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

摘要: 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+, 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 complex MIIIL TEMPOH tBu3PhOH. Solution equilibrium measurements for reaction [CoII(H2bim)3]2+ show a large, negative ground-state entropy transfer, −41 ± 2 cal mol−1 K−1. This is even more than ΔS°HAT −30 K−1 two iron complexes + TEMPO, 4.9 1.1 K−1, as reported earlier. Calorimetric quantitatively confirm enthalpy [FeII(H2bip)3]2+ thu...