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