pubs.acs.org/IC Published on Web 06/18/2009 r 2009 American Chemical Society 7466 Inorg. Chem. 2009, 48, 7466–7475 DOI: 10.1021/ic9006247 Thermodynamic and Kinetic Studies on Reactions of Fe III (meso-[tetra- (3-sulfonatomesityl)porphin]) with NO in an Ionic Liquid. Trace Impurities Can Change the Mechanism! Matthias Schmeisser and Rudi van Eldik* Inorganic Chemistry, Department of Chemistry and Pharmacy, University of Erlangen-N :: urnberg, Egerlandstrasse 1, 91058 Erlangen, Germany Received March 31, 2009 To elucidate the applicability and effects of ionic liquids as reaction media for bioinorganic catalysis, detailed kinetic and mechanistic studies on the reversible binding of NO to the monohydroxo ligated iron(III) phorphyrin, (TMPS)Fe III (OH) were performed in the ionic liquid [emim][NTf 2 ] as solvent. We report for the first time the determination of activation volumes via high pressure stopped flow methods in an ionic liquid. The studies clearly show that impurities of methylimidazole, present at the micromolar concentration level, can generate the 6-fold coordinated (TMPS)Fe III (OH)- (MeIm) complex and lead to a complete changeover in mechanism from associatively activated for (TMPS)Fe III (OH) to dissociatively activated for (TMPS)Fe III (OH)(MeIm). NMR measurements on the chemical shift of the β-pyrrole protons revealed a spin state change from high spin (S = 5/2 for (TMPS)Fe III (OH)) to an intermediate spin-state (S=5/2 and 3/2) following the coordination of methylimidazole. Because of the effect of the cationic component of the ionic liquid, Fe III (TMPS) also reacts with nitrite unlike the case in aqueous solution. Kinetic and thermodynamic studies on the reaction of (TMPS)Fe III (OH) with tetrabutylammonium nitrite allowed the determination of the equilibrium constant and thermodynamic parameters for the coordination of nitrite in [emim][NTf 2 ]. Introduction During the past decade ionic liquids have received growing attention as alternative solvents for various applica- tions. Because of their different tunable properties such as density, viscosity, polarity, melting point, and solubility, they enable the use of perfectly adapted reaction media and are even considered to be “designer solvents” of the future. 1-5 Although the use of some ionic liquids has already been well established in new technologies, 6,7 there is obviously still a lack of understanding of how these solvents actually influence or interact with substrates since some groups have reported an increase/decrease in reactivity or even a complete inhibition of reactions. 8,9 Therefore, we developed a general interest to quantify possible mechanistic changes and the reasons there for when a typical reaction of a catalytic active transition metal complex, extensively studied in convent- ional solvents before, is transferred into an ionic liquid. We recently reported on the significant effects of a series of ionic liquids, consisting of different anionic components, on substitution reactions of Pt II complexes. 10 Ionic liquids show a high solubility for several gases, especially for polar gases like CO 2 , as a result of their specific inner structure with empty cavities between the cationic and anionic components. More systematic investigations indicate that gas solubility is on the one hand an effect of the polarity and polarizability of a gas, and on the other hand depends on the nature and steric features of the cationic and anionic components. Thus, tailor-made ionic liquids are also con- sidered as storage media for harmful gases or as a separation agent for gas mixtures. 11-14 *To whom correspondence should be addressed. E-mail: vaneldik@ chemie.uni-erlangen.de. (1) Wasserscheid, P. Chemie in unserer Zeit 2003, 1, 52–63. (2) Wasserscheid, P.; Keim, W. Angew. Chem. 2000, 112, 3926–3945. 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