Disentangling Driving Force Effects, Polar Effects, e–/H+ Imbalance, and Other Influences on H-Atom Transfer Reactions

Abstract

Hydrogen atom transfer (HAT) reactions and their kinetic barriers ΔG_HAT^‡ are important in organic and inorganic chemistry. This study examines factors that influence ΔG_HAT^‡, reporting the kinetics and thermodynamics of HAT from various ruthenium bis(acetylacetonate) pyridine-imidazole complexes to nitroxyl radicals. Across these 36 reactions, the ΔG_PT^° and ΔG_HAT^° can be independently varied, with different sets of Ru complexes primarily tuning either their pK_as or their E°s. The ΔΔG_HAT^‡ are analyzed using multiple linear free energy relationships (LFERs), the first largely experimental study of its kind. The barriers vary most strongly with the overall driving force, ΔΔG_HAT^‡ = 0.28 × ΔΔG_HAT^°, but are also affected by HAT intrinsic barriers (λ), sterics, and the thermochemical e^–/H⁺ imbalance of the reactions, |ΔG_PT^° – ΔG_ET^°|. The latter is a small but significant effect, revealed only by comparing LFERs. The imbalance analysis is closely related to traditional explanations of polar effects, but it is quantitative: ΔG_HAT^‡ shifts by ∼4% with changes in |ΔG_PT^° – ΔG_ET^°|. This is the same dependence as was observed for purely organic HAT from toluenes─a remarkable result because traditional explanations of organic polar effects, e.g., using X–H bond polarities, do not apply to the Ru complexes in which the e^– and H⁺ are spatially separated. This work demonstrates the strong similarities between different kinds of HAT reactions when viewed through the lens of H⁺/e^– (PCET) free energies. This lens also shows that ΔG_HAT^‡ are ∼10-fold more sensitive to changes in ΔG_HAT^° and λ than to the e^–/H⁺ free-energy imbalance.