Arenes redox potential

The delocalised radical formed by protonation of the radical-anion is more easily reduced than the starting arene. For some polycyclic aromatic hydrocarbons, the redox potential for this radical species can be determined using a cyclic voltammetry technique [10]. Reduction in dimethylformamide is carried out to the potential for formation of the dianion. The dianion undergoes rapid monoprotonation and on the reverse sweep at a fast scan rate, oxidation of the monoanion to the radical can be observed. The radical intermediate from pyrene has E° = -1.15 V vs. see in dimethylformamide compared to E° = -2.13 V vs. see for pyrene,... 

Astruc has developed the concept of transition metal sandwiches acting as electron reservoir complexes. The characteristic of an electron reservoir is that the reduced form is easily generated and does not decompose to increase stability, the radical center can be sterically protected in the heart of a bulky molecular framework. The [FeCp(arene)] series of complexes, e.g. 7, are prime examples, for which variation of the arene structure modulates the redox potential. 

Tab. 1 Redox potentials of sonne bis(arene)vanadium complexes in DME versus SCE...

In a second example, Astruc et al. showed that the redox potential of FeCp(rjb-arene) core dendrimers (molecule 2 is the largest example studied) was similar to smaller molecules.86 The degree of hyperbranching is minimal in this molecule. Moreover, the authors also pointed out that the redox orbital is quite well buried at... 

Burke discussed recently that for easily oxidizable substances, such as alcohols and amines, the redox potentials of Table II might be irrelevant, as incipient formation of oxidic layers at lower oxidation potentials and not the bulk oxide may already cause the observed oxidative conversion (189). Nonetheless, for substances that are less easily oxidized for instance, olefines or arenes, the given redox potentials are certainly relevant with respect to the rate of their mediated oxidation. 

The complexes of the cis and trans ligands have the same redox potentials. c The presence of this couple was not recognized in the original paper, but it is clearly present in Fig. 6 of Ref. 71 as a small response at potentials more negative than the main response that is due to the [Os(NH3)5(T)2-(aI erce)-CH2=CHPh) ,+,2+ couple. As can be seen from its position, its redox potential is consistent with an Tj2-arene structure. d 9-Anthracenecarbonitrile. 

Redox Potentials (vs FcH/FcH+) of Selected Tethered Arene-Phosphine Complexes of Ruthenium (II)... 

At the singlet excited state, ortho and meta photocycloadditions are often competitive processes and physicochemical investigations were carried out to rationalize the modes of cycloaddition of arenes with alkenes. In the context of the study of photochemical electron transfer reactions, it has been proposed that the difference of the redox potentials of the reaction partners might play an important role in this competition [10]. Such a discussion involves the intervention of an exciplex as intermediate. The Rehm-Weller equation [11] was used to quantify the relationship. When an electron transfer process is strongly endergonic (AG>1.5eV), the meta cycloaddition should be favored. When such a process is less endergonic (1 < AG< 1.5 eY), the ortho addition dominate [12]. This means that the... 

Much of the work on model systems was stimulated by the observation of Udenfriend and co-workers in 19546S4a,b that a mixture of Fe(II), EDTA, ascorbic acid, and molecular oxygen could hydroxylate arenes to phenols under mild conditions. Udenfriend s reagent also hydroxylates alkanes to alcohols and epoxidizes olefins.670 6 74 The EDTA in Udenfriend s reagent probably reduces the redox potential of the Fe(II)/Fe(III) couple. The ascorbic acid functions as an electron donor, analogous to the cofactor in monooxygenases, and can be replaced by other enediols.672... 

Cationic sandwich complexes of the type CpCo(arene) + were first prepared by hydride abstraction from cyclohexadi-enyl cations (Section 7.1). They are accessible in broader variation from the reaction of CpCoX half-sandwich complexes with arene in the presence of AICI3. Their electrochemical reductions to the corresponding 19-electron monocations and to 20-electron neutral complexes have been studied. The stability of electron-rich sandwich complexes increases with increasing alkyl substitution in either ring despite the more negative redox potential mass spectrometry studies of bond dissociation energies of (arene)Co+ complexes corroborate these results. However, neutral sandwich complexes are not very stable in the polar solvents necessary for the reduction of mono- or dications and have been isolated only from alkyne trimerization with CpCo precursors in nonpolar solvents (Section 5.1.4). 

The 18-electron sandwich complexes [Cr( / -arene)2] have a range of standard redox potentials from = —1.15 V relative to FeCp2 in CH2C12 (arene = C6H6) to E° = -1.45 V (arene = CeMee) [136] and even —1.5 V (arene = C6H5NMe2) [137],... 

The most frequently used electron-reservoir complex for stoichiometric singleelectron transfer reactions is (Fe Cp( / -C6Me6)], because of its stability and ease of preparation, and since it has one of the most negative redox potentials in the series. It can reduce most inorganic and organometallic cations [2]. For instance, it is very useful to synthesize neutral 19-electron complexes (C in the equation below) such as other (Fe Cp( / -arene)] complexes and (Fe ( / -C6Me6H)( / -C6Me6)] from the 18-electron cationic precursors C[PF6 ... 

The free enthalpy of electron transfer can easily be calculated from the electrochemically measured redox potentials of the substrates and the excitation energy of the arene using... 

Mo(Tp )(E)(bdt)j, [Mo(Tp )(E)(tdt)], [Mo(Tp )(E)(bdtCl2)]82 83 (E = O, NO), and [MoO(qdt)(Tp )]84 have been investigated as models for various pyranopterin Mo enzyme active sites, including sulfite oxidase. Solution redox potentials and heterogeneous electron transfer rate constants for these species have been also reported.85 The interactions between the sulfur tt-orbitals of arene dithiolates and high-valent Mo in [MoO(Tp )(bdt)] have been investigated by gas-phase photoelectron spectroscopy and DFT methods in order to understand the properties of the active site of pyranopterin Mo-W enzymes.86 Temperature-dependent measurements of potential and electron-transfer rate constants are also reported for electrochemical reduction of a series of [MoO(Tp )(X,Y)] complexes.87 The molecular and electronic structures of the SO active site [MoO(Tp )(bdt)] have been also reported.88... 

Fig. 48. Redox potentials of bis( j -arene)ruthenium complexes as a function of the ligand...

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