Laser flash photolysis studies reveal that the cobalt-substituted polyoxotungstate Co6 undergoes a rapid, diffusion-controlled electron transfer with photogenerated RuIII(bpy)33+ under alkaline conditions (pH 8), yielding a bimolecular rate constant of approximately 2.8 × 10⁹ M⁻¹s⁻¹. This high reactivity is attributed to the formation of a transient encounter complex between the anionic POM and the cationic oxidant, followed by ultrafast electron transfer. At pH values above the pKa of 7.6, the dominant species is Co6(II)–OH, which reacts via a simple electron transfer pathway. However, at acidic pH (below pKa), where Co6(II)–OH₂ predominates, the rate drops significantly below the diffusion limit, indicating that the elementary step within the encounter complex becomes rate-determining.
This kinetic behavior strongly suggests a shift from a direct ET mechanism to a concerted proton-electron transfer (CPET) process involving water as the proton acceptor. The absence of any dependence on buffer base concentration rules out catalytic assistance from external bases such as acetate or phosphate. Instead, the observed H/D kinetic isotope effect (KIE ≈ 1.2–1.4) confirms that proton transfer is coupled to electron transfer and involves a hydrogen-bonded network centered on the Co(II)–OH₂ group. Deuterated experiments further support this interpretation, showing reduced rates consistent with the breaking of O–H bonds during the transition state.
The structural pre-organization of water molecules around the Co(II) site—facilitated by hydrogen bonding to adjacent W=O groups and embedded within surface water channels—creates a favorable environment for CPET. These water networks serve not only as proton relays but also stabilize the developing charges during the reaction.MICU1 Antibody Biological Activity The pH dependence of the rate constant further supports this model: while CPET mechanisms are generally expected to be pH-independent when water acts as the base, the observed decrease in k with decreasing pH reflects increasing competition from the reverse reaction—the backward CPET from RuII(bpy)₃²⁺ to Co6(III)–OH—where H₃O⁺ participates as a reactant.Albumin Antibody supplier This equilibrium effect explains the non-ideal pH profile and underscores the dynamic nature of the redox couple.PMID:34699050
These results demonstrate that the reactivity of Co6 is governed by a delicate balance between speciation, solvation structure, and electrostatic interactions. The ability of the polyoxometalate framework to maintain a structured hydration shell around the active site enables efficient, low-barrier CPET without requiring additional functional groups. This work provides a molecular-level understanding of how solvent organization can be harnessed to drive complex redox transformations, offering critical insights for the rational design of biomimetic catalysts for solar energy conversion and small molecule activation.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com