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Concerted proton-electron transfer in a ruthenium terpyridyl-benzoate system with a large separation between the redox and basic sites.

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). 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 A 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), DeltaG(o)(rxn) = -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) x 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.

SUBMITTER: Manner VW

PROVIDER: S-EPMC2765064 | biostudies-other | 2009 Jul

REPOSITORIES: biostudies-other

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Concerted proton-electron transfer in a ruthenium terpyridyl-benzoate system with a large separation between the redox and basic sites.

Manner Virginia W VW Mayer James M JM

Journal of the American Chemical Society 20090701 29

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)p ...[more]

PMID: 19569636

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Project description:Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and proton-coupled electron transfer (PCET) processes. Thermodynamic parameters including pK(a) and bond dissociation energy (BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes, [Ru(II)(Hdmp)(TPA)](ClO(4))(2) (1), [Ru(II)(Hdmdmp)(TPA)](ClO(4))(2) (2), [Ru(II)(dmp(-))(TPA)]ClO(4) (3), and [Ru(II)(dmdmp(-))(TPA)]ClO(4) (4) (Hdmp = 6,7-dimethylpterin, Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, TPA = tris(2-pyridylmethyl)amine), all of which had been isolated and characterized before. The BDE difference between 1 and one-electron oxidized species, [Ru(III)(dmp(-))(TPA)](2+), was determined to be 89 kcal mol(-1), which was large enough to achieve hydrogen atom transfer (HAT) from phenol derivatives. In the HAT reactions from phenol derivatives to [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants (k) were determined to exhibit a linear relationship with BDE values of phenol derivatives with a slope (-0.4), suggesting that this HAT is simultaneous proton and electron transfer. As for HAT reaction from 2,4,6-tri-tert-buthylphenol (TBP; BDE = 79.15 kcal mol(-1)) to [Ru(III)(dmp(-))(TPA)](2+), the activation parameters were determined to be DeltaH(double dagger) = 1.6 +/- 0.2 kcal mol(-1) and DeltaS(double dagger) = -36 +/- 2 cal K(-1) mol(-1). This small activation enthalpy suggests a hydrogen-bonded adduct formation prior to HAT. Actually, in the reaction of 4-nitrophenol with [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants exhibited saturation behavior at higher concentrations of the substrate, and low-temperature ESI-MS allowed us to detect the hydrogen-bonding adduct. This also lends credence to an associative mechanism of the HAT involving intermolecular hydrogen bonding between the deprotonated dmp ligand and the phenolic O-H to facilitate the reaction. In particular, a two-point hydrogen bonding between the complex and the substrate involving the 2-amino group of the deprotonated pterin ligand effectively facilitates the HAT reaction from the substrate to the Ru(III)-pterin complex.

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Concerted proton-electron transfer in a ruthenium terpyridyl-benzoate system with a large separation between the redox and basic sites.

Publications

Concerted proton-electron transfer in a ruthenium terpyridyl-benzoate system with a large separation between the redox and basic sites.

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