Long-range proton-coupled electron transfer in phenol–Ru(2,2′-bipyrazine)32+ dyads

Catherine Bronner,Oliver S Wenger

doi:10.1039/c3cp55071k

Abstract

Two dyads in which either 4-cyanophenol or un-substituted phenol is connected via a p-xylene spacer to a Ru(bpz)3(2+) (bpz = 2,2'-bipyrazine) complex were synthesized and investigated. Selective photo-excitation of Ru(bpz)3(2+) at 532 nm in a CH3CN-H2O mixture leads to the formation of 4-cyanophenolate or phenolate along with Ru(bpz)3(2+) in its electronic ground state. This apparent photoacid behavior can be understood on the basis of a reaction sequence comprised of an initial photoinduced proton-coupled electron transfer (PCET) during which 4-cyanophenol or phenol is oxidized and deprotonated, followed by a thermal electron transfer event in the course of which 4-cyanophenoxyl or phenoxyl is reduced by Ru(bpz)3(+) to 4-cyanophenolate or phenolate. Conceptually, this reaction sequence is identical to a sequence of photoinduced charge-separation and thermal charge-recombination events as observed previously for many electron transfer dyads, with the important difference that the initial photoinduced electron transfer process is proton-coupled. The dyad containing 4-cyanophenol reacts via concerted-proton electron transfer (CPET) whereas the dyad containing un-substituted phenol appears to react predominantly via a stepwise PCET mechanism. Long-range PCET is a key reaction in photosystem II. Understanding the factors that govern the kinetics of long-range PCET is desirable in the broader context of light-to-energy conversion by means of proton-electron separation across natural or artificial membranes.

Highlights

Many important redox reactions including water oxidation and carbon dioxide reduction must be coupled to acid/base chemistry in order to proceed efficiently.[1]
We aimed to explore how driving-force changes affect the proton-coupled electron transfer (PCET) mechanisms and rates in the two different settings shown in Scheme 1
The bimolecular reactions shown in Scheme 1a are clear-cut cases of concerted PCET processes

Summary

Introduction

Many important redox reactions including water oxidation and carbon dioxide reduction must be coupled to acid/base chemistry in order to proceed efficiently.[1] there is significant interest in understanding proton-coupled electron transfer (PCET) reactions at the most fundamental level both in artificial and biological systems.[2,3,4,5,6,7,8] Phenol molecules have played a prominent role in mechanistic PCET studies because their redox and acid/base chemistry are strongly interrelated.[9,10,11,12] Much research focused on bimolecular PCET in which phenols form hydrogen-bonded encounter complexes with reaction partners that can act as electron and/or proton acceptors.[13,14,15,16,17,18,19] In addition, there has been significant work on dyads in which phenols are connected covalently to an electron-accepting moiety and where the solvent (or added base) acts as a proton acceptor.[20,21,22,23,24,25,26,27] The importance of the distance between electron/proton donating and accepting sites in covalent PCET dyads has received increasing attention recently.[28,29,30,31,32,33,34] Long-range electron transfer which is not coupled to proton transfer is rather well understood.[35,36]

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Conclusion