title: Hydrogen atom transfer from iron(II)-tris[2,2'-bi(tetrahydropyrimidine)] to TEMPO: a negative enthalpy of activation predicted by the Marcus equation.
abstract: 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 k1 = 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: DeltaH1 = -2.7 +/- 0.4 kcal mol-1, DeltaS1 = -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: DeltaHcalcd = -3.5 +/- 0.5 kcal mol-1, DeltaScalcd = -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, DeltaH1 degrees = -9.4 +/- 0.6 kcal mol-1, rather than the small negative activation enthalpy for the H-atom self-exchange between the iron complexes.
authors.affiliation: Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700.
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Mader
authors.forename: Anna S
authors.initials: AS
authors.lastname: Larsen
authors.forename: James M
authors.initials: JM
authors.lastname: Mayer
mesh_terms: D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003497:Cyclic N-Oxides / Q000737:chemistry*; D004583:Electrons; D005296:Ferrous Compounds / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D007700:Kinetics; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D011522:Protons; D013816:Thermodynamics
chemical_list: D003497:Cyclic N-Oxides; D005296:Ferrous Compounds; D009942:Organometallic Compounds; D011522:Protons; C491624:tris(2,2'-bi(tetrahydropyrimidine))-Fe(II); D006859:Hydrogen; C003959:TEMPO