Spanning four mechanistic regions of intramolecular proton-coupled electron transfer in a Ru(bpy)3(2+)-tyrosine complex

Proton-coupled electron transfer (PCET) from tyrosine (TyrOH) to a covalently linked [Ru(bpy)(3)](2+) photosensitizer in aqueous media has been systematically reinvestigated by laser flash-quench kinetics as a model system for PCET in radical enzymes and in photochemical energy conversion. Previous kinetic studies on Ru-TyrOH molecules (Sjödin et al. J. Am. Chem. Soc. 2000, 122, 3932; Irebo et al. J. Am. Chem. Soc. 2007, 129, 15462) have established two mechanisms. Concerted electron-proton (CEP) transfer has been observed when pH 10. Here we compare the PCET rates and kinetic isotope effects (k(H)/k(D)) of four Ru-TyrOH molecules with varying Ru(III/II) oxidant strengths over a pH range of 1-12.5. On the basis of these data, two additional mechanistic regimes were observed and identified through analysis of kinetic competition and kinetic isotope effects (KIE): (i) a mechanism dominating at low pH assigned to a stepwise electron-first PCET and (ii) a stepwise proton-first PCET with OH(-) as proton acceptor that dominates around pH = 10. The effect of solution pH and electrochemical potential of the Ru(III/II) oxidant on the competition between the different mechanisms is discussed. The systems investigated may serve as models for the mechanistic diversity of PCET reactions in general with water (H(2)O, OH(-)) as primary proton acceptor

Probing Quantum and Dynamic Effects in Concerted Proton–Electron Transfer Reactions of Phenol–Base Compounds

The oxidation of three phenols, which contain an intramolecular hydrogen bond to a pendent pyridine or amine group, has been shown, in a previous experimental study, to undergo concerted proton−electron transfer (CPET). In this reaction, the electron is transferred to an outer-sphere oxidant, and the proton is transferred from the oxygen to nitrogen atom. In the present study, this reaction is studied computationally using a version of Hammes-Schiffer’s multistate continuum theory where CPET is formulated as a transmission frequency between neutral and cation vibrational-electronic states. The neutral and cation proton vibrational wave functions are computed from one-dimensional potential energy surfaces (PESs) for the transferring proton in a fixed heavy atom framework. The overlap integrals for these neutral/cation wave functions, considering several initial (i.e., neutral) and final (i.e., cation) vibrational states, are used to evaluate the relative rates of oxidation. The analysis is extended to heavy atom configurations with various proton donor–acceptor (i.e., O–N) distances to assess the importance of heavy atom “gating”. Such changes in <i>d</i>_ON dramatically affect the nature of the proton PESs and wave functions. Surprisingly, the most reactive configurations have similar donor–acceptor distances despite the large (∼0.2 Å) differences in the optimized structures. These theoretical results qualitatively reproduce the experimental faster reactivity of the reaction of the pyridyl derivative <b>1</b> versus the CH₂–pyridyl <b>2</b>, but the computed factor of 5 is smaller than the experimental 10². The amine derivative is calculated to react similarly to <b>1</b>, which does not agree with the experiments, likely due to some of the simplifying assumptions made in applying the theory. The computed kinetic isotope effects (KIEs) and their temperature dependence are in agreement with experimental results

Proton-coupled electron transfer in a series of ruthenium-linked tyrosines with internal bases:Evaluation of a tunneling model for experimental temperature-dependent kinetics

