title: Studies on the Reaction of Iron(II) with NO in a Noncoordinating Ionic Liquid.
abstract: In an earlier study we investigated the reaction of iron(II) chloride with NO in a strongly coordinating ionic liquid 1-ethyl-3-methylimidazolium dicyanamide [emim][dca] and showed that the actual reactive species in solution was [Fe(II)(dca)5Cl](4-). For the present report we investigated in detail how this reaction could proceed in a noncoordinating ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate [emim][OTf]. The donor ability of OTf(-) is much lower than that of dca(-), such that the solubility of FeCl2 in [emim][OTf] strongly depended on other donors like water or chloride ions present or added to the ionic liquid. On increasing the chloride concentration in [emim][OTf], the tetrachloridoferrate complex [emim]2[FeCl4] was formed, as verified by X-ray crystallography. This complex undergoes reversible binding of NO, for which the UV-vis spectral characteristics of the green-brown nitrosyl product resembled that found for the corresponding nitrosyl complexes formed in water and [emim][dca] as solvents. A detailed analysis of the spectra revealed that the {Fe-NO}(7) species has Fe(II)-NO(•) character in contrast to Fe(III)-NO(-) as found for the other solvents. The formation constant, however, is much higher than in [emim][dca], lying closer to the value found for water as solvent. Surprisingly, the Mössbauer spectrum found in [emim][OTf] is very unusual and unsimilar to that found in water and [emim][dca] as solvents, pointing at a different electron density distribution between Fe and NO in {Fe-NO}.7 First, the high isomer shift points to the presence of iron(II) species in solution, thus indicating that upon NO binding no oxidation to iron(III) occurs. Second, the negligible quadrupole splitting suggests a high local symmetry around the iron center. The nitrosyl product is suggested to be [Fe(II)Cl3NO](-), which is supported by electron paramagnetic resonance (EPR) and IR measurements. The nature of the Fe(II) complexes formed in [emim][OTf] depends on the additives required to dissolve FeCl2 in this ionic liquid.
authors.affiliation: †Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Egerlandstr. 1, 91058 Erlangen, Germany.
authors.forename: Svetlana
authors.initials: S
authors.lastname: Begel
authors.affiliation: †Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Egerlandstr. 1, 91058 Erlangen, Germany.
authors.forename: Ralph
authors.initials: R
authors.lastname: Puchta
authors.affiliation: †Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Egerlandstr. 1, 91058 Erlangen, Germany.
authors.forename: Jörg
authors.initials: J
authors.lastname: Sutter
authors.affiliation: †Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Egerlandstr. 1, 91058 Erlangen, Germany.
authors.forename: Frank W
authors.initials: FW
authors.lastname: Heinemann
authors.affiliation: †Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Egerlandstr. 1, 91058 Erlangen, Germany.
authors.forename: Lutz
authors.initials: L
authors.lastname: Dahlenburg
authors.affiliation: †Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Egerlandstr. 1, 91058 Erlangen, Germany.|‡Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland.
authors.forename: Rudi van
authors.initials: Rv
authors.lastname: Eldik