Concerted Proton−Electron Transfer in a Ruthenium Terpyridyl-Benzoate System with a Large Separation between the Redox and Basic Sites
- 1 July 2009
- journal article
- research article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 131 (29), 9874-9875
- https://doi.org/10.1021/ja902942g
Abstract
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). RuII(pydic)(tpy-PhCOOH) (RuIIPhCOOH), was synthesized in one pot from [(p-cymene)RuCl2]2, sodium 4′-(4-carboxyphenyl)-2,2′:6′,2′′-terpyridine ([Na+]tpy-PhCOO−), and disodium pyridine-2,6-dicarboxylate (Na2pydic). RuIIPhCOOH plus nBu4NOH in DMF yields the deprotonated Ru(II) complex, nBu4N[RuII(pydic)(tpy-PhCOO)] (RuIIPhCOO−). The Ru(III) complex (RuIIIPhCOO) has been isolated by one-electron oxidation of RuIIPhCOO− with triarylaminium radical cations (NAr3•+). The bond dissociation free energy (BDFE) of the O−H bond in RuIIPhCOOH is calculated from pKa and E1/2 measurements as 87 kcal mol−1, making RuIIIPhCOO a strong hydrogen atom acceptor. There are 10 bonds and ca. 11.2 Å separating the metal from the carboxylate basic site in RuIIIPhCOO. Even with this separation, RuIIIPhCOO oxidizes the hydrogen atom donor TEMPOH (BDFE = 66.5 kcal mol−1, ΔG°rxn = −21 kcal mol−1) by removal of an electron and a proton to form RuIIPhCOOH and TEMPO radical in a concerted proton−electron transfer (CPET) process. The second order rate constant for this reaction is (1.1 ± 0.1) × 105 M−1 s−1 with kH/kD = 2.1 ± 0.2, similar to the observed kinetics in an analogous system without the phenyl spacer, RuIII(pydic)(tpy-COO−) (RuIIICOO). In contrast, hydrogen transfer from 2,6-di-tert-butyl-p-methoxyphenol [tBu2(OMe)ArOH] to RuIIIPhCOO is several orders of magnitude slower than the analogous reaction with RuIIICOO.Keywords
This publication has 19 references indexed in Scilit:
- Slow Hydrogen Atom Transfer Reactions of Oxo- and Hydroxo-Vanadium Compounds: The Importance of Intrinsic BarriersJournal of the American Chemical Society, 2009
- Trends in Ground-State Entropies for Transition Metal Based Hydrogen Atom Transfer ReactionsJournal of the American Chemical Society, 2009
- Proton-Coupled Electron Transfer in Solution, Proteins, and ElectrochemistryThe Journal of Physical Chemistry B, 2008
- Facile Concerted Proton−Electron Transfers in a Ruthenium Terpyridine-4′-Carboxylate Complex with a Long Distance Between the Redox and Basic SitesJournal of the American Chemical Society, 2008
- Proton-Coupled Electron TransferChemical Reviews, 2007
- Large Ground-State Entropy Changes for Hydrogen Atom Transfer Reactions of Iron ComplexesJournal of the American Chemical Society, 2007
- Electrochemical Approach to Concerted Proton and Electron Transfers. Reduction of the Water−Superoxide Ion ComplexJournal of the American Chemical Society, 2005
- Excited State Electron Transfer from Ru(II) Polypyridyl Complexes Anchored to Nanocrystalline TiO2 through Rigid-Rod LinkersThe Journal of Physical Chemistry B, 2004
- Bond Dissociation Energies of O−H Bonds in Substituted Phenols from Equilibration StudiesThe Journal of Organic Chemistry, 1996
- The Chemistry of Stable Phenoxy RadicalsChemical Reviews, 1967