Hydrogen atom transfer (HAT) reactions and their kinetic barriers ΔGHAT‡ are important in organic and inorganic chemistry. This study examines factors that influence ΔGHAT‡, reporting the kinetics and thermodynamics of HAT from various ruthenium bis(acetylacetonate) pyridine-imidazole complexes to nitroxyl radicals. Across these 36 reactions, the ΔGPT° and ΔGHAT° can be independently varied, with different sets of Ru complexes primarily tuning either their pKas or their E°s. The ΔΔGHAT‡ 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, ΔΔGHAT‡ = 0.28 × ΔΔGHAT°, but are also affected by HAT intrinsic barriers (λ), sterics, and the thermochemical e–/H+ imbalance of the reactions, |ΔGPT° – ΔGET°|. 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: ΔGHAT‡ shifts by ∼4% with changes in |ΔGPT° – ΔGET°|. 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 ΔGHAT‡ are ∼10-fold more sensitive to changes in ΔGHAT° and λ than to the e–/H+ free-energy imbalance.
Groff et al. (Wed,) studied this question.