Redox metal centres

In this chapter, we overview first some recent examples of interfacial electrochemical ET of composite metalloproteins where molecular mechanistic detail has in some way been achieved. We discuss next some theoretical issues regarding in situ STM of large molecules, where resonance or environmentally activated tunnel channels are opened by the redox metal centre. This is followed by an overview of some recent achievements in the area of in situ STM/AFM of the single-metal proteins cytochrome c and azurin on polycrystalline and single-crystal platinum and gold surfaces. Such an integrated approach offers new perspectives for experimental and theoretical characterization of metalloproteins at solid surfaces in contact with the natural aqueous medium for metalloprotein function. 

The role of redox metal centres in the in situ STM configurations... 

Scheme 9.14 The catal5hic decomposition of peroxides over redox metal centres. ...

The combination of hard (A) and soft (5) coordination in the 1,5-P2N4S2 ring system leads to a diversity of coordination modes in complexes with transition metals (Lig. 13.1). In some cases these complexes may be prepared by the reaction of the dianion [Ph4P2N4S2] with a metal halide complex, but these reactions frequently result in redox to regenerate 13.3 (L = S, R = Ph). A more versatile approach is the oxidative addition of the neutral ligand 13.3 (L = S) to the metal centre. 

Polynuclear dendrimer complexes of this type can undergo redox reactions at the metal centre and have luminescent properties. They have been proposed as molecular photochemical devices, although no practical examples have yet been produced. 

In this chapter, for convenience of discussion, the redox behaviour of cyclic systems is divided into two categories. Initially, those reactions for which there is no marked alteration of the unsaturation pattern of the ligand are discussed. Subsequently, systems in which the final product involves an alteration in the ligand s unsaturation are treated. However, it is emphasized that a continuum exists between redox behaviour which is completely metal-centred and that which is solely ligand-based. 

The ability of thioether macrocyclic complexes (and especially those of [9]aneS3) to support multi-redox behaviour at the coordinated metal centre is particularly notable. This allows a series of reversible stepwise one-electron oxidation and/or reduction processes, and stabilization of highly unusual transition metal oxidation states e.g. mononuclear [Pd([9]aneS3)2]2+/3+/4+,149 [Au([9]aneS3)2]+/2+/3+,150 [Ni([9]aneS3)2]2+/3+,151 and [Rh([9]aneS3)2]+/2+/3+.152 It appears to be the ability of the crown thioethers to readily adjust their... 

Since many of the transformations undergone by metabolites involve changes in oxidation state, it is understandable that cofactors have been developed to act as electron acceptors/ donors. One of the most important is that based on NAD/NADP. NAD+ can accept what is essentially two electrons and a proton (a hydride ion) from a substrate such as ethanol in a reaction catalysed by alcohol dehydrogenase, to give the oxidized product, acetaldehyde and the reduced cofactor NADH plus a proton (Figure 5.2). Whereas redox reactions on metal centres usually involve only electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer but... 

Mangano C. (1998) Molecular Switches of Fluorescence Operating Through Metal Centred Redox Couples, Coord. Chem. Rev. 170, 31-46. 

In 1990 we reported the synthesis of new redox-responsive crown ether molecules that contain a conjugated link between the crown ether unit and a ferrocene redox-active centre (Beer et al., 1990a). Examples of some of the species synthesized are shown in Fig. 5. The electrochemical behaviour of these species was investigated and also the electrochemical behaviour of their analogues with a saturated link between the ferrocene unit and the crown ether. The changes in the CVs of [2a] upon addition of magnesium cations are shown in Fig. 6. The metal cation-induced anodic shifts of [2a], [2b] and also their saturated analogue [3] and vinyl derivatives [4a], [4b] are shown in Table 1. 

Compounds [54] and [55] have been shown to complex group 1 and 2 metal cations and also ammonium and alkylammonium cations by nmr and UV/Vis spectroscopies and also by a number of solid-state X-ray crystallographically determined structures. The quinone moieties in these molecules constitute not only the coordination site but also the redox-active centre. The complexation... 

CV and SWV were used to investigate the electrochemical anion recognition properties of these species in acetonitrile and the results are summarized in Tables 21 and 22. With reference to the known electrochemical properties of [Ru(bipy)3][PF6]2, the respective reversible oxidation and reduction redox couples exhibited by the receptors can be assigned to the metal-centred... 

All the three polypyridyl complexes display the reversible reduction sequence 2 + / + /0. The relative potential values are reported in Table 7. As far as the nature of such redox changes is concerned, it is important to recall the ambiguity that exists in attributing metal-centred or ligand-centred redox processes for metal-polypyridine complexes. 

The electrochemical reversibility of the 2 + /3+ transformation, believed to be the only metal-centred redox process, allows one to predict that the [FeIH(phen)3]3 + complex maintains the octahedral structure of the [Fen(phen)3]2+ complex virtually unaltered. As a matter of fact, this has been confirmed by the X-ray structure of [Feni(phen)3] [C104]3 (Fe-N 1.97 A).107... 

As a further example of redox-induced reorganisation processes we illustrate the reorientation of a /i-ethyne group bridging two metal centres from perpendicular to parallel orientation with respect to the metal centres, Scheme 11. 

More recent approaches to the effects of the ligands on the redox activity of metal complexes are based upon the assumption that the electrode potential of a redox change involving a metal complex is determined by the additivity of the electronic contribution of all the ligands linked to the metal centre, or to the overall balance between the c-donor and the 7r-acceptor capability of each ligand.3 In particular two ligand electrochemical parameters have gained popularity ... 

Technically important electrochemical reactions of pyrrole and thiophene involve oxidation in non-nucleophilic solvents when the radical-cation intermediates react with the neutral molecule causing polymer growth [169, 191], Under controlled conditions polymer films can be grown on the anode surface from acetonitrile. Tliese films exhibit redox properties and in the oxidised, or cation doped state, are electrically conducting. They can form the positive pole of a rechargeable battery system. Pyrroles with N-substituents are also polymerizable to form coherent films [192], Films have been constructed to support electroactive transition metal centres adjacent to the electrode surface fomiing a modified electrode,... 

The coordination chemistry of vanadium is strongly influenced by the oxidizing/reducing properties of the metallic centre, and the chemistry of vanadium ions in aqueous solution is limited to oxidation states +2, +3, +4 and +5, although V2+ can reduce water. Redox potentials are given in Table 1 and an E vs. pH diagram is shown in Figure 1. 

Redox chemistry of vanadium-catechol systems is complicated References 256, 497 and 499-508 discuss this subject in detail. In complexes, the metal centre may be in the +5, +4, +3 (and +2) formal oxidation state and quinones complex in three localized electronic forms ... 

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