title: Reactions of Superoxide with Iron Porphyrins in the Bulk and the Near-Surface Region of Ionic Liquids.
abstract: The redox reaction of superoxide (KO2) with highly charged iron porphyrins (Fe(P4+), Fe(P8+), and Fe(P8-)) has been investigated in the ionic liquids (IL) [EMIM][Tf2N] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and [EMIM][B(CN)4] (1-ethyl-3-methylimidazolium tetracyanoborate) by using time-resolved UV/vis stopped-flow, electrochemistry, cryospray mass spectrometry, EPR, and XPS measurements. Stable KO2 solutions in [EMIM][Tf2N] can be prepared up to a 15 mM concentration and are characterized by a signal in EPR spectrum at g = 2.0039 and by the 1215 cm(-1) stretching vibration in the resonance Raman spectrum. While the negatively charged iron porphyrin Fe(P8-) does not react with superoxide in IL, Fe(P4+) and Fe(P8+) do react in a two-step process (first a reduction of the Fe(III) to the Fe(II) form, followed by the binding of superoxide to Fe(II)). In the reaction with KO2, Fe(P4+) and Fe(P8+) show similar rate constants (e.g., in the case of Fe(P4+): k1 = 18.6 ± 0.5 M(-1) s(-1) for the first reaction step, and k2 = 2.8 ± 0.1 M(-1) s(-1) for the second reaction step). Notably, these rate constants are four to five orders of magnitude lower in [EMIM][Tf2N] than in conventional solvents such as DMSO. The influence of the ionic liquid is also apparent during electrochemical experiments, where the redox potentials for the corresponding Fe(III)/Fe(II) couples are much more negative in [EMIM][Tf2N] than in DMSO. This modified redox and kinetic behavior of the positively charged iron porphyrins results from their interactions with the anions of the ionic liquid, while the nucleophilicity of the superoxide is reduced by its interactions with the cations of the ionic liquid. A negligible vapor pressure of [EMIM][B(CN)4] and a sufficient enrichment of Fe(P8+) in a close proximity to the surface enabled XPS measurements as a case study for monitoring direct changes in the electronic structure of the metal centers during redox processes in solution and at liquid/solid interfaces.
authors.affiliation: †Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
authors.forename: Anne
authors.initials: A
authors.lastname: Dees
authors.affiliation: †Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
authors.forename: Norbert
authors.initials: N
authors.lastname: Jux
authors.affiliation: †Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
authors.forename: Oliver
authors.initials: O
authors.lastname: Tröppner
authors.affiliation: †Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Dürr
authors.affiliation: †Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
authors.forename: Rainer
authors.initials: R
authors.lastname: Lippert
authors.affiliation: ‡Fachbereich Chemie, Philipps-Universität Marburg, Marburg, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Schmid
authors.affiliation: §Institute for Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, 97074 Würzburg, Germany.
authors.forename: Bernd
authors.initials: B
authors.lastname: Küstner
authors.affiliation: §Institute for Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, 97074 Würzburg, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Schlücker
authors.affiliation: †Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
authors.forename: Hans-Peter
authors.initials: HP
authors.lastname: Steinrück
authors.affiliation: †Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.|‡Fachbereich Chemie, Philipps-Universität Marburg, Marburg, Germany.
authors.forename: J Michael
authors.initials: JM
authors.lastname: Gottfried
authors.affiliation: †Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
authors.forename: Ivana
authors.initials: I
authors.lastname: Ivanović-Burmazović
mesh_terms: D007093:Imidazoles / Q000737:chemistry*; D007094:Imides / Q000737:chemistry*; D052578:Ionic Liquids / Q000737:chemistry*; D058085:Iron Compounds / Q000737:chemistry*; D008665:Metalloporphyrins / Q000737:chemistry*; D009570:Nitriles / Q000737:chemistry*; D010084:Oxidation-Reduction; D013481:Superoxides / Q000737:chemistry*
chemical_list: C000597051:1-butyl-3-methylimidazolium tetracyanoborate; C506652:1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; D007093:Imidazoles; D007094:Imides; D052578:Ionic Liquids; D058085:Iron Compounds; D008665:Metalloporphyrins; D009570:Nitriles; D013481:Superoxides