Pourbaix diagrams in weakly coupled systems: a case study involving acridinol and phenanthridinol pseudobases

Abstract

The thermodynamics of proton‐coupled electron transfer (PCET) in weakly coupled organic pseudobases was investigated using 2,7‐dimethyl‐9‐hydroxy‐9‐phenyl‐10‐tolyl‐9,10‐dihydroacridine (AcrOH) and 6‐phenylphenanthridinol (PheOH) as model compounds. Pourbaix diagrams for two model compounds were constructed using the oxidation potentials and the pKa values obtained, respectively, from cyclic voltammetry and photometric titrations. Our comparative study reveals the importance of having the redox active –N center closer to –OH functionality on the thermodynamics of PCET process: PheOH exhibits a wider range of pH values (pH = 2.8 to 13.3) in which both the alcohol and the corresponding alkoxy radical are expected to coexist in solution. This result indicates that a concerted mechanism is more likely to be discovered in pseudobases analogous to PheOH. The thermochemical data also indicate that the concerted PCET mechanism cannot be achieved if water is used as the proton acceptor: assuming the pKa of hydronium ions as −1.7, the PCET involving PheOH or AcrOH as proton/electron donors and water as the proton acceptor is expected to follow the stepwise ET/PT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.

Toward a mechanistic understanding of electrocatalytic nanocarbon

“…Based on the Pourbaix diagrams, the authors identify anionic intermediate 2 as the key intermediate activating molecular oxygen towards reduction to hydroperoxide 3. Reactive sites are identified as carbon atoms adjacent to nitrogen atoms, which is in a good agreement with ORR studies on analogous metal-free models [80][81][82][83] . Subsequent computational investigation of the catalytic cycle 84 identified an unusual ether-like structure 4 as an intermediate, that ensures the four-electron pathway.…”

Cocatalysis: Role of Organic Cations in Oxygen Evolution Reaction on Oxide Electrodes

“…Thus, it is very likely that Xan 2+ and other cations presented in Scheme affect this step either by lowering the kinetic barrier for *O → *OOH conversion or by fully circumventing the formation of *OOH species (for example, by forming the *OOR intermediate, where R is the molecular cocatalyst). Both of these mechanisms are possible and were proposed previously. , However, our recent studies indicate that the formation of *OOR species is a less likely mechanism, since the formation of RO species is a proton-coupled process that is not feasible for all cations presented in Scheme . , It is more likely that the cocatalysis involves the lowering of kinetic barrier for *O → *OOH conversion, as presented in Scheme . According to this mechanism, an ether-like *OR 3+ intermediate makes an oxygen center highly electron-deficient and activates it toward nucleophilic attack by a water molecule to form the critical O–O bond.…”

“…11,12 However, our recent studies indicate that the formation of *OOR species is a less likely mechanism, since the formation of RO species is a proton-coupled process that is not feasible for all cations presented in Scheme 1. 27,28 It is more likely that the cocatalysis involves the lowering of kinetic barrier for *O → *OOH conversion, as presented in Scheme 2. According to this mechanism, an ether-like *OR 3+ intermediate makes an oxygen center highly electron-deficient and activates it toward nucleophilic attack by a water molecule to form the critical O− O bond.…”

Conformational analysis of diols: Role of the linker on the relative orientation of hydroxyl groups

“…Electrochemical OER was attempted with model dimers, but no catalysis was observed. Possible explanation is the fact that proton‐coupled oxidation of these alcohols is expected to take place in the pH = 14 to 16 range, which is not accessible under our experimental conditions.…”