The effect of several imidazolium-based ionic liquids on the mechanism of a classical ligand substitution reaction of [Pt(terpyridine)Cl] (+) with thiourea was investigated. A detailed kinetic study as a function of the nucleophile concentration and temperature was undertaken under pseudo-first-order conditions using stopped-flow techniques. Polarity measurements were performed for the employed ionic liquids on the basis of solvatochromic effects, and they show similarities with conventional polar solvents. Density-functional theory calculations (RB3LYP/LANL2DZp) were employed to predict the ion-pair stabilization energy between the ionic components of the ionic liquids and/or between the anions of the ionic liquids and the cationic Pt (II) complex. These data illustrate how the anions of the ionic liquids can affect the investigated substitution reaction. In general, the substitution mechanism in ionic liquids was found to have an associative character similar to that in conventional solvents. The observed deviations reflect the influence of the ionic liquid on the interaction between the anionic component of the liquid and the positively charged complex.

To elucidate the applicability and properties of ionic liquids (ILs) to serve as chemical reaction media for the activation of small molecules by transition-metal complexes, detailed kinetic and mechanistic studies were performed on the reversible binding of NO to FeCl(2) dissolved in the IL 1-ethyl-3-methylimidazolium dicyanamide ([emim][dca]) as a solvent. We report, for the first time, the application of laser flash photolysis at ambient and high pressure to study the kinetics of this reaction in an IL. The kinetic data and activation parameters for the "on" and "off" reactions suggest that both processes follow a limiting dissociative (D) ligand substitution mechanism, in contrast to that reported for the same reaction in aqueous solution, where this well-known "brown-ring" reaction follows an interchange dissociative (I(d)) ligand substitution mechanism. The observed difference apparently arises from the participation of the IL anion as a N-donor ligand, as evidenced by the formation of polymeric [Fe(dca)(3)Cl](x)[emim](2x) chains in the solid state and verified by X-ray crystallography. In addition, infrared (IR), Mössbauer, and EPR spectra were recorded for the monomeric reaction product [Fe(dca)(5)NO](3-) formed in the IL, and the parameters closely resemble those of the {FeNO}(7) unit in other well-characterized nitrosyl complexes. It is concluded that its electronic structure is best described by the presence of a high-spin Fe(III) (S = 5/2) center antiferromagnetically coupled to NO(-) (S = 1), yielding the observed spin quartet ground state (S(t) = 3/2).

The effect of several imidazolium-based ionic liquids on the rate and mechanism of the substitution reaction of [Pt(terpyridine)Cl](+) with thiocyanate was investigated as a function of thiocyanate concentration and temperature under pseudo-first-order conditions using stopped-flow and other kinetic techniques. The obtained rate constants and activation parameters showed good agreement with the ion-pair stabilization energies between the anions of the ionic liquids and the cationic Pt(II) complex derived from density-functional theory calculations (RB3LYP/LANL2DZp) and with parameters derived from the linear solvation energy relationship set by the Kamlet-Taft beta parameter, which is a measure of a solvent's hydrogen bonding acceptor ability. In general, the substitution reactions followed an associative mechanism as found for conventional solvents, but the observed rate constants showed a significant dependence on the nature of the anionic component of the ionic liquid used as solvent. The second order rate constant measured in [emim][NTf(2)] is 2000 times higher than the one measured in [emim][EtOSO(3)]. This difference is much larger than observed for a neutral entering nucleophile studied before.

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.

The aim of this study was to determine rate constants for the release of NO from an iron(II) nitrosyl complex dissolved in a typical imidazolium-based ionic liquid, [emim][dca], using an NO trapping technique combined with stopped-flow measurements. The anionic component of the selected ionic liquid, viz. [dca] (dicyanamide), was found to coordinate to the Ru(III)(edta) complex used as trapping agent, and hindered its successful application. Further investigations in aqueous solution revealed thermodynamic and kinetic data for the formation of the [Ru(III)(edta)(dca)](2-) complex and its reaction with NO to form the corresponding nitrosyl complex. The results obtained under conditions similar to those in the ionic liquid indicated a much slower formation of the nitrosyl complex in [emim][dca] compared to that reported for the formation of this complex in aqueous medium.