Redox active metals

Belouzov-Zhabotinsky reaction [12, 13] This chemical reaction is a classical example of non-equilibrium thermodynamics, forming a nonlinear chemical oscillator [14]. Redox-active metal ions with more than one stable oxidation state (e.g., cerium, ruthenium) are reduced by an organic acid (e.g., malonic acid) and re-oxidized by bromate forming temporal or spatial patterns of metal ion concentration in either oxidation state. This is a self-organized structure, because the reaction is not dominated by equilibrium thermodynamic behavior. The reaction is far from equilibrium and remains so for a significant length of time. Finally,... 

As a result of strong electronic interactions between the two metalloporphyrin units, there is a substantial uncertainty in assigning oxidation states in mixed-valence group 2 complexes of redox-active metals, such as Co. Thus, although reduced neutral C02 derivatives can be reasonably well described as those of Co the location (metal versus porphyrin) of the electron hole(s) in the singly and doubly oxidized derivatives is not known definitively, and may be very sensitive to the medium [LeMest et al., 1996, 1997]. For example, in benzonitrile, the UV-vis spectmm of [(FTF4)Co2]" ... 

Much effort has been expanded in drawing mechanistic inferences from the observation that cofacial bismetalloporphyrins containing a non-redox-active metal ion are fairly selective catalysts (e.g., (DPA)CoM, where M = Lu, Sc, Al, Ag, Pd, 2H, i.e., monometallic porphyrins Fig. 18.15). At least two hypotheses have been proposed (i) polarization of the 0-0 bond in catalytic intermediates by the second ion (on an N-H moiety) acting as a Lewis acid [CoUman et al., 1987, 1994] and (ii) spatial positioning of H+ donors especially favorable for proton transfer to the terminal O atoms of coordinated O2 [Ni et al., 1987 Rosenthal and Nocera, 2007]. To the best of my knowledge, neither hypothesis has yet been convincingly proven nor resulted in improved ORR catalysts. When seeking stereoelectronic rational of the observed av values, it is useful to be mindful that a fair number of simple Co porphyrins are also relatively selective ORR catalysts (Section 18.4.2). 

Substitution-inert complexes have also recently been introduced into DNA as modified-base phosphoramidites. Interest here is generally focused on photo- and redox-active metal species for use as probes for sensing applications (165) and in studies on DNA-mediated electron... 

The nature of the ligand donor atom and the stereochemistry at the metal ion can have a profound effect on the redox potential of redox-active metal ions. The standard redox potentials of Cu2+/Cu+, Fe3+/Fe2+, Mn3+/Mn2+, Co3+/Co2+, can be altered by more than 1.0 V by varying such parameters. A simple example of this effect is provided by the couple Cu2+/Cu+. These two forms of copper have quite different coordination geometries, and ligand environments, which are distorted towards the Cu(I) geometry, will raise the redox potential, as we will see later in the case of the electron transfer protein plastocyanin. 

There is considerable evidence that defective homeostasis of redox-active metals, i.e. iron and copper, together with oxidative stress, contributes to the neuropathology of AD. The characteristic histology of AD is the deposition of both Ap, as neurotic plaques (Figure 18.12a), and of the protein tau, as neurofibrillary tangles NFT (Figure 18.12b), predominantly in the cerebral cortex and hippocampus. 

Table 1. Representative metalloproteins that bind redox-active metals and that have been studied by site-directed mutagenesis...

One example of the combined use of site-directed mutagenesis with chemical modification has been provided by the work of Bowler et al. with yeast iso-l-cytochrome c [15]. These workers introduced a His residue at position 62 to provide a site for attachment of a pentammineruthenium complex. Introduction of a second redox-active metal center to the protein at this position permitted... 

The chemistry presented herein has heen presented on the occasion of the SFB symposium Redox active metal complexes - Control of Reactivity via Molecular Architecture. ... 

Thus, superoxide can react with almost all redox-active metal centers (Scheme 1). In general, going through similar redox reaction steps metal complexes can interact with superoxide either as catalysts for its dismutation (superoxide dismutase (SOD) mimetics), or in a stoichiometric manner (Scheme 1). 

The author gratefully acknowledges financial support from the Deutsche Forschungsgemeinschaft within SFB 583 Redox-active Metal Complexes . 

Early attempts at observing electron transfer in metalloproteins utilized redox-active metal complexes as external partners. The reactions were usually second-order and approaches based on the Marcus expression allowed, for example, conjectures as to the character and accessibility of the metal site. xhe agreement of the observed and calculated rate constants for cytochrome c reactions for example is particularly good, even ignoring work terms. The observations of deviation from second-order kinetics ( saturation kinetics) allowed the dissection of the observed rate constant into the components, namely adduct stability and first-order electron transfer rate constant (see however Sec. 1.6.4). Now it was a little easier to comment on the possible site of attack on the proteins, particularly when a number of modifications of the proteins became available. 

Bielawski et al. have developed Janus-head dicarbene ligands which are able to act as a bridge between two metal centers, thereby leading to dinuclear complexes of type 96 [58-60] (Fig. 32). More recently homonuclear bimetallic ruthenium(II) and iron(II) complexes 97 have been synthesized. It was hoped that the dicarbene ligand would interconnect the redox-active metal centers, but the... 

The insertion electrochemistry of PCMs Most of the PCMs possess two characteristic properties, they have redox-active metal centers and they have an open... 

Metal hexacyanoferrates possessing only one kind of redox-active metal ions Most of the metal hexacyanoferrates show only... 

Metal hexacyanoferrates possessing two kinds of redox-active metal ions Here we consider those hexacyanoferrates that... 

First of all, I would like to express my gratitude to the institutions that facilitated this work. Studies related to the formation, properties, and reactivity of metalloporphyrins were supported by grants from the Alexander von Humboldt Stiftung (fellowship 2005-2006), by the Deutsche Forschungsgemeinschaft through SFB 583, Redox-Active Metal Complexes Control of Reactivity via Molecular Architectures and by a grant of computer time on the Hochstleistungsrechner in Bayern II (HLRB II). 

The authors gratefully acknowledge financial support from the Deutsche Forschungsge-meinschaft through SFB 583 on Redox-Active Metal Complexes. 

---