Large Ground-State Entropy Changes for Hydrogen Atom Transfer Reactions of Iron Complexes

Abstract

Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(α-diimine) complexes. Fe^II(H₂bip) (iron(II) tris[2,2‘-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and Fe^II(H₂bim) (iron(II) tris[2,2‘-bi-2-imidazoline]diperchlorate) both transfer H^• to TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) to yield the hydroxylamine, TEMPO-H, and the respective deprotonated iron(III) species, Fe^III(Hbip) or Fe^III(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both systems using both static and kinetic measurements. For Fe^II(H₂bip) + TEMPO, ΔG° = −0.3 ± 0.2 kcal mol^-1, ΔH° = −9.4 ± 0.6 kcal mol^-1, and ΔS° = −30 ± 2 cal mol^-1 K^-1. For Fe^II(H₂bim) + TEMPO, ΔG° = 5.0 ± 0.2 kcal mol^-1, ΔH° = −4.1 ± 0.9 kcal mol^-1, and ΔS° = −30 ± 3 cal mol^-1 K^-1. The large entropy changes for these reactions, |TΔS°| = 9 kcal mol^-1 at 298 K, are exceptions to the traditional assumption that ΔS° ≈ 0 for simple HAT reactions. Various studies indicate that hydrogen bonding, solvent effects, ion pairing, and iron spin equilibria do not make major contributions to the observed ΔS°_HAT. Instead, this effect arises primarily from changes in vibrational entropy upon oxidation of the iron center. Measurement of the electron-transfer half-reaction entropy, |ΔS°_Fe(H2bim)/ET| = 29 ± 3 cal mol^-1 K^-1, is consistent with a vibrational origin. This conclusion is supported by UHF/6-31G* calculations on the simplified reaction [Fe^II(H₂NCHCHNH₂)₂(H₂bim)]²⁺···ONH₂ ⇆ [Fe^II(H₂NCHCHNH₂)₂(Hbim)]²⁺···HONH₂. The discovery that ΔS°_HAT can deviate significantly from zero has important implications on the study of HAT and proton-coupled electron-transfer (PCET) reactions. For instance, these results indicate that free energies, rather than enthalpies, should be used to estimate the driving force for HAT when transition-metal centers are involved.