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 M~(II)LH and oxyl radicals. [Fe~(II)(H_2bip)_3]~(2+), [Fe~(II)(H_2bim)_3]~(2+), [Co~(II)(H_2bim)_3]~(2+), and Ru_(II)(acac)_2(py-imH) [H_2bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine, H_2bim = 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 ~tBu_3PhO· (2,4,6-tri-tert-butylphenoxyl) to give the deprotonated, oxidized metal complex M_(III)L and TEMPOH or ~(t)Bu_3PhOH. Solution equilibrium measurements for the reaction of [Co~(II)(H_2bim)_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 Ru~(II)(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 [Fe~(II)(H_2bip)_3]~(2+) + TEMPO, thus also confirming ΔS°_(HAT)- Calorimetry on TEMPOH + ~tBu_3PhO' 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 ~tBu_3PhOH 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 IΔS°_(HAT)I for reactions with TEMPO, Ru~(II)(acac)_2(py-imH)<[Fe_(II)(H_2bip)_3]~(2+) = [Fe~(II)(H_2bim)_3]~(2+) < [Co~(II)(H_2bim)_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.