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

Hydrogen atom transfer (HAT) reactions and their kinetic barriers Δ are important in organic and inorganic chemistry. This study examines factors that influence Δ, reporting the kinetics and thermodynamics of HAT from various ruthenium bis(acetylacetonate) pyridine-imidazole complexes to nitroxyl radicals. Across these 36 reactions, the Δ and Δ can be independently varied, with different sets of Ru complexes primarily tuning either their ps or their °s. The ΔΔ 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, ΔΔ = 0.28 × ΔΔ, but are also affected by HAT intrinsic barriers (λ), sterics, and the thermochemical /H imbalance of the reactions, |Δ - Δ|. 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: Δ shifts by ∼4% with changes in |Δ - Δ|. 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 and H are spatially separated. This work demonstrates the strong similarities between different kinds of HAT reactions when viewed through the lens of H/ (PCET) free energies. This lens also shows that Δ are ∼10-fold more sensitive to changes in Δ and λ than to the /H free-energy imbalance.