Electrochemical access

The coordination of transition metal ions in acidic chloroaluminate melts has not been firmly established. However, in the case of AICb-EtMelmCI. the E0 values of simple redox systems that are electrochemically accessible in both acidic and basic melt, e.g., Hg(II)/Hg [51], Sb(III)/Sb [52], and Sn(II)/Sn [53] exhibit a large positive potential shift on going from basic melt, where metal ions are known to exist as discrete anionic chloride complexes, to acidic melt. Similar results were observed for Cu(I) in AlCh-NaCl [48]. This dramatic decrease in electrochemical stability isprima facie evidence that metal ions in acidic melt are probably only weakly solvated by anionic species such as AICI4 and AECI-. Additional evidence for this is derived from the results of EXAFS measurements of simple metal ions such Co(II), Mn(II), and Ni(II) in acidic AlCh-EtMelmCl, which indicate that each of these ions is coordinated by three bidentate AICI4 ions to give octahedrally-coordinated species such as [ M (AIC14) 2 ] [54]. Most transition metal chloride compounds are virtually... 

Redox molecules are particularly interesting for an electrochemical approach, because they offer addressable (functional) energy states in an electrochemically accessible potential window, which can be tuned upon polarization between oxidized and reduced states. The difference in the junction conductance of the oxidized and the reduced forms of redox molecules may span several orders of magnitude. Examples of functional molecules used in these studies include porphyrins [31,153], viologens [33, 34,110,114,154,155], aniline and thiophene oligomers [113, 146, 156, 157], metal-organic terpyridine complexes [46, 158-163], carotenes [164], nitro derivatives of OPE (OPV) [165, 166], ferrocene [150, 167, 168], perylene tetracarboxylic bisimide [141, 169, 170], tetrathia-fulvalenes [155], fullerene derivatives [171], redox-active proteins [109, 172-174], and hydroxyquinones [175]. 

The second oxidation, which involves the Fe(III)/Fe(IV) change, is accompanied by a relatively slow degradation of the electrogenerated Fe(IV) complex. The electrochemical access to the high-valent Fe(IV) oxidation state is rather uncommon, even if a few bis(/x-oxo)diiron complexes have displayed the (III,III)/(III/IV) redox changes.116,117... 

Nitroso groups are well known as good electrophilic reagents and can be used to prepare various heterocycles. However, a direct electrochemical access to nitroso derivatives from the corresponding nitro compounds is not usual. Generally, nitroso compounds are obtained in two consecutive steps as summarized in Scheme 34. 

The cephalosporins bearing a hydrogen atom at C-3 position play an important role in antibiotics research. Electrochemical access from C-3-substituted cephems 48 has been developed. The reductive removal of the C-3 substituents of 48 (Y = Cl, MsO, TsO) has been carried out in a Mcllvain buffer solution (pH 8-7) in the presence of tetrabutylammonium iodide to give the compounds 49 in good yields (Scheme 2-17) i2). 

Some simple biphenols equipped with methyl groups, e.g., 3,3, 5,5 -tetramethyl-2,2 -biphenol 38, have attracted attention as important components of highly potent ligand systems [75-86]. Remarkably, the sustainable synthesis of such biphenols is rather challenging despite their simple scaffolds. In particular, methyl-substituted phenols are prone to side reactions. This is especially the case when 2,4-dimethyl-phenol (37) is oxidatively treated. Upon anodic conversion 37 is preferably transformed into polycyclic architectures [87]. Direct electrolysis in basic media provided only traces of the desired biphenol 38 and the dominating components of the product mixture consisted of Pu in meter s ketone 39 and the consecutive pentacyclic spiro derivative 40 [88]. For an efficient electrochemical access to 3,3, 5,5 -tetramethyl-2,2,-biphenol (38) we developed a boron-based template strategy [89, 90]. This methods requires a multi-step protocol but can be conducted on a multi-kilogram scale (Scheme 17). 

Polypyrroles and polythiophenes are particularly adaptable to such studies. They can be polymerized by an oxidative polymerization mechanism that proceeds through an electrochemically accessible radical cation intermediate [66]. 

Electrochemical accessibility to high-area materials can be achieved in another way by dispersing them in the form of a fluidized bed in a stirred electrolytic solution provided with a current-collector electrode. This system. 

Although the standard potentials, at least in aqueous solutions, are always related to reaction (2), that is, the standard hydrogen electrode (SHE) (see (Ih. 18.3), it does not mean that other reference systems cannot be used or AG of any electrochemically accessible reaction cannot be determined by measuring cell potential, ceii, when both electrodes are at equilibrium. The cell as a whole is not at equilibrium (for if the cell reaction reach es its equilibrium then E ell = Gcell = however, no current flows through the external circuit, with all local charge-transfer equilibria across phase boundaries (except at electrolyte-electrolyte junctions) and local chemical equilibria within phases being established. 

Fig. 8.6 The molecular Lego approach applied to the scaffold of P450 BM3 a to generate a P450 catalytic domain electrochemically accessible through the fusion with the electron transfer protein flavodoxin b to solubilize the human membrane-bound P450 2E1 by fusion with select-...

Where more than two redox states are electrochemically accessible in a given electrolyte solution, the electrochromic material may exhibit several colors and be termed polyelectrochromic, a frequent property of electrochromic pol5uners (2). 

---