Tuning the electronic environment of cations and anions using ionic liquid mixtures

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Accepted Manuscript
Chemical Science www.rsc.org/chemicalscience Introduction Ionic liquids, low temperature molten salts composed entirely of mobile ions, exhibit a large range of properties that make them useful in a wide variety of fields such as synthesis and catalysis, 1 biocatalysis, 2 electrochemistry, 3 separation technology 4 or materials science. 5 One of the reasons for this wide span of applications is their tunability. Every combination of ions that give rise to an ionic liquid has its own set of properties. The number of combinations can reach the order of trillions if ionic liquids mixtures are considered. 6 In the last years, several applications and fundamental studies of ionic liquid mixtures have appeared. 7 Very promising examples can be found in their application in lithium battery cells, 8 magnesium-based rechargeable batteries, 9 dye-sensitised solar cells, 10 and in the electrodeposition of metals including magnesium. 11 The use of tuneable ionic liquid mixtures as chromatographic stationary phases, 12 and solvents for the controlled selfassembly of amphiphilic materials, 13 or enzymatic mediated reactions 14 also highlights the potential of mixtures to improve the performance of existing ionic liquid-based systems.
The potential to fine tune the properties of ionic liquid media by the use of mixtures is clear and the ability to understand and predict these properties would constitute an invaluable tool. However, for applications of ionic liquid mixtures to become widespread, a deep understanding of their properties and the molecular level structure that determines them is needed. The physical and chemical properties of these systems have begun to be consistently measured and simulated. The properties of several ionic liquid mixtures have been found to follow an ideal or quasi-ideal mixing behaviour. 15 Ionic liquid mixtures have also been found to exhibit enhanced properties when compared to the neat ionic liquids used in their preparation. 15c,16 Structurally, ionic liquid mixtures seem to consist of mixtures of randomly distributed ions. 7,17 More in detail, their molecular level structure might be dominated by one of the ionic liquids. 7,17d,18 The electronic environment of the ions constituting ionic liquids plays a dominant role in determining the molecular level structure, macroscopic properties and the interaction of ionic liquids with other molecules. For instance, accurate reproduction of molecular charge distributions using point charges is critical in classical molecular simulations. In ionic liquids this charge distribution plays an essential role. There is on-going debate as to which force fields most adequately describe ionic liquids. 19 Models that allow for polarisation and charge transfer in ionic liquids have recently gained recognition. Traditionally, the total charge on individual ions has been assumed to be an integer. 20 However, reduced charges have been used to account for partial charge transfer between the anion and the cation. 21 Polarisable force fields, which can artificially reproduce charge transfer effects, have also been used but at much greater computational expense. 19c,22 Experimentally obtained charge distributions would be an invaluable piece of information in parameterising accurate electrostatic interactions for ionic liquid systems.
The distribution of electron density also has a significant impact in the way molecules interact. Ionic liquid solute interactions can have an enormous impact on several applications such as catalysis or electrochemistry that depend on the electronic distribution of specific species in solution. For example, the electronic environment of the metal centre of catalysts is a major factor controlling its catalytic activity. The nature of the ionic liquid anion has been proven to have a determining influence in the activity and stability of several cationic catalytic systems. 23 In terms of electrochemical applications, ionic liquid mixtures have also been used to tune the redox potentials of different compounds in solution. 24 These examples evidence that the ability to measure and control the electronic environment of ions within ionic liquids, and therefore programming its properties and potential interactions, will be of utmost importance for the ionic liquid field. X-ray photoelectron spectroscopy (XPS) can be used to monitor the electronic environment of the composition. These changes can be used to tune the properties and interactions of a particular ionic liquid system. To demonstrate the power of this tool, we use an ionic liquid mixture to fine tune the electronic environment of a catalytic metal centre in an ionic liquid solution and achieve distinct turnover frequencies. Clanions, the cations within the mixture will have more capacity to accept electron density from Clanions. Consequently, the Clanions will be able to transfer more electronic charge to these cations.

