Hydrogen Atom Transfer from Iron(II)−Tris[2,2‘-bi(tetrahydropyrimidine)] to TEMPO:  A Negative Enthalpy of Activation Predicted by the Marcus Equation

The transfer of a hydrogen atom from iron(II)−tris[2,2‘-bi(tetrahydropyrimidine)], [FeII(H2bip)3]2+, to the stable nitroxide, TEMPO, was studied by stopped-flow UV−vis spectrophotometry. The products are the deprotonated iron(III) complex [FeIII(H2bip)2(Hbip)]2+ and the hydroxylamine, TEMPO−H. This reaction can also be referred to as proton-coupled electron transfer (PCET). The equilibrium constant for the reaction is close to 1; thus, the reaction can be driven in either direction. The rate constants for the forward and reverse reactions at 298 K are k 1 = 260 ± 30 M-1 s-1 and k - 1 = 150 ± 20 M-1 s-1. Interestingly, the rate constant for the forward reaction decreases as reaction temperature is increased, implying a negative activation enthalpy: ΔH 1 ⧧ = −2.7 ± 0.4 kcal mol-1, ΔS 1 ⧧ = −57 ± 8 cal mol-1 K-1. Marcus theory predicts this unusual temperature dependence on the basis of independently measured self-exchange rate constants and equilibrium constants: ΔH calcd ⧧ = −3.5 ± 0.5 kcal mol-1, ΔS calcd ⧧ = −42 ± 10 cal mol-1 K-1. This result illustrates the value of the Marcus approach for these types of reactions. The dominant contributor to the negative activation enthalpy is the favorable enthalpy of reaction, ΔH 1 ° = −9.4 ± 0.6 kcal mol-1, rather than the small negative activation enthalpy for the H-atom self-exchange between the iron complexes.