With Inorganic Acceptors

ID-DADA chains of alternating donors and acceptors, as depicted in Fig, 8-9. The two materials exhibit positive values of the Curie-Weiss constant (0 = -f 6.6 K and 9 = +9.8 K, respectively) and their main magnetic coupling is of the ferromagnetic type, although no 3D cooperative effeets could be observed [43]. 

The same ligand [C2S2(Cp3)2] coordinated to molybdenum forms pseudo-octahedral (the actual structure is intermediate between octahedral and trigonal prismatic) complexes of the type [Mo C2S2(CF3)2 3] , which have been employed as acceptors [44]. Two CT complexes with different stoichiometries with [FeCpf] have been described. The 1 1 material shows in the solid state a ID-DADA type structure and turns out to be ferromagnetic 9 = + 8.4 K, but no bulk ferromag- 

One ligand at molybdenum has been omitted for clarity (reprinted with permission from ref. [44], 

Complexes of the well-known and ubiquitous ligand dmit (l,3-dithiole-2-thione- 

S-S intermolecular eontacts. In this context it is interesting to note that ferrocene also forms CT eomplexes with [Au(dmit)2] of 1 3 and 1 4 stoichiometries similar to those of [FeCpf] and that such materials possess very similar electrical properties [48]. 

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Single-Stack Donor. Ion-radical salts can also be formed from electron donors such as tetrathiafulvalene (TTE) or TMPD (N,N,N N-tetramethyl- phenylene diamine) with inorganic acceptors such as halogens. The resulting stmcture of compounds such as TTE(A)... 

Decamethylferrocene has also been used as a donor for the formation of CT complexes with inorganic acceptors. Such acceptors are mainly composed of late transition metal complexes containing planar ligands. Some of these acceptors are illustrated in Scheme 8-3. As for their organic counterparts, the inorganic partners have at least two reversibly accessible oxidation states. The reduced form present in the CT complex is usually a radical anion. 

These association reactions can be controlled. Acetone or acetonylacetone added to the solution of the polymeric electron acceptor prevents insolubilization, which takes place immediately upon the removal of the ketone. A second method of insolubiUzation control consists of blocking the carboxyl groups with inorganic cations, ie, the formation of the sodium or ammonium salt of poly(acryhc acid). Mixtures of poly(ethylene oxide) solutions with solutions of such salts can be precipitated by acidification. 

Many inorganic species show charge-transfer absorption and are called charge-transfer complexes. For a complex to demonstrate charge-transfer behavior one of its components must have electron donating properties and another component must be able to accept electrons. Absorption of radiation then involves the transfer of an electron from the donor to an orbital associated with the acceptor . ... 

Here CH2O represents oxidizable organic matter. In reality the reactions with inorganic terminal electron acceptors, particularly Fe(lll) and S04 , should also be considered. But in the absence of a complete understanding of these processes (see Chapter 5), and for the sake of simplicity, we exclude them. 

As discussed, there are various methods of cation-radical generation. Every individual case needs its own appropriate method. A set of these methods is continuously being supplemented. For example, it was very difficult to prepare the cation-radicals of benzene derivatives with strong acceptor groups. However, some progress has been achieved, thanks to the use of fluorosulfonic acid, sometimes with addition of antimony pentafluoride, and lead dioxide (Rudenko 1994). As known, superacids stabilize cationic intermediates (including cation-radicals) and activate inorganic oxidants. The method mentioned is effective at -78°C. Meanwhile, -78°C is the boundary low temperature because the solution viscosity increases abruptly. This leads to the anisotropy of a sample and a sharp deterioration in the ESR spectrum quality. 

Pyruvate oxidase. The soluble flavoprotein pyruvate oxidase, which was discussed briefly in Chapter 14 (Fig. 14-2, Eq. 14-22), acts together with a membrane-bound electron transport system to convert pyruvate to acetyl phosphate and C02.319 Thiamin diphosphate is needed by this enzyme but lipoic acid is not. The flavin probably dehydrogenates the thiamin-bound intermediate to 2-acetylthiamin as shown in Eq. 15-34. The electron acceptor is the bound FAD and the reaction may occur in two steps as shown with a thiamin diphosphate radical intermediate.3193 Reaction with inorganic phosphate generates the energy storage metabolite acetyl phosphate. 

The appearance of similar absorption bands has also been observed upon the formation of a complex between the reduced form of cytochrome c and the simple inorganic acceptor Fe(III)(CN)6[106]. The tunneling distance evaluated from the intensity of this band amounts to 7—10 A. However, more recent experiments have failed to detect such a band [107]. The situation is more favourable in the system [cytochrome c/P870] of the Chromatium reaction centre, where the intensity of the charge transfer band centred at 200 nm could be correlated with the data obtained in kinetic experiments [108]. 

Dithiafulvene derivatives behave as -electron donors and form stable charge-transfer complexes and radical ion salts with a wide variety of organic and inorganic acceptor species. 

SQR (respiratory complex II) is involved in aerobic metabolism as part of the citric acid cycle and of the aerobic respiratory chain (Saraste, 1999). QFR participates in anaerobic respiration with fumarate as the terminal electron acceptor (Kroger, 1978 Kroger etal., 2002) and is part of the electron transport chain catalyzing the oxidation ofvarious donor substrates (e.g., H2 or formate) by fumarate. These reactions are coupled via an electrochemical proton potential (Ap) to ADP phosphorylation with inorganic phosphate by ATP synthase (Mitchell, 1979). 

Organolithium species may form coloured and EPR-active intermediates not only with conventional acceptors such as carbonyl or imine compounds [14-16] but also with unsaturated inorganic ions such as ambidentate thiocyanate-SCN the reaction between n-BuLi and NH4SCN in the presence of strongly ion-stabilizing hexamethylphosphoric triamide 0 = P(NMe2)3 (HMPA) eventually produced LiNCS 2 HMPA via some unusually colored intermediates and thiocyanate-based radical species [62]. 

Figure 21. Two triads with inorganic chromophore and organic donor-acceptor units built around a central lysine core.

Some references of reviews besides the ones already cited are given [1,3, 5-9, 19, 23-25, 28, 31, 33]. Organometallic photochemistry [36] was excellently treated in [37] and may be compared with inorganic photochemistry to gain further inspiration [38-40]. A recent multiauthored book strongly overlaps with the subject matter of the present section, and should certainly be consulted [41]. Electron transfer reactions play a determinant role in many photocatalytic processes several recent reviews and books may be cited on this topic [42-44]. The photochemistry of the M-CO bond [45] and the theme of photocatalysis by transition metal complexes [46] have recently been reviewed. Covalently linked donor-acceptor systems for mimicry of photosynthetic energy transfer have been discussed in [47]. Several special issues of Coordination Chemistry Reviews have been devoted to the photochemistry and photophysics of coordination compounds [48-50], and a special issue to photochemistry [51]. Further developments in photochemistry were the subject of a special issue of Chemical Reviews [52]. Practical considerations useful for designing photochemical experiments may be found in [53]. 

Powerful inorganic acceptors like the halogens, certain transition metal complexes, and negatively charged counterions with negligible nucleophilic tendency like BFT, CIOT, PFg, AsFg, NOj, CH SO, etc. result in fully ionic phenazine compounds. The physical properties of phenazine salts with inorganic anions have not been extensively or systematically studied. We mention below materials whose structures have been determined. 

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