Proton-Coupled Electron Transfer of Ruthenium(III)−Pterin Complexes: A Mechanistic Insight

Abstract

Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and protoncoupled electron transfer (PCET) processes. Thermodynamic parameters including pK a , bond dissociation energy (BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes, [Ru II (Hdmp) (Hdmp = 6, Hdmdmp = N,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 ΔH ‡ = 1.6 ± 0.2 kcal mol −1 and ΔS ‡ = −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.kojima@chem.tsukuba.ac.jp (T. K.), mayer@chem.washington.edu (J. M. M.) and fukuzumi@chem.eng.osaka-u.ac.jp. † Osaka University ‡ Present address: Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennou-dai, Tsukuba, Ibaraki 305-8571, Japan. § University of Washington Supporting Information Available: UV-vis spectra in the spectroscopic titrations to determine the pK a values, the ESR spectrum of 2,4,6-tri-tert-butyl phenoxyl radical obtained by the hydrogen atom transfer reaction from 2,4,6-tri-tert-butyl phenol to [Ru III (dmp − ) (TPA)] 2+ , the mass spectrum of the reaction product of 4-nitrophenol with [Ru III (dmp − )(TPA)] 2+ . This material is available free of charge via the Internet at

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

“…Exploiting the concept of bond-weakening upon coordination to redox-active metals [223][224][225][226][227][228][229][230][231][232], our lab demonstrated that a redox-active titanium catalyst Cp* 2 Ti III Cl and a weak hydrogen atom acceptor TEMPO enable intramolecular additions of amides to Michael acceptors ( Figure 32) [223]. The substrate scope allows for N-H activation of amides, carbamates, thiolcarbamates, and ureas bearing phenyl or electron-rich arenes in uniformly high yield.…”

Proton-coupled electron transfer at modified electrodes by multiple pathways

“…Electrocatalysis by deliberate introduction of PCET pathways at modified electrode surfaces has been reported (6)(7)(8)(9)(10)(11)(12)(13)(14)(15). This study is important in extending solution reactivity, for example, toward water and hydrocarbon oxidation catalysis, to electrode interfaces in device configurations.…”

“…The role of PCET in electrochemical reactivity has been discussed in a series of papers (6)(7)(8)(9)(10)(11)(12)(13)(14)(15). In a cyclic voltammetry (CV) study on the Os-based couples cis-½Os IV ðbpyÞ 2 ðpyÞðOÞ 2þ ∕ cis-½Os III ðbpyÞ 2 ðpyÞðOHÞ 2þ (where bpy is 2,2′-bipyridine) and cis-½Os III ðbpyÞ 2 ðpyÞðOHÞ 2þ ∕cis-½Os II ðbpyÞ 2 ðpyÞðOH 2 Þ 2þ , Savéant and coworkers (7) concluded that oxidation of Os II Rate accelerations were observed with added proton acceptor bases, the Britton-Robinson buffer (phosphate, citrate, borate, and acetate), accompanied by the appearance of a solvent KIE of 2-2.5.…”

Synthesis, Characterization of New Tungsten(IV)?Pterin Complexes and their Reactivity Studies towards Trimethylamine N-Oxide, Sodium Borohydride and Potassium Ferricyanide

“…In the above enzymes, this redox capability of the pterin moiety is matched by the ability of the metal centres to display multiple oxidation states [5]. This aspect has catalysed research work on the coordination chemistry of pteridines in general and pterins in particular [5][6][7][8]. Literature survey reveals the existence of only a limited amount of data on the synthetic tungstenpterin complexes [9] and provides with the impetus for the present endeavour using 2-pivaloylamino-6-acetonyl-isoxanthopterin (1, H 2 L) ( Figure 1).…”