Spanning four mechanistic regions of intramolecular proton-coupled electron transfer in aRu(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 <pK_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 (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 (H₂O, OH^-) as primary proton acceptor.