PCET proton-coupled electron thermodynamics

To study the mechanism by which HCIO4 catalyzes electron transfer, the kinetic order in acid was measured by performing a series of Stem-Volmer luminescence quenching experiments at various concentrations of HCIO4. These experiments revealed a linear relationship between k t and [HCIO4] (Fig. 42) showing electron transfer from [Ru (bpy)3] " is first order in acid. In this work, the authors proposed a stepwise ET/PT mechanism in which protonation of the ketyl radical anion provides additional thermodynamic driving force which causes an increase in et- This work predated the wide-spread acceptance of concerted proton-coupled electron transfer as an elementary step, however these seminal observations provided the conceptual framework for PCET to be applied further in contemporary synthetic chemistry. 

Lastly, electron transfer in D—[H]—A assemblies is not a perquisite of the excited states of metal complexes. Organic ensembles 38 and 39 (R = SiMe2 Bu), containing a dimethylaniline-anthracene redox pair, have been synthesized recently [124]. Preliminary time-resolved and steady-state fluorescence experiments indicate the occurrence of photoinduced electron transfer. In work related to Watson Crick base-paired systems, the excited state of the fluorescent pyrene derivative 40 is efficiently quenched (94-99 %) by 2 -deoxyguanosine (dG), 2 -deoxycytidine (dC), or 2 -deoxythymidine (dT) in aqueous solution [125]. A PCET mechanism is thought to be responsible for this process, as the thermodynamics of electron transfer are unfavorable unless coupled to a rapid proton-transfer step. The quenched lifetime of 40 in the presence of dC and dT in H2O is significantly extended by a factor of 1.5-2.0 in D2O this isotope effect is similar to that observed in the kinetics studies of 1 [70]. The invoked PCET reaction mechanism also accounts for the inability of dC and dT to quench the fluorescence of 40 in the aprotic organic solvent DMSO. 

PCET can occur when the electron and proton are site-differentiated on both the donor and acceptor sides of the reaction. The PT coordinate must still be constrained to a hydrogen bond length scale, however, it is feasible for the ET coordinate to span an extended distance [79-81]. Nevertheless, coupling between the electron and proton may be strong since the redox potentials depend on the protonation state and the pfQ,s depend on the redox state. Consequently, the square scheme of Eig. 17.1 must be used to evaluate the attendant thermodynamics. 

As demonstrated in this chapter, there have always been the fundamental mechanistic questions in oxidation of C-H bonds whether the rate-determining step is ET, PCET, one-step HAT, or one-step hydride transfer. When the ET step is thermodynamically feasible, ET occurs first, followed by proton transfer for the overall HAT reactions, and the HAT step is followed by subsequent rapid ET for the overall hydride transfer reactions. In such a case, ET products, that is, radical cations of electron donors and radical anions of electron acceptors, can be detected as the intermediates in the overall HAT and hydride transfer reactions. The ET process can be coupled by proton transfer and also by hydrogen bonding or by binding of metal ions to the radical anions produced by ET to control the ET process. The borderline between a sequential PCET pathway and a one-step HAT pathway has been related to the borderline between the outer-sphere and inner-sphere ET pathways. In HAT reactions, the proton is provided by radical cations of electron donors because the acidity is significantly enhanced by the one-electron oxidation of electron donors. An electron and a proton are transferred by a one-step pathway or a sequential pathway depending on the types of electron donors and acceptors. When proton is provided externally, ET from an electron donor that has no proton to be transferred to an electron acceptor (A) is coupled with protonation of A -, when the one-electron reduction and protonation of A occur simultaneously. The mechanistic discussion described in this chapter will provide useful guide to control oxidation of C-H bonds. 

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