Facile Concerted Proton−Electron Transfers in a Ruthenium Terpyridine-4′-Carboxylate Complex with a Long Distance Between the Redox and Basic Sites

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Abstract
We have designed and prepared ruthenium complexes with terpyridine-4′-carboxylate (tpyCOO) ligands, in which there are six bonds between the redox-active Ru and the basic carboxylate. The protonated Ru(II) complex, RuII(dipic)(tpyCOOH) (RuIICOOH), is prepared in one-pot from [(p-cymene)RuCl2]2, tpyCOONa, and then sodium pyridine-2,6-dicarboxylate [Na(dipic)]. A crystal structure of the deprotonated Ru(II) complex, RuIICOO−, shows a distance of 6.9 Å between the metal and basic sites. The Ru(III) complex (RuIIICOO) has been isolated by one-electron oxidation of RuIICOO− with triarylaminium radical cations (NAr3•+). RuIIICOO has a bond dissociation free energy (BDFE) of 81 ± 1 kcal mol−1, from pKa and E1/2 measurements. It oxidizes 2,4,6-tri-tert-butylphenol (BDFE = 77 ± 1 kcal mol−1) by removal of e− and H+ (≡ H•) to form 2,4,6-tri-tert-butylphenoxyl radical and RuIICOOH, with a second-order rate constant of (2.3 ± 0.2) × 104 M−1 s−1 and a kH/kD of 7.7 ± 1.2. Thermochemical analysis suggests a concerted proton−electron transfer (CPET) mechanism for this reaction, despite the 6.9 Å distance between the redox-active Ru and the H+-accepting oxygen. RuIIICOO also oxidizes the hydroxylamine TEMPOH to the stable free radical TEMPO and xanthene to bixanthyl. These reactions appear to be similar to processes that have been previously termed hydrogen atom transfer.

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