Reactions of Superoxide with Iron Porphyrins in the Bulk and the Near-Surface Region of Ionic Liquids

• Published in: Inorganic chemistry 2015-07, Vol.54 (14), p.6862-6872
• Main Authors: Dees, Anne, Jux, Norbert, Tröppner, Oliver, Dürr, Katharina, Lippert, Rainer, Schmid, Martin, Küstner, Bernd, Schlücker, Sebastian, Steinrück, Hans-Peter, Gottfried, J. Michael, Ivanović-Burmazović, Ivana
• Format: Article
• Language: English
• Subjects: Imidazoles - chemistry; Imides - chemistry; Ionic Liquids - chemistry; Iron Compounds - chemistry; Metalloporphyrins - chemistry; Nitriles - chemistry; Oxidation-Reduction; Superoxides - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.5b00770

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+): k 1 = 18.6 ± 0.5 M–1 s–1 for the first reaction step, and k 2 = 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.