Elektronentransfer und Ionenpaar-Bildung, 28. ESR/ENDOR-Untersuchungen an Radikalanionen und Kontakt-Ionenpaaren der Naturstoff-Chinone Mitomycin C, Streptonigrin, Entobex® und 10,11-Dioxobrucin/Electron Transfer and Ion Pairing, 28. ESR/ENDOR Studies of Radical Anions and Contact Ion Pairs of the Naturally Occurring Quinones Mitomycine C, Streptonigrine, Entobex® and 10,11-Dioxobrucine

1992 ◽  
Vol 47 (4) ◽  
pp. 533-546 ◽  
Author(s):  
H. Bock ◽  
P. Hänel ◽  
H.-F. Herrmann
Keyword(s):  
Electron Transfer ◽  
Mitomycin C ◽  
Ion Pairs ◽  
Radical Cations ◽  
Paramagnetic Species ◽  
Radical Anions ◽  
Multiply Charged ◽  
Naturally Occurring ◽  
Contact Ion Pairs ◽  
Triple Ion

Abstract Reduction of naturally occurring para-and ortho-benzoquinone derivatives M to their respective radical anions M·⊖ can be accomplished under largely aprotic conditions either by cautious low-temperature reaction in THF containing an excess of (2.2.2) cryptand at a potassium mirror or by using the "mild" single electron transfer reagent tetrabutylammonium boranate R4N⊕BH4⊖ in DMF. On addition of soluble alkali tetraphenylborates Me⊕[B(C6H5)4]⊖ , their hitherto unknown radical ion pairs [M·⊖ Me⊕]· and/or triple ion radical cations [Me⊕M·⊖Me⊕]·⊕ form, which might be of biological relevance in molecular carrier and "turn off -turn on" switch processes. On addition of metal perchlorates Me⊕n(ClO4⊖)n with multiply charged counter cations Me⊕n the respective paramagnetic species [M·⊖Me⊕n]·(n-1)⊕ result. Assuming exclusive one-electron transfer reductions without any redox fragmentation reac­tions, ESR, ENDOR and GENERAL TRIPLE spectra are presented and discussed for the following radical anions and radical ion pairs: mitomycin C (M·⊖ and [M·⊖Mex⊕]·(x-1)⊕ with Me⊕ = Li⊕, Na⊕), streptonigrine (M·⊖ and [M·⊖Lix⊕]·(x-1)⊕), Entobex® (M·⊖ and [M·⊖Me⊕n]·(n-1)⊕ with Me⊕n = Li⊕, Na⊕, Cd⊕⊕, (H5C6)2Tl⊕) as well as brucinequinone ([M·⊖ Me⊕n]·(n-1)⊕ with Me⊕n = Li⊕, Cd⊕⊕, Pb⊕⊕, La⊕⊕⊕).

Elektronentransfer und Ionenpaar-Bildung, 18 [1 -3] / Electron Transfer and Ion Pairing, 18 [ 1 - 3]

1991 ◽  
Vol 46 (3) ◽  
pp. 326-338 ◽  
Author(s):  
H. Bock ◽  
P. Dickmann ◽  
H.-F. Herrmann
Keyword(s):  
Electron Transfer ◽  
Metal Cation ◽  
Reduction Potential ◽  
Ion Pairs ◽  
Radical Cations ◽  
Membered Ring ◽  
Cation Complexation ◽  
Metal Cation Complexation ◽  
Contact Ion Pairs ◽  
Triple Ion

The redox behaviour of aza-substituted naphtho- and anthraquinones, which offer O=C–C=N– chelate tongs for an advantageous five-membered ring metal cation complexation, is investigated by a combination of cyclovoltammetric and ESR/ENDOR spectroscopic measurements. The formation of paramagnetic contact ion pairs like [(quinoline-5,8-semiquinone)·⊖Me⨁]·, with Me⨁ = Li⨁, Na⨁, Tl⨁, or of triple ion radical cations like [(1,4-diazo-9,10-anthrasemiquinone)eMe⨁]·⨁, with Me⨁ = Li⨁, Na⨁ is corroborated both by shifts of the second reduction potential of up to 0,67 V for e.g. quinoline 5,8-quinone upon addition of Li⨁[B(C6H5)4]⊖ to its DMF solution and by the observation of ESR/ENDOR metal couplings.