Photoinitiated proton-coupled electron transfer (PCET) kinetics has been investigated in a series of four modified tyrosines linked to a ruthenium photosensitizer in acetonitrile, with each tyrosine bearing an internal hydrogen bond to a covalently linked pyridine or benzimidazole base. After correcting for differences in driving force, it is found that the intrinsic PCET rate constant still varies by 2 orders of magnitude. The differences in rates, as well as the magnitude of the kinetic isotope effect (KIE = k(H)/k(D)), both generally correlate with DFT calculated proton donor-acceptor distances. An Arrhenius analysis of temperature dependent data shows that the difference in reactivity arises primarily from differences in activation energies. We use this kinetic data to evaluate a commonly employed theoretical model for proton tunneling which includes a harmonic distribution of proton donor-acceptor distances due to vibrational motions of the molecule: Applying this model to the experimental data yields the conclusion that donor-acceptor compression is more facile in the Compounds with shorter PT distance; however, this is contrary to independent calculations for the same compounds. This discrepancy is likely because the assumption in the model of Morse-shaped proton potential energy surfaces is inappropriate for (strongly) hydrogen-bonded systems. These results question the general applicability of this, model. The results also suggest that a correlation of rate vs proton tunneling distance for the series of compounds is complicated by a concomitant variation of other relevant parameters

Isolating the Effects of the Proton Tunneling Distance on Proton-Coupled Electron Transfer in a Series of Homologous Tyrosine-Base Model Compounds

The distance dependence of concerted proton-coupled electron transfer (PCET) reactions was probed in a series of three new compounds, where a phenol is covalently bridged by a 5, 6, or 7 membered carbocycle to the quinoline. The carbocycle bridge enforces the change in distance between the phenol oxygen (proton donor) and quinoline nitrogen (proton acceptor), <i>d</i>_O···N, giving rise to values ranging from 2.567 to 2.8487 Å, and resulting in calculated proton tunneling distances, <i>r</i>₀, that span 0.719 to 1.244 Å. Not only does this series significantly extend the range of distances that has been previously accessible for experimental distance dependent PCET studies of synthetic model compounds, but it also greatly improves the isolation of <i>d</i>_O···N as a variable compared to earlier reports. Rates of PCET were determined by time-resolved optical spectroscopy with flash-quench generated [Ru­(bpy)₃]³⁺ and [Ru­(dce)₃]³⁺, where bpy = 2,2′-bipyridyl and dce = 4,4′-dicarboxyethylester-2,2′-bipyridyl. The rates increased as <i>d</i>_O···N decreased, as can be expected from a static proton tunneling model. An exponential attenuation of the PCET rate constant was found: <i>k</i>_PCET(<i>d</i>) = <i>k</i>⁰_PCETexp­[−β­(<i>d</i> – <i>d</i>₀)], with β ∼ 10 Å^–1. The observed kinetic isotope effect (KIE = <i>k</i>_H/<i>k</i>_D) ranged from 1.2 to 1.4, where the KIE was observed to <i>decrease</i> slightly with increasing <i>d</i>_O···N. Both β and KIE values are significantly smaller than what is predicted by a static proton tunneling model. We conclude that vibrational compression of the tunneling distances, as well as higher vibronic transitions, that contribute to concerted proton coupled electron transfer must also be considered

Spanning Four Mechanistic Regions of Intramolecular Proton-Coupled Electron Transfer in a Ru(bpy)₃²⁺–Tyrosine Complex

Proton-coupled electron transfer (PCET) from tyrosine (TyrOH) to a covalently linked [Ru­(bpy)₃]²⁺ photosensitizer in aqueous media has been systematically reinvestigated by laser flash-quench kinetics as a model system for PCET in radical enzymes and in photochemical energy conversion. Previous kinetic studies on Ru–TyrOH molecules (Sjödin et al. <i>J. Am. Chem. Soc.</i> <b>2000</b>, <i>122</i>, 3932; Irebo et al. <i>J. Am. Chem. Soc.</i> <b>2007</b>, <i>129</i>, 15462) have established two mechanisms. Concerted electron–proton (CEP) transfer has been observed when pH < p<i>K</i>_a(TyrOH), which is pH-dependent but not first-order in [OH^–] and not dependent on the buffer concentration when it is sufficiently low (less than ca. 5 mM). In addition, the pH-independent rate constant for electron transfer from tyrosine phenolate (TyrO^–) was reported at pH >10. Here we compare the PCET rates and kinetic isotope effects (<i>k</i>_H/<i>k</i>_D) of four Ru–TyrOH molecules with varying Ru^III/II oxidant strengths over a pH range of 1–12.5. On the basis of these data, two additional mechanistic regimes were observed and identified through analysis of kinetic competition and kinetic isotope effects (KIE): (i) a mechanism dominating at low pH assigned to a stepwise electron-first PCET and (ii) a stepwise proton-first PCET with OH^– as proton acceptor that dominates around pH = 10. The effect of solution pH and electrochemical potential of the Ru^III/II oxidant on the competition between the different mechanisms is discussed. The systems investigated may serve as models for the mechanistic diversity of PCET reactions in general with water (H₂O, OH^–) as primary proton acceptor

