Stabilizing Extreme Redox States

1) the values of the oxidation and redox potentials are much lower than those for linear 7t-systems  

2) the number of transferred electrons increases strongly thus the quaterrylene 44c exhibits a sequence of 8 separate redox states ranging from the trianion to the tetracation  

3) plotting the energy differences between the first oxidation and reduction steps as a function of the number of naphthalene repeating units, one does not observe a true convergence, although - different from 64 and 65 - a significant decrease is observed. By extrapolation one obtains a band gap AE of less than 1.0 eV for the polymer which is very low in comparison with all other model systems discussed above [4, 210-212]. 

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E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

We report here studies on a polymer fi1m which is formed by the thermal polymerization of a monomeric complex tris(5,5 -bis[(3-acrylvl-l-propoxy)carbonyll-2,2 -bipyridine)ruthenium(11) as its tosylate salt,I (4). Polymer films formed from I (poly-I) are insoluble in all solvents tested and possess extremely good chemical and electrochemical stability. Depending on the formal oxidation state of the ruthenium sites in poly-I the material can either act as a redox conductor or as an electronic (ohmic) conductor having a specific conductivity which is semiconductorlike in magnitude. 

It is often difficult to measure stability constants directly for Fe + complexes of natural and model siderophores, since their very high stabilities mean there is extremely little free Fe + in equilibrium with the complex. It is therefore common to use the link between redox potentials and stability constants in the two oxidation states involved in estimating values for logAiFeinL- The correlation of logAlpeniL with redox potential, established over a range of 10 in can thus... 

A number of unique difficulties pertain to oxidation states of metal ions encountered in molten salt solutions. For example, for first-row transition metals, the highest oxidation state prevailing is often +3, as in the case of Fe and Cr. Frequently, for chlorides in particular, the +3 state compounds are volatile at suitable operating temperatures and, hence, their solutions are thermally unstable.Other problems encountered include rapidly dispropor-tionating states, the formation of oxyhalides, and precipitation of complexes by reaction with the melt. While redox reactions per se involve very fast charge transfer steps, these may occur at the extremes of the range of electrochemical stability, thus leading to concomitant solvent melt decomposition. Nevertheless, suitable processes such as Fe /Fe on vitreous carbon in chloride melts can be employed to determine the effective electrochemical areas of electrodes where diffusion coefficients are accurately known. ... 

The relative stability of the oxidation states is extremely important and is usually discussed in terms of standard electrode potentials (Chapter 19). Standard electrode potentials can be used to establidi whether a redox reaction is feasible or spontaneous (under standard thermodynamic conditions). 

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