Concerted Proton−Electron Transfer in a Ruthenium Terpyridyl-Benzoate System with a Large Separation between the Redox and Basic Sites

Abstract

In order to understand how the separation between the electron and proton-accepting sites affects proton-coupled electron transfer (PCET) reactivity, we have prepared ruthenium complexes with 4′-(4-carboxyphenyl)terpyridine ligands, and studied reactivity with hydrogen atom donors (H-X). Ru II (pydic)(tpy-PhCOOH) (Ru II PhCOOH), was synthesized in one pot from [(p-cymene) RuCl 2 ] 2 , sodium 4′-(4-carboxyphenyl)-2,2′:6′,2″-terpyridine ([Na + ]tpy-PhCOO − ), and disodium pyridine-2,6-dicarboxylate (Na 2 pydic). Ru II PhCOOH plus n Bu 4 NOH in DMF yields the deprotonated Ru(II) complex, n Bu 4 N[Ru II (pydic)(tpy-PhCOO)] (Ru II PhCOO − ). The Ru(III) complex (Ru III PhCOO) has been isolated by one-electron oxidation of Ru II PhCOO − with triarylaminium radical cations (NAr 3 •+ ). The bond dissociation free energy (BDFE) of the O-H bond in Ru II PhCOOH is calculated from pK a and E 1/2 measurements as 87 kcal mol -1 , making Ru III PhCOO a strong hydrogen atom acceptor. There are 10 bonds and ca. 11.2 Å separating the metal from the carboxylate basic site in Ru III PhCOO. Even with this separation, Ru III PhCOO oxidizes the hydrogen atom donor TEMPOH (BDFE = 66.5 kcal mol -1 , ΔG°r xn = -21 kcal mol -1 ) by removal of an electron and a proton to form Ru II PhCOOH and TEMPO radical in a concerted proton-electron transfer (CPET) process. The second order rate constant for this reaction is (1.1 ± 0.1) × 10 5 M -1 s -1 with k H /k D = 2.1 ± 0.2, similar to the observed kinetics in an analogous system without the phenyl spacer, Ru III (pydic)(tpy-COO) (Ru III COO − ). In contrast, hydrogen transfer from 2,6-di-tert-butyl-p-methoxyphenol [ t Bu 2 (OMe)ArOH] to Ru III PhCOO is several orders of magnitude slower than the analogous reaction with Ru III COO.Coupling electron transfer to proton transfer is key to a wide range of chemical and biochemical processes, such as converting solar energy to chemical fuels. 1 While many of the fundamentals of electron transfer (ET) are well understood, the principles of proton-coupled electron transfer (PCET) are still being developed. The effects of increasing the distance between reaction centers has been much studied for ET, 2 and we have started to explore PCET systems in which the electron-and proton-accepting sites are increasingly separated. 3 PCET processes with large separations appear to be important in a number of biological systems, such as ribonucleotide reductases and photosystem II. 4 They may also be involved in charge injection into oxide semiconductors from ruthenium polypyridyl-carboxylate complexes. 5 In our previously reported ruthenium terpyridine-4′-carboxylate complex Ru III COO (Scheme 1), the Ru is six bonds and 6.9 Å removed from the basic carboxylate oxygen atoms. 3 Despite this separation, the reported reactions occur with H + and e − transferring in the same kinetic step, by concerted proton-electron transfer (CPET). 1,3,[6][7][8] In this report, the distance between the metal and basic Email: mayer@chem.washington.edu. Supporting Information A...

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