Extended redox sequences

3 Highly charged states via extended redox sequences 

The first question is whether the redox systems can be subjected to successive electron-transfer reactions in extended redox sequences. What one needs to know thereby are the number of charges that can be transferred and what is the Coulombic repulsion arising between the charged subunits. The experimental methods that have to be applied are obvious. Cyclic 

Accordingly, dianthryl compounds with a close proximity, such as [6] and even [18], are capable of accepting four electrons although an appreciable electrostatic repulsion is built up (Becker et al., 1991 Huber and Mullen, 1980). When considering the question of how a bis-electrophore accommodates the extra charge it is important to note that, for example, the tetra-anion of di(9-anthryl)ethane [6] adopts an anti-conformation with respect to the central C—C bond, thus minimizing the electrostatic repulsion (Huber et al., 1983). 

In this context it is noteworthy to refer to the unsaturated analogue l,2-di(9-anthryl)ethene [32] (Weitzel and Mullen, 1990 Weitzel et al., 1990). Like [6] (Becker et al., 1991), compound [32] forms a stable dianion and tetra-anion upon reduction. In the cyclic voltammogram of [32], the first two electrons are transferred at nearly the same potential, pointing to an effective minimization of the Coulombic repulsion between the charged anthryl units (Bohnen et al, 1992). This situation, which again corresponds to that in [6], could imply a torsion about the central olefinic bond (Bock et al., 1989). 

In contrast to the conformationally mobile dianthrylethanes, the rigid cyclophane [11], with a face-to-face arrangement of the rc-layers, is electrostatically less favourable for reduction and only gives rise to a dianion upon alkali-metal reduction (Huber et al., 1983). 

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Fig. 6. Sequence comparisons of Rieske proteins from spinach chloroplasts, beef heart mitochondria, green sulfur bacteria, and firmicutes. The extended insertion of proteobacterial Rieske proteins as compared to the mitochondrial one is indicated by a dotted arrow. The redox-potential-influencing Ser residue is marked by a vertical arrow. The top and the bottom sequence numberings refer to the spinach and bovine proteins, respectively. Fully conserved residues are marked by dark shading, whereas the residues conserved in the b6f-group are denoted by lighter shading.

The electron transfer Au(R2voltametric measurements 163). The half-wave potentials of the quasi-reversible process depends on the substituent R according to the Taft relation, as was described for Mo, W and Mn 37). The value of p decreases in the series Au > Mn > Mo = W, which indicates that in this sequence the mixing of ligand orbitals into the redox orbital decreases. The dominant ligand character of the unpaired electron MO in Au(R2dtc)2 relative to those in copper and silver compounds is found from Extended Hiickel MO calculations, as will be discussed later on. 

This enables one to use aliphatic systems as precursors to the radicals X-Y whose solvolytic (= redox) behavior can then be studied. Equations 2a, c describe what may be called oxidative solvolysis . This reaction sequence, the first step of which is in many cases induced by the OH radical, is of great importance in radical (and radiation) chemistry. It extends from /8-elimination reactions of monomeric radicals [6, 7] to the mechanism of DNA strand breakage [8]. An example for Eq. 2 in which it is shown that the radical XY can be produced by either step a or b is given in section 3.3. 

A more recent investigation has been carried out on the homogeneous ET to an extended series of diaryl disulfides (X = NH2, OMe, H, F, C02Et, CN. NO2) in DMF. The redox catalysis approach was applied extensively. The mechanism of the homogeneous reaction between electrogenerated radical anion donors D and (ArS)2 takes place according to the sequence (equations 80-83) ... 

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