Mechanistic Studies on Fast Ligand Substitution Reactions of Pt(II) in Different Ionic Liquids: Role of Solvent Polarity and Ion-Pair Formation

Inorganic reaction mechanisms. A personal journey

This review covers highlights of the work performed in the van Eldik group on inorganic reaction mechanisms over the past two decades in the form of a personal journey. Topics that are covered include, from NO to HNO chemistry, peroxide activation in model porphyrin and enzymatic systems, the wonder-world of Ru^III(edta) chemistry, redox chemistry of Ru(iii) complexes, Ru(ii) polypyridyl complexes and their application, relevant physicochemical properties and reaction mechanisms in ionic liquids, and mechanistic insight from computational chemistry. In each of these sections, typical examples of mechanistic studies are presented in reference to related work reported in the literature.

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

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+): k1 = 18.6 ± 0.5 M(-1) s(-1) for the first reaction step, and k2 = 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.

Elucidation of inorganic reaction mechanisms in ionic liquids: the important role of solvent donor and acceptor properties

In this article, we focus on the important role of solvent donor and acceptor properties of ionic liquids in the elucidation of inorganic reaction mechanisms. For this purpose, mechanistic and structural studies on typical inorganic reactions, performed in ionic liquids, have been conducted. The presented systems range from simple complex-formation and ligand-substitution reactions to the activation of small molecules by catalytically active complexes. The data obtained for the reactions in ionic liquids are compared with those for the same reactions carried out in conventional solvents, and are discussed with respect to the donor and acceptor properties of the applied ionic liquids. The intention of this perspective is to gain more insight into the role of ILs as solvents and their interaction with metal ions and complexes in solution.

Coordination of terpyridine to Li+ in two different ionic liquids

On the basis of (7)Li NMR experiments, the complex-formation reaction between Li(+) and the tridentate N-donor ligand terpyridine was studied in the ionic liquids [emim][NTf2] and [emim][ClO4] as solvents. For both ionic liquids, the NMR data implicate the formation of [Li(terpy)2](+). Density functional theory calculations show that partial coordination of terpyridine involving the coordination of a solvent anion can be excluded. In contrast to the studies in solution, X-ray diffraction measurements led to completely different results. In the case of [emim][NTf2], the polymeric lithium species [Li(terpy)(NTf2)]n was found to control the stacking of this complex, whereas crystals grown from [emim][ClO4] exhibit the discrete dimeric species [Li(terpy)(ClO4)]2. However, both structures indicate that each lithium ion is formally coordinated by one terpy molecule and one solvent anion in the solid state, suggesting that charge neutralization and π stacking mainly control the crystallization process.

Understanding chemical reaction mechanisms in ionic liquids: successes and challenges

The focus of this article is an examination of chemical reaction mechanisms in ionic liquids. These mechanisms are compared with those pertaining to the same reactions carried out in conventional solvents. In cases where the mechanisms differ, attempts to provide an explanation in terms of the chemical and physicochemical properties of the reactants and of the components of the ionic liquids are described. A wide range of reactions from different branches of chemistry has been selected for this purpose. A sufficient background for student readers has been included. This tutorial review should also be of interest to kineticists, and to both new and experienced investigators in the ionic liquids field.