Preface

This is the second edition of a book, that originates in the Baker Lectures that one of us (JMS) gave at Cornell in the fall of 2002. Besides corrections and updates of the content of the first edition, substantial additions have been introduced. They mostly concern proton-coupled electron transfer reactions and molecular catalysis of electrochemical reactions. Proton-coupled electron transfers have attracted considerable attention since a long time, but recent conceptual breakthroughs amply justify the dedication of a whole chapter to this subject. A chapter was already devoted to molecular catalysis in the first edition, but attention to this domain has been considerably boosted by its connection with modern energy challenges, resulting in the necessity of reporting new conceptual perspectives backed by illustrating experiments.

A key idea of the whole book is that electrochemistry might be one of the best approaches to electron transfer chemistry, as illustrated by the work of many researchers all over the world.

One important facet of electron transfer chemistry concerns reactions in which the injection or removal of one electron into or from a molecule leaves the nuclear skeleton intact. Such "outersphere" electron transfers leading to a stable species are well documented in inorganic chemistry and also in organic chemistry, albeit to a somewhat lesser extent. They are quite important in natural processes. In many instances however, injection or removal of one electron into or from a molecule trigger drastic changes in the nuclear framework, as drastic as bond cleavage and bond formation. The interest in this area of electron transfer chemistry is twofold. One is that a wealth of reactions can be triggered in this way, which associates radical and acid-base (in the general sense) reactions to electron transfer. Besides photolysis and thermolysis, this is a conspicuous route to radical chemistry, with, in many cases, the advantage of a better control of the reactivity. The second aspect is more fundamental in nature. The understanding of the effects of injecting one electron into (or removing one electron from) a molecular edifice is a crucial milestone en route to a general comprehension of chemical reactions where the reshuffling of electrons bonds involves breaking and formation of bonds. It is remarkable in this connection that general reactivity models have been built for electron-transfer reactions with more success than for other, more complex, reactions.

There are several manners of injecting an electron in (or remove an electron from) a molecule. Properly set up, electrochemical approaches may be considered as the simplest and most controlled ways of investigating one-electron transfer chemistry in a quantitative fashion. The main reason for this derives from the possibility of finely tuning the electron donating or accepting capacity of the electrode by precisely setting or programming the value of its potential. Achieving such a wide and almost continuous variation of the electron-transfer driving force in homogeneous thermal or photoinduced electron transfers would require resorting to a huge number of electron donors or acceptors. A second favorable feature is that the current flowing through the electrode surface - an easy-to-gauge quantity - is a direct measure of the reaction kinetics. Current-potential curves are thus the main tools giving access to mechanisms and rate constants. In the most common case, where reactants stand in the solution, the extraction of these features implies determination and elimination of the contribution of mass transport from the global electrochemical response. Mass transport is not merely an unavoidable burden one has to get rid of. In fact, the electrochemical response is, in many cases, the result of a competition between mass transport and the reaction, or set of reactions, under examination. The possibility of a quantitative control of mass transport over an extended range of rates thus amounts to have at disposal of a wide and adjustable time-scaling tool for investigating the reaction kinetics.

The precise control of the electrode potential and measure of the current response is one of the requirements molecular electrochemistry, i.e. of this segment of electrochemistry where attention is primarily focused on the molecular changes brought about by electron transfer to or from an electrode. Another requisite is thus that the electrode material does not appreciably interfere chemically in the course of the electrochemical process. What is then sought after is an "innocent" electrode simply behaving as an electron source or sink.

The stage is set in the first chapter with the depiction of a typical electrochemical experiment, and application to the determination of the thermodynamic and kinetic characteristics of outersphere electron transfer reaction with no further chemical steps in the reaction mechanism. In this chapter as well as in the others, we describe both the experimental data and the methods by which they can be gathered.

In this respect, rather than providing a survey of all electrochemical techniques, we choose to focus on the most popular of them, namely cyclic voltammetry. This option is based on the notion that all techniques are essentially equivalent in the sense that their limit are caused by the same phenomena. This equivalence becomes even more evident after convoluting the current responses with a diffusion characteristic function. It may, nevertheless, happen that one technique is more convenient than cyclic voltammetry for a particular purpose. This is the reason that we briefly address the use of techniques such as potential-step chronoamperometry and rotating disk electrode voltammetry in these circumstances.

The experimental data we discuss in this first chapter pertain to two problems. One concerns the relationships that exist, for a simple outersphere electron transfer between activation and driving force, or in other words between kinetics and thermodynamics. The models on which these relationships are based are described, and the experiments we report are selected so as to illustrate the main predictions of these models. The second problem deals with the factors that make the injection (or removal) of a second electron more difficult or easier than the first.

The second chapter is devoted to the association between electrode electron transfers and chemical reactions. This is the heart of molecular electrochemistry since the way in which these reactions are triggered by the electrode electron transfers govern the fate of the molecules that are reduced or oxidized. These accompanying reactions and the way in which they are coupled with the electrode electron transfers may be categorized according to two points of view. One relates to the type of chemistry that is being initiated by the electrode electron transfer. The other concerns the way in which they can be identified and kinetically characterized experimentally. For the same reasons as before, we privilege the use of cyclic voltammetry, giving however examples where the complementary use of other techniques may be helpful. The first part of the chapter is thus dedicated to establishing mechanism diagnostic criteria and procedures for kinetic characterization for the main reaction schemes that may be encountered in practice. The limitations of the direct electrochemical techniques in terms of measurement of large rate constants is discussed and a section is devoted to indirect electrochemical techniques, based on redox catalysis, which allows one to overcome these limitations. Besides the nondestructive investigation techniques, such as cyclic voltammetry, used directly or indirectly, we added a section devoted to preparative-scale electrolysis, where mechanisms are translated into competitions between pathways that govern the final distribution of products. The interest in this discussion of conditions that are the exact opposite of the nondestructive regimes is twofold. On the one hand, it is related to cases where product distribution and its variations with concentrations and electrolysis rate may provide a mechanistic answer, while the use of a nondestructive technique cannot, or the two techniques can be combined to obtain the desired answer. On the other hand, the same analysis provide a rational basis for product optimization.

The experimental examples we report and discuss in this chapter are selected so as to illustrate the chemical aspects of the problems rather that the methodological aspects just alluded to. Uncovering of chemically important issues and depiction of a restricted number of well-established illustrating examples is favored over a systematic literature coverage, explaining, together with laziness, the large number of self-citations.

Besides the nature of the accompanying reaction, the prediction that electron transfer chemistry triggers a radical chemistry or an acid-base chemistry depends upon the redox characteristic of transient intermediates, such as unstable radicals. The methods that may be used to achieve this difficult task are described and illustrated by experimental examples.

The third chapter focuses on one particular type of accompanying reactions, namely breaking (or formation) of a chemical bond. This is the occasion to address a new type of electron transfer, in which a bond is not simply elongated (or shortened) or twisted upon electron transfer, but bluntly broken concertedly with electron transfer. The discussion concerns bonds linking heavy atoms, i.e. all atoms besides hydrogen, whose case is treated in...