Thermodynamic and Kinetic Studies on Reactions of FeIII(meso-[tetra(3-sulfonatomesityl)porphin]) with NO in an Ionic Liquid. Trace Impurities Can Change the Mechanism!

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][NTf2] 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)(Melm) 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)(Melm). NMR measurements on the chemical shift of the beta-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][NTf2].