Elektronentransfer und Ionenpaar-Bildung, 7 [1, 2] Kontakt-Ionenpaare Schwefel enthaltender Radikalanionen / Electron Transfer and Ion Pairing, 7 [1, 2] Contact Ion Pairs of Sulfur-Containing Radical Anions

1988 ◽  
Vol 43 (10) ◽  
pp. 1240-1246 ◽  
Author(s):  
Hans Bock ◽  
Peter Hänel ◽  
H.-F Herrmann ◽  
Heindirk torn Dieck
Keyword(s):  
Electron Transfer ◽  
Ion Pairs ◽  
Ion Pairing ◽  
Radical Anions ◽  
Contact Ion Pairs ◽  
Sulfur Containing

The structurally different radical anions M⊖ of peralkylated 1-sila-2,5-diazacyclopentane-3,4-dithione and of tetrakis(isopropylthio)-p-benzoquinone are generated by reduction with potassium/2.2.2-cryptand under aprotic conditions in THF solution. On addition of Li⊕B(C6H5)4⊖, both form hitherto elusive sulfur-containing contact ion pairs, which are characterized by their ESR/ENDOR spectra.

Elektronentransfer und Ionenpaare, 15 [ 1– 3] Radikalanion und Radikal-Kontakt-Ionenpaare von Dimesityl-tetraketon / Electron Transfer and Ion Pairing, 15 [1-3] Radical Anion and Radical Ion Pairs of Dimesityl Tetraketone

1990 ◽  
Vol 45 (8) ◽  
pp. 1197-1204 ◽  
Author(s):  
H. Bock ◽  
P. Hänel ◽  
H.-F. Herrmann
Keyword(s):  
Electron Transfer ◽  
Radical Anion ◽  
Ion Pairs ◽  
Ion Pairing ◽  
Single Electron ◽  
Single Electron Transfer ◽  
Temperature Dependent ◽  
Contact Ion Pairs

The radical anion of dimesityltetraketone (ERed, I = -0.40 V) is easily generated in THF by potassium mirror/[2.2.2]-cryptand reduction. Its contact ion pairs with Na⊕, Cs⊕ and Ba⊕⊕ counter cations, prepared in THF solution by single electron transfer from the respective metals, are characterized by their ESR/ENDOR spectra, which exhibit temperature-dependent metal couplings of aNa⊕ = 0.061 mT (190 K), aCs⊕ = 0.021 mT (190 K), and aBa⊕⊕ = 0.145 mT (295 K).

Regio- and stereoselective functionalization of electron-deficient alkenes by organosilicon compounds via photoinduced electron transfer

2003 ◽  
Vol 75 (8) ◽  
pp. 1049-1054 ◽  
Author(s):  
K. Mizuno ◽  
T. Hayamizu ◽  
Hajime Maeda
Keyword(s):  
Electron Transfer ◽  
Photoinduced Electron Transfer ◽  
Radical Cations ◽  
Reactive Intermediates ◽  
Radical Anions ◽  
Organosilicon Compounds ◽  
Allylic Silanes ◽  
Carbon Radicals

Regio- and stereoselective photoallylation of electron-deficient alkenes by use of allylic silanes via photoinduced electron transfer has been described. Similar photoinduced  functionalization reactions such as arylmethylation, alkylation, and silylation can be achieved by using a variety of organosilicon compounds. These photoreactions proceed via radical cations of organosilicon compounds and radical anions of electron-deficient alkenes as reactive intermediates. The key step of the photoreactions is the attack of carbon radicals, which are generated from the radical cations of organosilicon compounds, on the radical anions of alkenes. The mechanism of the regio- and stereoselective photofunctionalization is discussed.

Elektronentransfer und Ionenpaar-Bildung, 27 [1, 2] Darstellung von Semichinon-Ionenpaaren durch Reduktion von Chinonen mit Tetraalkylammonium-boranat in aprotischen Lösungen: 1,10-Phenanthrolin-5,6-dion / Electron Transfer and Ion Pairing, 27 [1, 2] Preparation of Semiquinone Ion Pairs by Reduction of Quinones Using Tetraalkylammonium Boranate in Aprotic Salt Solutions: 1,10-Phenanthrolin-5,6-dione

1992 ◽  
Vol 47 (2) ◽  
pp. 288-300 ◽  
Author(s):  
H. Bock ◽  
P. Hänel
Keyword(s):  
Radical Anion ◽  
Nuclear Charge ◽  
Ion Pairs ◽  
Radical Cations ◽  
Ion Pair ◽  
Metal Salts ◽  
Multiply Charged ◽  
Counter Cation ◽  
Cation Charge ◽  
Spin Densities

