Trends in Ground-State Entropies for Transition Metal Based Hydrogen Atom Transfer Reactions

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Abstract
Reported herein are thermochemical studies of hydrogen atom transfer (HAT) reactions involving transition metal H-atom donors MIILH and oxyl radicals. [FeII(H2bip)3]2+, [FeII(H2bim)3]2+, [CoII(H2bim)3]2+, and RuII(acac)2(py-imH) [H2bip = 2,2′-bi-1,4,5,6-tetrahydropyrimidine, H2bim = 2,2′-bi-imidazoline, acac = 2,4-pentandionato, py-imH = 2-(2′-pyridyl)imidazole)] each react with TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) or tBu3PhO• (2,4,6-tri-tert-butylphenoxyl) to give the deprotonated, oxidized metal complex MIIIL and TEMPOH or tBu3PhOH. Solution equilibrium measurements for the reaction of [CoII(H2bim)3]2+ with TEMPO show a large, negative ground-state entropy for hydrogen atom transfer, −41 ± 2 cal mol−1 K−1. This is even more negative than the ΔS°HAT = −30 ± 2 cal mol−1 K−1 for the two iron complexes and the ΔS°HAT for RuII(acac)2(py-imH) + TEMPO, 4.9 ± 1.1 cal mol−1 K−1, as reported earlier. Calorimetric measurements quantitatively confirm the enthalpy of reaction for [FeII(H2bip)3]2+ + TEMPO, thus also confirming ΔS°HAT. Calorimetry on TEMPOH + tBu3PhO• gives ΔH°HAT = −11.2 ± 0.5 kcal mol−1 which matches the enthalpy predicted from the difference in literature solution BDEs. A brief evaluation of the literature thermochemistry of TEMPOH and tBu3PhOH supports the common assumption that ΔS°HAT ≈ 0 for HAT reactions of organic and small gas-phase molecules. However, this assumption does not hold for transition metal based HAT reactions. The trend in magnitude of |ΔS°HAT| for reactions with TEMPO, RuII(acac)2(py-imH) ≪ [FeII(H2bip)3]2+ = [FeII(H2bim)3]2+ < [CoII(H2bim)3]2+, is surprisingly well predicted by the trends for electron transfer half-reaction entropies, ΔS°ET, in aprotic solvents. This is because both ΔS°ET and ΔS°HAT have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between ΔS°HAT and ΔS°ET provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.

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