Effect of Basic Site Substituents on Concerted Proton–Electron Transfer in Hydrogen-Bonded Pyridyl–Phenols

Separated concerted proton–electron transfer (sCPET) reactions of two series of phenols with pendent substituted pyridyl moieties are described. The pyridine is either attached directly to the phenol (<b>HOAr-pyX</b>) or connected through a methylene linker (<b>HOArCH</b>_<b>2</b><b>pyX</b>) (X = 4-NO₂, 5-CF₃, 4-CH₃, and 4-NMe₂). Electron-donating and -withdrawing substituents have a substantial effect on the chemical environment of the transferring proton, as indicated by IR and ¹H NMR spectra, X-ray structures, and computational studies. One-electron oxidation of the phenols occurs concomitantly with proton transfer from the phenolic oxygen to the pyridyl nitrogen. The oxidation potentials vary linearly with the p<i>K</i>_a of the free pyridine (pyX), with slopes slightly below the Nerstian value of 59 mV/p<i>K</i>_a. For the <b>HOArCH</b>_<b>2</b><b>pyX</b> series, the rate constants <i>k</i>_sCPET for oxidation by NAr₃^•+ or [Fe­(diimine)₃]³⁺ vary primarily with the thermodynamic driving force (Δ<i>G</i>°_sCPET), whether Δ<i>G</i>° is changed by varying the potential of the oxidant or the substituent on the pyridine, indicating a constant intrinsic barrier λ. In contrast, the substituents in the <b>HOAr-pyX</b> series affect λ as well as Δ<i>G</i>°_sCPET, and compounds with electron-withdrawing substituents have significantly lower reactivity. The relationship between the structural and spectroscopic properties of the phenols and their CPET reactivity is discussed

Effect of Basic Site Substituents on Concerted Proton–Electron Transfer in Hydrogen-Bonded Pyridyl–Phenols

Separated concerted proton–electron transfer (sCPET) reactions of two series of phenols with pendent substituted pyridyl moieties are described. The pyridine is either attached directly to the phenol (<b>HOAr-pyX</b>) or connected through a methylene linker (<b>HOArCH</b>_<b>2</b><b>pyX</b>) (X = 4-NO₂, 5-CF₃, 4-CH₃, and 4-NMe₂). Electron-donating and -withdrawing substituents have a substantial effect on the chemical environment of the transferring proton, as indicated by IR and ¹H NMR spectra, X-ray structures, and computational studies. One-electron oxidation of the phenols occurs concomitantly with proton transfer from the phenolic oxygen to the pyridyl nitrogen. The oxidation potentials vary linearly with the p<i>K</i>_a of the free pyridine (pyX), with slopes slightly below the Nerstian value of 59 mV/p<i>K</i>_a. For the <b>HOArCH</b>_<b>2</b><b>pyX</b> series, the rate constants <i>k</i>_sCPET for oxidation by NAr₃^•+ or [Fe­(diimine)₃]³⁺ vary primarily with the thermodynamic driving force (Δ<i>G</i>°_sCPET), whether Δ<i>G</i>° is changed by varying the potential of the oxidant or the substituent on the pyridine, indicating a constant intrinsic barrier λ. In contrast, the substituents in the <b>HOAr-pyX</b> series affect λ as well as Δ<i>G</i>°_sCPET, and compounds with electron-withdrawing substituents have significantly lower reactivity. The relationship between the structural and spectroscopic properties of the phenols and their CPET reactivity is discussed