Proton-coupled electron transfer metal complexes

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... 

Metal-oxygen intermediates react with inorganic or organic substrates via various reaction pathways, such as oxygen atom transfer, hydrogen atom transfer, hydride transfer, electron transfer, proton-coupled electron transfer, free radical reactions, and others.14-16 The preferential reactivity pathways depend on the nature and oxidation state of the metal, the nuclearity of the complex, and the coordination mode and protonation state of coordinated oxygen-derived ligand(s). 

Metal-oxo complexes abstract hydrogen atom from substrates faster than their protonated metal-hydroxo counterparts, as was shown for Mn(IV) compounds with crossed-bridged cyclam.136 Hydrogen atom abstraction is characterized by a fairly large H/D kinetic isotope effects KIE usually exceeds 3, and sometimes reaches values as high as 30 -0.58 In contrast, a modest kinetic isotope effect of about 1.3-1.5 is usually observed for proton-coupled electron transfer reactions.137,138... 

The authors noticed no C-H/C-D isotope effect for the reaction of 13 with methanol and ferf-butanol, but saw a KIE k Jk = 1.4) for the O-H/O-D bond, suggesting that the stronger O-H bond is activated preferentially over the weaker C-H bonds (Pig. 12). In addition, the authors observed the formation of acetone upon the oxidation of tert-butanol. Upon comparison of rate constants (which have been normalized to account for the amount of hydrogens available for abstraction), tert-butanol reacts 50 times faster than cyclohexane. The authors propose a proton-coupled electron transfer event is responsible for the observed selectivity this complex represents a rare case in which O-H bonds may be homolyzed preferentially to C—H bonds. In further study, 13 was shown to oxidize water to the hydroxyl radical by PCET [95]. Under pseudo-first-order conditions, conversion of 13 to its one-electron reduced state was found to have a second-order dependence on the concentration of water, in stark contrast to the first-order dependence observed for aUphatic hydrocarbons and alcohols. Based on the theimoneutral oxidation of water (2.13 V v. NHE in MeCN under neutral conditions [96]) by 13 (2.14 V V. NHE in MeCN under neutral conditions) and the rate dependence, the authors propose a proton-coupled electron transfer event in which water serves as a base. While the mechanism for O-H bond cleavage of alcohols and water is not well understood in these instances, the capacity to cleave a stronger O-H bond in the presence of much weaker C-H bonds is a tremendous advance in metal-oxo chemistry and represents an exciting avenue for chemoselective substrate activation. 

An essential feature of reactions catalyzed by metal-sulfur oxidoreductases is the coupling of proton- and electron-transfer processes. In this context, an important question is how primary protonation of metal-sulfur sites influences the metal-sulfur cores, small molecules bound to them, and the subsequent transfer of electrons. In order to shed light upon this question, protonations, isoelectronic alkylations, and redox reactions of [M(L) (S )] complexes were investigated (M = Fe, Ru, Mo L = CO, NO S = Sj, US24 ). The CO and NO ligands served as infrared (IR) probe for the electron density at the metal centers. Resulting complexes were characterized as far as possible by X-ray crystallography. Scheme 23 shows examples of such complexes. 

Are the mechanisms described here applicable to cells operating in nonaque-ous environments It is conceivable that the sequence described by Eqs. (12)-( 14) occurs under certain conditions. The more complex sequence involving coupled electron and cation transfer probably does not. Although Li+ (the electrolyte cation most often used in Gratzel-type cells) is known to intercalate into high-area metal oxide semiconductors [49,90,108-111], the rate is probably too slow to be coupled to injection and back ET in the same way that aqueous proton uptake and release are coupled to these processes. The ability to use water itself as a proton source means that solution-phase diffusional limitations on proton uptake are absent. Alkali metal ion uptake from nonaqueous solutions, on the other hand, clearly is subject to diffusional limitations. 

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. 

---