Organic electron acceptors

The ET reaction between aqueous Fe(CN)g and the neutral species, TCNQ, has been investigated extensively with SECM, in parallel with microelectrochemical measurements at expanding droplets (MEMED) [84], which are discussed in Chapter 13. In the SECM studies, a Pt UME in the aqueous phase generated Fe(CN)g by reduction of Fe(CN)g. TCNQ was selected as the organic electron acceptor, because the half-wave potential for TCNQ ion transfer from DCE to water is 0.2 V more positive than that for ET from Fe(CN)g to TCNQ [85]. This meant that the measured kinetics were not compromised by TCNQ transfer from DCE to the aqueous phase within the potential window of these experiments. 

Charge-transfer adducts are formed with the gold(I) trimers [Au3(MeN=COR)3] (R = Me, Et), which act as electron donors, and organic electron acceptors as nitro-9-fluorenes. The structures of these adducts involve mixed stacks in which the gold trimers and the planar nitro-fluorenes are interleaved.3130 No luminescence has been observed from these solid charge-transfer adducts, which is not surprising since the luminescence of [Au3(MeN=COR)3] is a property that is associated with the supramolecular organization in the solid. 

Table 4 Reduction potential and electron affinity of various classes of organic electron acceptors. ...

This idea was put forward first by Scott, Miller and Labes [57] for the polymerization of N-vinylcarbazole by organic electron-acceptors. It was then applied to initiation by the tropylium and other ions [52,58] by reducible metal ions but with emphasis on a possible radical reaction [59] and by sodium chloroaurate in which Au(III) is reduced (see also Section 4.5) [60]. Then Plesch suggested [6] an application of the idea of catalysis by metal halides generally, giving as an example, the following hypothetical scheme ... 

Monocyclic azines are very weak rr-donors and behave mostly as n-donors on interaction with electrophiles. However, ir-donor character is significantly increased in their benzo-derivatives. For instance, acridine forms with chloranil a highly colored 1 1 molecular complex. Perimidine is one of the strongest heterocyclic ir-donors which gives deeply colored molecular complexes with a variety of organic electron acceptors. On the other hand, the rr-acceptor ability of perimidine is moderate. 

Interaction of 1-vinylindole with halogenated organic electron acceptors, even such weak acceptors as alkyl halides, results in the formation of polymeric CT complexes (28) of varying composition, all of which contain paramagnetic centers and display resistivities in the insulator range (74IZV1837). 

Figure 4B. Plots of log kc, for the photoinduced electron transfer reactions from [Ru(bpy)3]2+ to organic electron acceptors (Nos. 1-16) in the absence of HC104 and acetophenone derivatives (Nos. 17-21) in the presence of HC104 (0.10 mol dm-3) in MeCN vs. the difference between the one-electron redox potentials of [Ru(bpy)3]2 + and the electron acceptors in the absence of HC104 in MeCN, [42].

The MLCT basis for the reactive excited state leads to interesting consequences with respect to the orbitals involved in photoinduced forward and back ET in metal complex dyads. In order to categorize this difference we define two categories of dyad systems Type 1 dyads contain an electron acceptor covalently attached to the d6 metal chromophore and type 2 dyads contain an electron donor covalently attached to the d6 metal chromophore (see Fig. 1). In the type 1 dyads, photoinduced forward ET involves transfer of an electron from a tt orbital localized on the acceptor ligand, L, to a ir orbital on the organic electron acceptor, A. Back ET involves transfer of an electron from a it orbital of the organic electron acceptor, A, to the -shell of the transition metal center. By contrast, in the type 2 dyads photoinduced forward ET involves transfer of an electron from a tt orbital on the organic donor, D, into the hole in the d-shell of the... 

Figure 1 Photoinduced electron transfer schemes for type 1 and type 2 metal-organic dyads. Key L is a diimine ligand such as 2,2 -bipyridine M is a transition metal dn indicates the electron count in the valence shell d-orbitals of M A is an organic electron acceptor D is an organic electron donor FET is forward ET BET is back ET.

Organisms with anaerobic mitochondria can be divided into two different types those which perform anaerobic respiration and use an alternative electron acceptor present in the environment, such as nitrate or nitrite, and those which perform fermentation reactions using an endogenously produced, organic electron acceptor, such as fumarate (Martin et al. 2001 Tielens et al. 2002). An example of the first type is the nitrate respiration that occurs in several ciliates (Finlay et al. 1983), and fungi (Kobayashi et al. 1996 Takaya et al. 2003), which use nitrate and/or nitrite as the terminal electron acceptor of their mitochondrial electron-transport chain, producing nitrous oxide as... 

Cholesterol (see Fig. 4) also acts as an electron donor with I2 and various organic electron acceptors which lower the activation energy and resistivity [1,133]. The cholesterols were studied in various states dry and fully and partially hydrated. The fully hydrated cholesterol complexes had the lowest activation energies and resistivities [1,133]. 

Table 3 Reduction Potentials of Organic Electron Acceptors ...

Scheme 8-2. Some organic electron-acceptors used for the formation of CT complexes.

Hydrogenation of n-AIkenes. - The activity of oxides with alkali metals in the hydrogenation of alkenes is similar to the activity of EDA complexes of alkali metals with organic electron acceptors described by Tamam. Hydrogenation of alkenes occurs at 423-473 K under normal pressure. In Table 5 are given the initial rates of hydrogenation reactions of alkenes in the presence of oxides doped with sodium and potassium vapours. 

Similar hydrogen exchange reactions take place over the electron donor-acceptor complexes of the metallocenes with various organic electron acceptors such as quinones, and nitro- and cyano-substituted compounds (7). The hydrogen exchange reaction between acetylene or molecular hydrogen, and various EDA complexes of metallocenes proceeded at room temperature via bonded hydrogen (HZ) in the complexes, but did not take place over either metallocenes or quinones alone under the same reaction conditions. 

Kang and coworkers76 have also used organic electron acceptors such as 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and chloranil. A solvent effect was observed when polymerization was carried out using DDQ. Polymerization in acetonitrile gave the lowest conductivity, and in water it was slightly better. A similar solvent dependence was observed for chloranil oxidations. 

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