ECE mechanisms

The ECE mechanism [54] unifies the previously elaborated EC and CE mechanisms. It is represented by the following scheme  

I and h are current contributions arising from electrode reactions (2.58) and (2.60), respectively. Of course, in the experiment, only the total current / = h +h is observable, cpi = E -Ef) and p2 = ) are relative dimen- 

F = Wi + W2. The integration factors Sm and Mm are defined by (2.40) and (2.41), respectively. Note, that in the present case, the dimensionless chemical kinetic parameter e is defined as e =.  

Particnlarly interesting is the case when the second electrode reaction reqnires lower energy than the first one, i.e., R2 is more easily oxidized than Ri. In this case the total response consists of a single peak. The exact shape and position of this peak and its forward and reverse components reflect the relative contribntions of the redox conples Ri/Oi and R2/O2 over a narrow range of potentials dictated by the oxidation of Ri. As a conseqnence, the response due to Ri/Oi masks the response 

The experimental model used to illustrate the ECE mechanism was the reduction of p-nitrosophenol at a mercury electrode, in which the chemical step is dehydration [54]. The experimental data have been analyzed by best-fitting curve pro- 

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Multiple electron transfer with intervening chemical reaction—ECE mechanism... 

S-C sMes)Ir1"(L)C1]1 complexes (L = bpy and phen derivatives) form stable hydrides upon oxidative addition of a proton to the doubly reduced [(775-C5Me5)Ir1(L)]° intermediate (Equations (14) and (15)),23 24 26 28 which have been characterized by absorption spectroscopy and H-NMR spectrometry. Additional electrochemical activation is thus required for efficient H2 evolution, by either spontaneous decomposition (Equation (17)) or protonation (Equation (18)) of the reduced hydride complex (ECE mechanism) 24,26,28... 

The electroreduction of NO to NH2OH and NH3 in aqueous media is also catalyzed by Mnin-TMPyP complexes.334 In contrast to the behavior found with iron porphyrin complexes, the reaction proceeds through an ECE mechanism, in which reduction of Mn111 to Mn11 is followed by NO coordination and then by electroreduction of [Mnn(NO)(TMPyP)]. 

The natural assumption made by a large number of researchers in the field of electrochemical C02 reduction was that the intermediate was C02, as postulated by Haynes and Sawyer (1967). The observation of oxalate as a major product in addition to, or in competition with, the formation of CO, CO, HCOj and HCOO , increased the attention focused on the reactive intermediate and the mechanisms by which it reacted. However, controversy has arisen over whether the subsequent reaction of the CO 2 was via dimerisation (the EC mechanism) or via attack on another C02 molecule (the ECE mechanism). In addition, the existence of such species as CO 2 (ads) and HCOO (ads) have also been suggested but, as we shall see, these are not now thought to play a major role on simple metals. 

By employing an extinction coefficient for the anion radical obtained from the pulse radiolysis experiments, the concentration of the radical could be calculated, and plotted against /c. The straight line plot so obtained was taken as strong evidence for the ECE mechanism, i.e. the solution phase attack of C02 on C02, thus fully resolving the controversy over the identity and state of the intermediate. From the slope of the plot the authors obtained the rate constant k2 as 7.5 x I03dm3mol 1 s 1. 

FIGURE 2.10. ECE mechanism. Variations potential (b) with the kinetic parameter, A. 

It should be noted that the conditions that make possible the occurrence of an ECE mechanism, involving the reduction of C at the electrode surface, involve the possibility of another mechanism in which the second electron is transferred from B to C rather than from the electrode as pictured in Scheme 2.5. This homogeneous electron transfer reaction may be viewed as a disproportionation reaction insofar that A has one oxidation number more than B and C, and D, one oxidation number less. 

Since the occurrence of the ECE mechanism implies that yD , B, it follows that Kd 1, meaning that the disproportionation reaction is strongly exergonic. Since we have assumed that the two electrode electron transfer reactions are fast, the same is true for the disproportionation... 

Once a DISP mechanism has been recognized, the procedures for determining the rate constant of the follow-up reaction and the standard potential of the A/B couple from peak current and/or peak potential measurements are along the same lines as the procedures described above for the ECE mechanism. A distinction between the ECE and DISP mechanisms cannot be made when the pure kinetic conditions are achieved since the peak height, peak width, and variations of the peak potential with the scan rate and rate constant are the same, and so is its independence vis-a-vis the concentration of substrate. The only difference is then the absolute location of the peak, which cannot be checked, however, unless the standard potential of the A/B couple and the follow-up rate constant are known a priori. 