Tuning the Electronic Environment of Ions
This larger donation of charge will leave the Clanion with less electron density and therefore, their electrons will exhibit higher binding energies than the Clanion electrons in neat [C 8 C 1 Im]Cl. In low basicity anions and they do not participate in significant charge transfer. 28a Moreover, as the negative charge associated with [Tf 2 N]is delocalised across many atoms, any change in electron density will be distributed around many atoms and therefore, will be too small to be measured directly by XPS, i.e. it will be within the error of the measurement.
In Figure 1c and Figure 1f  This data on binding energies also offers invaluable information to computational simulations of ionic liquids. It is especially relevant in order to evaluate point charges in computational force field models. XPS data shows that charge transfer from anion to cation occurs for ionic liquid mixtures as well as for neat ionic liquids and it is characteristic for every combination of ions. Therefore, when carrying out simulations of ionic liquids and ionic liquid mixtures, reduce charge or polarisable force fields will more accurately describe the real charge distribution within ionic liquids. In addition, the distinct binding energies and narrow peaks (with similar FWHM as for neat ionic liquids) obtained for ionic liquid mixtures suggest that the electronic environment of the ions within ionic liquid mixtures is distinct and unique. This cannot be the consequence of a mixture of different local distribution of ions but the result of the sum of long range anion-cation interactions and charge transfer events which results in this distinct and unique electronic environment. Consequently, simulations based on small clusters (maybe <10 ions) will not be able to capture the real electronic environment of ionic liquids as it is the combination of several solvent shells that appears to influence the electron density on the ions. Moreover, binding energies could be used as a useful solvation parameter. For neat ionic liquids, the binding energy of N cation 1s has been found to linearly correlate with the hydrogen bond donor ability of the anion, β, calculated by solvatochromic methods. 28a,b For mixtures, β has not been measured as determining β for mixtures is more challenging than for pure samples as one of the ions normally preferentially interacts with the probing dye giving a wrong measure of the average hydrogen donor ability of the mixture. 29 XPS binding energies could also be used as an alternative to β in both neat ionic liquids and ionic liquid mixtures.

b) N 1s and e) Cl 2p3/2 for [C 8 C 1 Im]Cl x [TfO] 1-x and c) N 1s and f) Cl 2p3/2 for
Reference data recorded for the neat ionic liquids is also included for comparison. In this case the intensity of the photoemission peaks have been normalised to the intensity of the N cation 1s peak, so the natural change in peak areas with composition can be also visualised. All XP spectra are charge corrected by setting C alkyl 1s = 285.0 eV and offset vertically for clarity.

Tuning the Electronic Environment of a metal catalyst in solution:
The electronic environment of the ions that constitute ionic liquids also has a determining influence in the interaction of these ions with solutes. 30 Changing the interaction of ions with starting materials, reactive intermediates or catalysts can have a determining effect in the outcome of chemical reactions. 31 Ionic liquids can act as ligands of charged catalysts. 23b-d Some cationic catalyst have been found to be stable, unstable, active or even change coordination depending on the constituent anion of the ionic liquid media. 23b-d For example, it has been shown that a mixture of Pd(PPh 3 ) 4 /NaCl/[C 8 C 1 Im][A] and aryl bromide renders an stable phosphine-imidazolydidene palladium complex that successfully catalyses the Suzuki cross-coupling reaction of the aryl bromide with phenylboronic acid over different cycles. 23b,c The formation of the final catalytic complex and its activity have been found to depend on the nature of the anion. The complex is not formed when PF 6 and SbF 6 anions are used and different turn over frequencies (TOF) are obtained for different ionic liquids in which the active complex is formed. It has been suggested that anions of high basicity interact more strongly with the catalyst and change the electronic environment of the catalyst and hence its activity and TOF.
The electronic environment of the metal centre can also be probed directly using XPS. 32 Figure 3 shows that there is a lineal correlation between the Pd 3d 5/2 binding energy and the TOF for these systems. These measurements exemplify how the combination of two different ionic liquids can be used to fine tune the anion-catalyst interactions and therefore, the electronic environment of the metal centre of a catalyst, in order to modify the TOF of the reaction. XPS is also proven as an ideal technique to measure these changes in electronic environment in ionic liquid media.

Conclusions
Changes in the composition of ionic liquid mixtures can be used to tune the electronic environment