Proton-Coupled Electron Transfer Reaction Basd On "Acid-Reductant" Pre-complex Mode

Proton-coupled electron transfer reactions（PCET）has drawn extensive attention for its widespread occurrence in chemistry,biology,and material science.However,traditional PCET reaction always depends on the hydrogen-bonding interaction between proton donor and proton acceptor,thus limiting the strategy to the activation of polar bonds（non-polar bonds cannot form hydrogen bond interactions as described above）.To settle the key scientific problem of how to realize the PCET activation of non-polar bonds,three experimental projects were carried out as follows:1.We proposed an innovative“proton donor-electron donor”pre-complexation model and developed a corresponding reaction model to achieve PCET activation of non-polar bonds.In this study,we used the hydrogen bond formed between DMBIH and carboxylic acid to achieve multiple-site concerted proton-electron-transfer（MS-CPET）reduction of non-polar O=O bonds that cannot be achieved by traditional methods.The concerted transfer of proton and electron avoided the formation of high-energy intermediate superoxide radical anion and lower the reaction barrier effectively.2.Developing different reaction system featuring“proton donor-electron donor”hydrogen-bonding pre-complexation mode.Based on the first work we next developed the new model system where aryl thiol serves as proton donor and aryl thiolate serves as electron donor.The hydrogen bond complex formed between the two can achieve O₂reduction through the MS-CPET pathway.However,in the absence of thiolate anions,thiol alone cannot reduce oxygen through the hydrogen-atom-transfer（HAT）pathway,even though the thermodynamic driving forces of the two pathways are identical.Mechanistic studies show that the reaction undergoes an electron transfer-dominated transition state,which is the true reason why MS-CPET and HAT show completely different reactivities.This work perfectly demonstrates the importance of reaction paths in PCET reactions.Moreover,the“Ar SH-Ar S^-”precomplex can serve as one kind of effective reductive PCET reagent whose hydrogen-atom donating ability is comparable to metal hydride species and the precomplex is capable to reduce azobenzenes the corresponding N-H bond with bond dissociation free energy as low as 56 kcal/mol.3.Exploring the PCET oxidation of low-polar C-H bond.Compared with O-H,N-H bond,C-H bond which acts as a proton donor can’t form interaction with base.We found that the oxidant whose oxidation ability is far from enough to oxidase C-H bond can effectively realize the homolysis of C-H bond under the existence of base.Mechanism analysis indicates that the reaction follows a pre-equilibrium ET-PT pathway(ET_pre-PT).In summary,this paper proposes for the first time the“proton donor-electron donor”hydrogen-bonding pre-complexation model and demonstrates the validity of this model in PCET activation of non-polar bonds through two different reaction systems.At the same time,we provide the first ET_pre-PT-characterized C-H bond PCET oxidation reaction model supported by detailed experimental evidence.The mechanism analysis in the above three reaction systems further deepened our understanding to the coupling between protons and electrons in PCET reactions.