Dissociative proton-coupled electron transfer

Protons are in general indispensable for the dismutation of superoxide (Eq. (4)). Also in the case of its dismutation catalyzed by a metal center, two protons are needed for the dissociation of the product (H2O2) from the metal center (Scheme 9). Therefore, a complex which can accept two protons upon reduction and release them upon oxidation is an excellent candidate for SOD activity. The studies on proton-coupled electron transfer in Fe- and Mn-SODs 48), demonstrated that the active site of MnSOD consists of more than one proton acceptor (Scheme 10). Since the assignment of species involved in proton transfer is extremely difficult in the case of enzymatic systems, relevant investigations on adequate model complexes could be of vast importance. H2dapsox coordinates to Fe(II) in its neutral form, whereas in the case of Fe(III) it coordinates in the dapsox form. Thus, oxidation and reduction of its iron complex is a proton-coupled electron transfer process 46), which as an energetically favorable... 

Proton-coupled electron transfer (pcet) is an important mechanism for charge transfer in biology. In a pcet reaction, the electron and proton may transfer consecutively (et/pt or pt/et) or conceitedly (etpt). These mechanisms are analyzed and expressions for their rates presented. Features that lead to dominance of one mechanism over another are outlined. Dissociative etpt is also discussed, as well as a new mechanism for highly exergonic proton transfer. 

As outlined in the Introduction, a couple of suggested pathways have been proposed for the first electron transfer step (a) dissociative chemisorption of O2 (rds) probably accompanied by e-transfer and followed by proton transfer (b) simultaneous proton and electron transfer to a weakly adsorbed O2 molecule. We have recently shown through CPMD [21,69] and DFT [75] results that both pathways may take place under different conditions of the interfadal structure i.e., proton transfer may be involved in the first reduction step depending on the relative location of the O2 molecule with respect to the surface and to the proton, on the degree of proton hydration, and on the surface charge which is dependent on the electrode potential. Moreover, it was shown that proton transfer may precede or follow the first electron transfer, but in most cases the final product of the first step is an adsorbed HOO. ... 

The ORR proceeds in four or five steps. Four of these elementary steps involve proton and electron transfer. The main ORR mechanisms are known as dissociative and associative mechanism. In the dissociative mechanism, the oxygen molecule first adsorbs onto the metal and then dissociates by the breaking of the 0-0 bond. Dissociation of O2 is followed by the transfer of two electrons and two protons to form two adsorbed OYiad- Another coupled proton-electron transfer process transforms each of the OH into water. However, DFT studies have shown that direct dissociation of O2 has an activation barrier of >0.5 eV, rendering this process an unlikely reaction step (Hyman and Medlin, 2005). 

It is also common to measure by voltammetry the thermodynamic properties of purely chemical reactions that are in some way coupled to the electron transfer step. Examples include the determination of solubility products, acid dissociation constants, and metal-ligand complex formation constants for cases in which precipitation, proton transfer, and complexation reactions affect the measured formal potential. Also in these instances, studies at variable temperature will afford the thermodynamic parameters of these coupled chemical reactions. 

The ET processes under discussion here correspond by definition to pure ET, in which molecular or medium coordinates may shift (the polaron response) [17], but no overall bonding rearrangements occur. More complex ET processes accompanied by such rearrangements (e.g., coupled electron/proton transfer and dissociative ET) are of great current interest, and many theoretical approaches have been formulated to deal with them, including quantum mechanical methods based on DC treatment of solvent [31,32],... 

For solution redox couples uncomplicated by irreversible coupled chemical steps (e.g. protonation, ligand dissociation), a standard (or formal) potential, E°, can be evaluated at which the electrochemical tree-energy driving force for the overall electron-transfer reaction, AG c, is zero. At this potential, the electrochemical rate constants for the forward (cathodic) and backward (anodic) reactions kc and ka (cms-1), respectively, are equal to the so-called "standard rate constant, ks. The relationship between the cathodic rate constant and the electrode potential can be expressed as... 

Equation (1) describes the chemisorption of O2 on a surface site A of a metal (Ma) in an acid medium, where coupled to a proton and an electron transfer leads to the formation of an adsorbed end-on complex HOO-Ma. The unstable intermediate subsequently dissociates into two adsorbed species, one adsorbing on A sites, 0-Ma and the OH species adsorbing on B sites, HO-Mb (Eq. 2). In the rest of the electroreduction steps, represented by Eq. (3), adsorbed O and OH are reduced to H2O and the water molecules are eventually desorbed from the metal surface. Actually, Eqs. (1)-(3) can also be used to interpret the ORR activity for Pt-skin surfaces. The electronic sfructures of surface Pt atoms are not identical due to the existence of 3d metal in the sublayers. Ma and Mb can be looked as two Pt surface sites with different activities for reactions (l)-(3). Ma site possess better performance for the formation of the OOH complex, and Mb site may enhance the dissociation of OOH. The overall ORR is thus facilitated by the skin sfructures. 

Another area of increasing emphasis is the elucidation of chemical bonding rearrangements either initiated by or accompanying ET for example, coupled proton- (or other ion ) electron transfer cpet) [20, 22] and dissociative ET [80]. Such a focus, of course, lies at the heart of much current research in solar-energy conversion. An especially exciting recent development is the construction of a functioning biomimetic photon-driven proton pump [81]. 

ELECTRON TRANSFER DISSOCIATION (ETD) COUPLED WITH PROTON TRANSFER (PTR)... 

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