Ion pairs of 1,10-phenanthrolin-5,6-dione radical anion [M · ⊖Me⊕n] ·⊕(n−1) with Me⊕n = Mg⊕⊕, Ca⊕⊕, Sr⊕⊕, Zn⊕⊕, Cd⊕⊕, Pb⊕⊕ and La⊕⊕⊕ are advantageously prepared in aprotic DMF solution containing appropriate metal salts Me⊕nX⊖ by using the ‘mild’ single-electron reducing agent tetra(n-butyl)ammonium-boranate R4N⊕BH4⊖ . For comparison, the ‘naked’ radical anion with the largely interaction-free [K⊕(2.2.2)-cryptand]⊕ counter cation is chosen, which is formed on reduction with potassium in THF solution of (2.2.2)-cryptand. Addition of excess Na⊕[B(C6H5)4]⊖ to the reduction solution only yields a solvent-separated ion pair (M · ⊖)DMF ··· (Na⊕)DMF, whereas in the presence of multiply charged counter cations Me⊕n the respective contact ion pair radical cations [M · ⊖Me⊕n] · ⊕(n−1) are formed. Their g values decrease with increasing nuclear charge of Me⊕n and their metal-s-spin densities increase with the effective counter cation charge n⊕/rMe⊕n. The ESR /ENDOR data recorded suggest Me⊕n complexation by the δ⊖OC -COδ⊖ chelate tongs and the ion pair stability, which is modified by the dielectric properties of the solvent used, may be rationalized by the Coulombic attraction between the radical anion M · ⊖ and the counter cations Me⊕n.

Elektronentransfer und Ionenpaar-Bildung, 33 [1, 2]. Die Einelektronen-Reduktion von Tetraphenyl-p-benzochinon mit Alkalimetallen: ENDOR-Spektren von Kontaktionen-Paaren sowie Tripelionen in Lösung und Einkristallstrukturen der Neutralverbindung und ihres Natrium-Salzes / Electron Transfer and Ion Pair Formation, 33 [1,2]. The Single Electron Reduction of Tetraphenyl-p-benzoquinone by Alkali Metals: ENDOR Spectra of Contact Ion Pairs as well as Triple Ions in Solution and Single Crystal Structures of Both the Neutral Compound and its Sodium Salt

1994 ◽  
Vol 49 (4) ◽  
pp. 529-541 ◽  
Author(s):  
Hans Bock ◽  
Andreas John ◽  
Markus Kleine ◽  
Christian Näther ◽  
Jan W. Bats
Keyword(s):  
Single Crystal ◽  
Radical Anion ◽  
Alkali Metals ◽  
Ion Pairs ◽  
Radical Cations ◽  
Chair Conformation ◽  
Reduction Potentials ◽  
Half Wave ◽  
Electron Reduction ◽  
Contact Ion Pairs

Tetraphenyl-p-benzoquinone, according to its single crystal structure, shows some steric congestion: its quinone ring is distorted by 7° to a chair conformation, and its phenyl substituents are twisted around their CC axes between 46° and 72°. The half-wave reduction potentials of -0.57 and -1.25 V in acetonitrile confirm negligible π interaction of the phenyl substituents. Addition of alkalimetal tetraphenylborate salts lowers the second reduction potential due to contact ion formation, which can be confirmed by UV/VIS spectra recorded under aprotic conditions. Extensive ESR/ENDOR investigations prove the formation of the following species in THF solution: Tetraphenyl-p-benzosemiquinone radical anion contact ion pairs [M·⊖ Me⊕solv]' (Me⊕: Li⊕, Na⊕, Rb⊕, Cs⊕) and contact triple ion radical cations both with identical cations [M·⊖ (Me⊕solv)2]·⊕ (Me⊕: Li⊕, Na⊕, Cs⊕) and different cations [M·⊖ (Li⊕solv)(Me⊕solv)]·⊕ (Me⊕: Na⊕, Cs⊕). Addition of crown ethers can lead to external solvation of the Me⊕ counter cations, whereas cryptands form internal solvation complexes. The radical anion of 2,6-diphenyl-p-benzosemiquinone adds cations at its phenyl-free molecular half. The radical anion salt [tetraphenyl-p-benzosemiquinone·⊖ (Na⊕(tetrahydropyrane) 2)] could be crystallized and its structure determined at 200 K. In agreement with the Hirota sign rules for contact radicals in solution, the Na⊕ ion is found 62 pm above the π plane and 29° outside the axis of the CO bound, which is elongated due to one-electron reduction by 5 pm to 127 pm.

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