Proton-coupled electron transfer with photoexcited ruthenium(II), rhenium(I), and iridium(III) complexes

• PCET between phenols and photoexcited Ru(2,2′-bipyrazine)32+ is concerted.
• Phenol-Ru(2,2′-bipyridine)32+ dyads exhibit apparent photoacid behavior.
• Thiophenols undergo PCET via stepwise or concerted mechanisms.

The field of excited-state proton-coupled electron transfer (PCET) with d6 metal complexes is reviewed. This includes mostly work on Ru(α-diimine)32+ complexes and rhenium(I) tricarbonyl diimines. In many cases the metal complexes were designed such that they can exhibit acid/base chemistry in addition to their well-known photoredox behavior. Upon photoexcitation the resulting complexes can then act either as combined electron/proton donors or as combined electron/proton acceptors. This review aims to illustrate the usefulness of such complexes for mechanistic studies of excited-state PCET. In other studies the photoactive d6 metal complexes merely act as ordinary electron donors or acceptors, but their reaction partners undergo PCET chemistry, i.e., they release a proton upon oxidation or they are protonated in the course of reduction. This design principle permits the investigation of long-range electron transfer which is coupled to proton transfer, for example in phenol-ruthenium(II) or phenol-rhenium(I) dyads, and this represents another focal point of this review. PCET is an elementary reaction step in biological photosynthesis and plays an important role in water oxidation as well as in CO2 reduction or N2 fixation. Mechanistic studies of excited-state PCET are therefore of interest in the greater contexts of solar energy conversion and small molecule activation.

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