Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes.

Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes. Research Abstract Details 

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Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes. Abstract Text:

Elizabeth A Mader,Ernest R Davidson,James M Mayer,

Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(alpha-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, DeltaG degrees = -0.3 +/- 0.2 kcal mol-1, DeltaH degrees = -9.4 +/- 0.6 kcal mol-1, and DeltaS degrees = -30 +/- 2 cal mol-1 K-1. For FeII(H2bim) + TEMPO, DeltaG degrees = 5.0 +/- 0.2 kcal mol-1, DeltaH degrees = -4.1 +/- 0.9 kcal mol-1, and DeltaS degrees = -30 +/- 3 cal mol-1 K-1. The large entropy changes for these reactions, |TDeltaS degrees | = 9 kcal mol-1 at 298 K, are exceptions to the traditional assumption that DeltaS degrees approximately 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 DeltaS degrees 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, |DeltaS degrees 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 left arrow over right arrow [FeII(H2N=CHCH=NH2)2(Hbim)]2+...HONH2. The discovery that DeltaS degrees 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.

Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes. Publishing Authors By Initials

EA Mader,ER Davidson,JM Mayer,

For similar chemical actions and uses: specialty uses of chemicals: solvents research abstracts see: chemical actions and uses: specialty uses of chemicals: solvents research

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Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes. Journal Published:

PUBLICATION TYPE: Research Support, Non-U.S. Gov

Journal: Journal of the American Chemical Society

VOLUME: 129

Page Numbers: 5153-66

Journal Abbreviation:

ISSN: 0002-7863

DAY: 3

MONTH: 04

YEAR: 2007

Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes. Information

Number of References:

LANGUAGE: eng

NlmUniqueID: 7503056

Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes. Keywords Mesh Terms:

KEYWORDS: Solvents

MESH TERMS: chemistry

Chemical & Substance for Abstract: Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes. Information

Substance Name: Hydrogen

Registry Number: 1333-74-0

Grant and Affiliation Information for Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes.

AFFILIATION: University of Washington, Campus Box 351700, Seattle, Washington 98195-1700, USA.

Country: United States

AGENCY: United States NIGMS

GRANT: GM50422

ACRONYM: GM

MEDLINETA: J Am Chem Soc

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