The fact that the normalized current ratio becomes negative at intermediate values of X with the ECE mechanism and not with the DISP mechanism stems from the same phenomenon as the one causing the tracecrossing behavior in cyclic voltammetry (Figure 2.9) (i.e., continuation of the reduction of C during the anodic scan). 

Two-Electron Catalytic Reactions In a number of circumstances, the intermediate C formed upon transformation of the transient species B is easily reduced (for a reductive process, and vice versa for an oxidative process) by the active form of the mediator, Q. This mechanism is the exact counterpart of the ECE mechanism (Section 2.2.2) changing electron transfers at the electrode into homogeneous electron transfers from Q, as depicted in Scheme 2.9. In most practical circumstances both intermediates B and C obey the steady-state approximation. It follows that the current is equal to what it would be for the corresponding EC mechanism with a... 

The transition between the two limiting situations is a function of the parameter (k-e/kc)Cp. The ratio between the catalytic peak current, ip, and the peak current of the reversible wave obtained in the absence of substrate, Pp, is thus a function of one kinetic parameter (e.g., Xe) of the competition parameter, (k e/A c)c and of the excess ratio y = C /Cp, where and Cp are the bulk concentrations of the substrate and catalyst, respectively. In fact, as discussed in Section 2.6, the intermediate C, obtained by an acid-base reaction, is very often easier to reduce than the substrate, thus leading to the redox catalytic ECE mechanism represented by the four reactions in Scheme 2.13. Results pertaining to the EC mechanism can easily be transposed to the ECE mechanism by doubling the value of the excess factor. 

Comparison of equations (6.85) and (6.86) with equations (6.83) and (6.84) shows that the previous analysis of the catalytic EC mechanism is applicable to the catalytic ECE mechanism after replacement of y by 2y. 

A Chemical Reaction Interposed Between Two Electron Transfers. An electrochemical process in which the product of the electron transfer undergoes a chemical reaction that generates a species which in turn is electrochemically active is defined as an ECE mechanism. It is commonly schematized as ... 

Amongst the ECE mechanisms, this one is perhaps the most commonly encountered in inorganic electrochemistry. 

Standard potential of the second electron transfer more anodic than that of the first electron transfer (AE01 positive). The case in which the product Ox, generated by the chemical reaction following the first electron transfer, is more easily reduced than the original species Ox constitutes another common ECE mechanism in inorganic electrochemistry. 

Figure 53 Potentiostatic behaviour for an ECE mechanism as a function of the rate constant kfof the interposed chemical reaction...

The appearance of free fulleride reoxidations in the backscan (starred peaks) is accounted for by the following ECE mechanism ... 

Both the rhodium atoms assume a tetrahedral geometry with respect to the RI12P2 plane (the TTD label derives from the tetrahedral-tetrahedral geometry of the two rhodium atoms in the dianion). On this basis, the overall electrode process involves the ECE mechanism illustrated in Scheme 4, where TPA = tetrahedral-planar monoanion, TTA = tetrahedral-tetrahedral monoanion. 

The most important are cases in which the product of the C-step is again electroactive [ECE mechanism, Reaction (12)] ... 

Oxidations of heterocycles can afford formations of double bonds. This is illustrated by the anodic oxidation of dihydropy-ridines (Scheme 11) [16] for which pyri-dinium cations are produced according to an ECE mechanism. Unsubstituted dihy-dropyridines at carbon 4 give pyridines. 

The formation of a double bond during anodic oxidations can result from eliminations of protons, carbon dioxide or acylium cations. The electrooxi dative aromatization of dihydropyridine derivatives and heterocycles containing nitrogen atom (di-hydroquinoxalines, tetrahydrocinnolines) involves an ECE mechanism as previously... 

The mechanism which could explain the formation of these products is described in Scheme 27. In an EC mechanism, the intermediate radical cation 48a could undergo a follow-up reaction with water as a nucleophile to form radical 48b which could than dimerize through S-N or S-S bond formation or react with 48a to yield 50 and 51 as the fianl one-electron oxidation products. In an ECE mechanism, intermediate 48b is further oxidized to 48c which reacts with acetonitrile as a solvent to give 49 as the final two-electron oxidation product. The cation intermediate 48c can react with the parent molecule 48 through [2 -f 3]-cycloaddition to give the final products 50 and 51. The [2 -f 3]-... 

As can be seen from the model, each redox reaction is attributed with different set of kinetic parameters. The current contributions are designated with I and h-As for the ECE mechanism considered in Sect. 2.4.3, the total current that can be experimentally observed is a sum of distinct current contributions, / = I +I2. Solutions for the surface concentration of the electroactrve species are given by ... 

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