Experimental and theoretical studies on the mechanistic behaviour of catalytic-relevant metal complexes in ionic liquids
Final Report Abstract
The general goal of this project was to contribute to an improved understanding of the mechanistic behaviour of catalytic-relevant metal complexes in ionic liquids. It was the objective of the proposal to perform experimental and theoretical studies on the fundamental influence of ionic liquids on the mechanistic behaviour of transition metal complexes in reactions that involve fast ligand substitution, activation of small molecules such as NO and CO2, and electron transfer processes. As part of the project, typical ligand substitution reactions of Pt(II) and Pd(II) of which the underlying reaction mechanisms were well understood for conventional solvents, were reinvestigated in different ionic liquids (ILs). The results in terms of rate constants and activation parameters were found to be very similar to those obtained for ethanol as solvent. containing anions that are strong nucleophiles can slow down the substitution rate as a result of the direct coordination of such anions to the labile metal centre. In the case of complex formation reactions of Li+ with bipyridine and terpyridine chelates, it could be shown that most of the anionic components of ILs studied can coordinate directly to the metal centre in the crystalline state. These studies were the first to demonstrate that the anionic component of an IL, and thereby the IL itself, can be non-innocent and enter the coordination sphere of metal ions and complexes. Subsequent studies involved the activation of NO and CO2 by transition metal ions and complexes in different ILs. In order to dissolve a metal salt such as FeCl2, an IL is required that contains a strong anionic nucleophile in order to stabilize the metal ion in solution. In addition, trace impurities of the starting material present in the synthesized IL can have a major influence on the kinetic parameters and underlying reaction mechanism especially when dealing with porphyrin complexes in the micro-molar concentration range. The employed ILs show a significantly higher solubility of NO and CO2 than conventional solvents which can be ascribed to empty pockets formed between the anionic and cationic components of the IL. This can have meaningful advantages in the application of ILs in the activation of small gaseous molecules like NO, CO2, O2, H2, CH4, etc. Finally, a few heterogeneous electron transfer reactions were studied in ILs by way of comparison with earlier studies in conventional solvents. ILs have the advantage that a much larger potential range can be used in such studies. Our work mainly involved the study of proteins and enzymes attached to a self-assembled-monolayer (SAM) on the surface of gold electrodes in an IL medium. This approach enabled a systematic study of electron transfer as a function of the distance between the electrode surface and the redox centre of the biological molecule by tuning the length of the SAM. The employed ILs added stability to the SAM and enabled a cycling of the reaction parameters including temperature and pressure over a wide range. All in all, the project has been very successful and revealed important properties of ILs that should be kept in mind when dealing with the chemistry of transition and main group metal ions and complexes dissolved in ILs.
Publications
- Coordination of 1,10-phenanthroline and 2,2’-bipyridine to Li+ in different ionic liquids. How innocent are ionic liquids? Inorg. Chem., 50, 6685-6695 (2011)
M. Schmeisser, F.W. Heinemann, P. Illner, R. Puchta, A. Zahl and R. van Eldik
- The classic “brown-ring” reaction in a new medium: Kinetics, mechanism, and spectroscopy of the reversible binding of nitric oxide to iron(II) in an ionic liquid, Inorg. Chem., 50, 3946-3958 (2011)
S. Begel, F.W. Heinemann, G. Stopa, G. Stochel and R. van Eldik
- Thermodynamic and kinetic studies on the interaction of RuIII(edta) with NO in an ionic liquid, Dalton Trans., 40, 4892-4897 (2011)
S. Begel and R. van Eldik
- Gutmann donor and acceptor numbers for ionic liquids, Chem. Eur. J., 18, 10969-10982 (2012)
M. Schmeisser, P. Illner, R. Puchta, A. Zahl and R. van Eldik
(See online at https://doi.org/10.1002/chem.201200584) - Mechanistic insight from activation parameters for the reaction of a ruthenium hydride complex with CO2 in conventional solvents and an ionic liquid, Inorg. Chem., 51, 7340-7345 (2012)
S. Kern and R. van Eldik
(See online at https://doi.org/10.1021/ic300718v) - Coordination of terpyridine to Li+ in two different ionic liquids, Inorg. Chem., 52, 13167-13178 (2013)
K. Pokorny, M. Schmeisser, F. Hampel, A. Zahl, R. Puchta and R. van Eldik
(See online at https://doi.org/10.1021/ic4020724) - Electron transfer with azurin at Au/SAM junctions in contact with a protic ionic melt: Impact of glassy dynamics, Phys. Chem. Chem. Phys., 15, 16515-16526 (2013
D.E. Khoshtariya, T.D. Dolidze, T. Tretyakova, D.H. Waldeck and R. van Eldik
(See online at https://doi.org/10.1039/c3cp51896e) - Elucidation of inorganic reaction mechanisms in ionic liquids: The important role of solvent donor and acceptor properties, Perspective article in Dalton Trans., 43, 15675-15692 (2014)
M. Schmeisser and R. van Eldik,
(See online at https://doi.org/10.1039/c4dt01239a)
