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

Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic
iron tris(α-diimine) complexes. FeII(H2bip) (iron(II) tris[2,2‘-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and
FeII(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,
FeIII(Hbip) or FeIII(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both
systems using both static and kinetic measurements. For FeII(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 FeII(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 [FeII(H2NCHCHNH2)2(H2bim)]2+···ONH2 ⇆ [FeII(H2NCHCHNH2)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.