Photoinduced PCET in Ruthenium–Phenol Systems: Thermodynamic Equivalence of Uni- and Bidirectional Reactions

Abstract

Six termolecular reaction systems comprised of Ru(4,4′-bis(trifluoromethyl)-2,2′-bipyridine)32+, phenols with different para substituents, and pyridine in acetonitrile undergo proton-coupled electron transfer (PCET) upon photoexcitation of the metal complex. Five of these six phenols are found to release in concerted fashion an electron to the ruthenium photooxidant and a proton to the pyridine base. The kinetics for this concerted bidirectional PCET process and its relationship to the reaction free energy were compared to the driving-force dependence of reaction kinetics for unidirectional concerted proton–electron transfer (CPET) between the same phenols and Ru(2,2′-bipyrazine)32+, a combined electron/proton acceptor. The results strongly support the concept of thermodynamic equivalence between separated electron/proton acceptors and single-reagent hydrogen-atom acceptors. A key feature of the explored systems is the similarity between molecules employed for bi- and unidirectional CPET.

Separating Proton and Electron Transfer Effects in Three-Component Concerted Proton-Coupled Electron Transfer Reactions

“…8,24,25 The following sections present studies of the hydrogen-bond equilibria, then kinetic measurements and thermochemical analyses.…”

“…For the oxidations of phenols by an excited ruthenium oxidant and pyridine bases, Nomrowski and Wenger reported a slope of 0.52 ± 0.05. 8 For MS-CPET oxidation of a tungsten hydride complex, Bourrez, Hammarström et al reported a slope for ∂ln( k PCET )/∂Δ G ° PCET of (68 meV) –1 , 28 which is equivalent to a Brønsted α of 0.66. With one series of tethered phenol-CH 2 -pyridine compounds, MS-CPET oxidations showed a linear Brønsted plot (α = 0.54) with variation of both the oxidant and internal base, 21 while a related series containing conjugated phenol-pyridines showed a poorer correlation, likely due to communication between the phenol and the base.…”

“…26,27 More recently, Wenger and co-workers have observed similar rates for bimolecular and termolecular photochemical MS-CPET reactions in the presence of added pyridine using para -substituted phenols as donors. 8 Bourrez, Hammarström, et al have reported MS-CPET from a tungsten hydride complex with a series of bases and oxidants. 28 …”

“…This study also provides additional evidence against the intuition that MS-CPET reactions are less facile than related hydrogen atom transfer processes, following Wenger’s related study with excited state ruthenium oxidants. 8 …”

Direct observation of light-driven, concerted electron–proton transfer

“…Spectrophotometric analyses of complex formation, with the evaluation of equilibrium constants for association, K A , were carried out under conditions with less than 100% complex formation by the method of Curtis and Meyer (34). Based on the K A values, free energies of formation for the series of H-bonded adducts were ∼2 kcal/mol (700 cm −1 ) ( Table 1), within the range expected for hydrogen bond interactions 1-5 kcal/mol (300-1,500 cm −1 ) (Supporting Information) (16,35). Absorption spectral profiles were analyzed by a Gaussian deconvolution procedure that provided absorption band maxima (E op ), band widths at half height (Δ υ 1/2 ), and integrated intensities (Fig.…”