rh-diazo radical could be an σ-state of diazo radical 24 . The smaller a N should be attributed to the terminal 14 N nucleus which has a typical “p-character” while the larger a N should be assigned to the central 14 N nucleus which has a substantial “s-character”.…”

Section: Resultsmentioning

confidence: 94%

“…Remarkably, in all of the tested [RhCl(cod)] 2 -, [Co(por)]- and PdCl 2 -catalyzed systems in the presence of DMPO, only the [RhCl(cod)] 2 -catalyzed α-carbonyl diazomethane systems showed a striking quintet EPR spectrum ( Fig. 2a ), which obviously arises from coupling of two 14 N-nuclei triplets 24 . Further simulation of the EPR spectrum obtained from the [RhCl(cod)] 2 -PDA system ( Fig.…”

Section: Resultsmentioning

confidence: 99%

“…Because of the vigorous N 2 evolution and high efficiencies of these reactions, few studies have reported such radicals 20 21 22 23 . The EPR signals of some diazo radical cations of diphenyldiazomethanes and their derivatives have been recorded after electrochemical oxidation or irradiation in Freon matrices 24 25 26 . In frozen toluene at 40 K, a carbon-centered Co-carbene radical was detected by EPR spectroscopy, along with several other paramagnetic cobalt species 17 , which complicated the spectral analysis.…”

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Metal-Diazo Radicals of α-Carbonyl Diazomethanes

Xiao

Liu

2016

Sci Rep

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Metal-diazo radicals of α-carbonyl diazomethanes are new members of the radical family and are precursors to metal-carbene radicals. Herein, using electron paramagnetic resonance spectroscopy with spin-trapping, we detect diazo radicals of α-carbonyl diazomethanes, induced by [Rh Radicals are highly reactive intermediates, long sought because of their importance in understanding reaction mechanisms and finding new reactions 1 . However, their extremely short lifetimes make their direct detection difficult and impede the development of radical chemistry. Carbenes, from nitrogen-releasing diazo compounds 2 , are the versatile active species in a broad range of reactions, including C-H functionalization 3,4 , carbon-carbon bond formation 5,6 , cyclopropanation 7,8 and polycarbene construction 9-11 . Usually, these reactions are carried out with metal catalysts, and the intermediates, generally considered to be metal-carbenes, have attracted much attention over the past decade [12][13][14][15][16] . Recent studies have shown that some metal-carbenes within paramagnetic metal catalysts (such as Co II , Rh II , and Ir II ) are carbon-centered metal-carbene radicals, not closed-shell metal-carbenes [17][18][19] . As such, their metal-diazo-adduct precursors are reasonably postulated to also possess radical character.Because of the vigorous N 2 evolution and high efficiencies of these reactions, few studies have reported such radicals [20][21][22][23] . The EPR signals of some diazo radical cations of diphenyldiazomethanes and their derivatives have been recorded after electrochemical oxidation or irradiation in Freon matrices [24][25][26] . In frozen toluene at 40 K, a carbon-centered Co-carbene radical was detected by EPR spectroscopy, along with several other paramagnetic cobalt species 17 , which complicated the spectral analysis. Diazo radicals may coexist with carbene radicals within reaction systems, and distinguishing them from one another is likewise difficult. Thus, the detection of metal-diazo radicals and their transformations into metal-carbene radicals remain unsolved challenges in diazo chemistry.To overcome these challenges, we employed room-temperature electron paramagnetic resonance (RT-EPR) spectroscopy with a spin-trapping technique 27 , which offers a convenient and powerful way to identify both diazo radicals and carbene radicals and to simultaneously investigate their behaviors. Because of the persistent radical effects of the redox-active metal and nitroxide 1 , the transient radical intermediates can be either directly detected by EPR or captured by radical traps (Fig. 1). In this paper, we report that the use of [RhCl(cod)] 2 (cod = 1,5-cyclooctadiene) allowed the RT-EPR detection of a rather stable dinitrogen radical of α -carbonyl diazomethane as a unique quintet signal. Density function theory (DFT) calculations support the EPR analyses, assigning a significant amount of spin density (97.2%) to the diazo moiety. To our knowledge, this work represents the first unequivocal spectroscopic confi...

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Section: Resultsmentioning

confidence: 94%

Section: Resultsmentioning

confidence: 99%

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confidence: 99%

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Metal-Diazo Radicals of α-Carbonyl Diazomethanes

Xiao

Liu

2016

Sci Rep

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“…92,93 The p-radical cations are expected to have a linear CNN group which should be bent in their s counterparts. 92,93 The p-radical cations are expected to have a linear CNN group which should be bent in their s counterparts.…”

Section: Overall Performance In Modeling Hfccs Of Nitrogen Containingmentioning

confidence: 99%

Modeling EPR parameters of nitrogen containing conjugated radical cations

2015

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The accuracy of DFT methodology on modeling isotropic hyperfine coupling constants (hfccs or a iso ) of conjugated radical cations containing 14 N nucleus, intermediates involved in many biological and chemical processes, is investigated. A set of 222 hfccs, belonging to 50 radical species, are computed at the following levels of theory: PBE0/N07D, B3LYP/6-31G*, B3LYP/N07D, B3LYP/TZVP, and B3LYP/EPR-III, and compared to the available experimental values. In general, these five combinations of methods and basis sets estimate a iso ( 1 H) in good agreement with experimental data. Conversely, selection of the basis set is of fundamental importance for accurate prediction of the 14 N hfccs. A thorough analysis of the computed data allows us to establish that the best functional/basis set combination for obtaining accurate nitrogen coupling constants in conjugated radical cations is PBE0/N07D//B3LYP/6-31G*, which provides values with a discrepancy smaller than 1.5 G for most of constants. The model has been successfully tested on the calculation of 14 N hfccs for different radical cation derivatives of syn-1, 6:8,13diimino[14]annulene, which is a highly demanding test because the strong experimental change in the a iso ( 14 N) values critically depends on the local geometry of the diimino fragment which is function of the polymethylene chain length. Furthermore, a whole analysis of the computed structural parameters, NBO and distribution of the spin density of these annulene radicals has allowed to explain the huge increase of the a iso ( 14 N) as well as their stability just for certain polymethylene chain length evidencing the formation of a N-N three electron s bond. Fig. 1 Scheme of amine synthetic photo-redox method. View Article Online a MAD ðMean Absolute DeviationÞ ¼ 1 N X N i ja iso ðcalcÞ À a iso ðexpÞj:This journal is

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“…We thus take the DMPO-N• and DMPO-C• signals as an indication of the trapped N-centred diazo radical and C-centred carbene radical adducts. 24,25 The appearance of the three-line DMPOX signal, which was also detected with the reaction system of oxygenation of iridium(II)-ethene (or "iridium(III)-ethyl radical") 26 in the presence of DMPO could support the generation of the open-shell copper complexes with the poly(imidazole-Cu)-EDA system. After reaction, we found the colour of poly(imidazole-Cu) changed from blue to green ( Fig.…”

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confidence: 91%