When talking about homogeneous catalyst systems, it has long been assumed that the system at hand consists of a transition metal complex in solution with the liquid interface representing the composition of the bulk solution. Now, in light of considerable developments in the study of metal complexes dissolved in ionic liquids with their negligible vapor pressures, more detailed studies of the composition at the liquid/gas interface became possible. These investigations revealed pronounced surface enrichment and segregation effects of high relevance for practical applications. This article reviews recent advancements in tailoring the interfacial composition of ionic liquid-based catalytic systems. A particular focus is dedicated to surface enrichment phenomena, and a variety of parameters are presented for deliberate control of the local concentration of the complexes at the surface, that is, the nature of the ligands, the bulk concentration, the temperature, and the nature of the IL solvent. As experimental methods, angle-resolved X-ray photoelectron spectroscopy (ARXPS) and vacuum-based pendant-drop surface tension measurements were applied. The reviewed results are intended to provide the basis for the advancement of catalytic systems with high surface areas, such as in supported ionic liquid phase (SILP) catalysis, where the interface design is directly interconnected with catalytic performance.

AbstractIn der homogenen Katalyse geht man üblicherweise davon aus, dass sich die Konzentrationen der Übergangsmetallkomplexe an der Gas‐Flüssig‐Grenzfläche und im Volumen der Lösung nicht unterscheiden. Tatsächlich zeigen aber umfangreiche Untersuchungen an gelösten Metallkomplexen in ionischen Flüssigkeiten (Englisch: Ionic Liquids, ILs) ausgeprägte Oberflächenanreicherungs‐ und Segregationseffekte, die auch für praktische Anwendungen von großer Bedeutung sein können. Diese Untersuchungen basieren auf Methoden der Oberflächenforschung, die aufgrund der vernachlässigbaren IL‐Dampfdrücke unter wohldefinierten Vakuumbedingungen anwendbar sind. Dieser Übersichtsartikel bietet einen Überblick über die jüngsten Fortschritte bei der Steuerung der Grenzflächenzusammensetzung von katalytischen Systemen auf der Basis ionischer Flüssigkeiten. Ein besonderer Schwerpunkt liegt auf der gezielten Oberflächenanreicherung. Insbesondere werden eine Vielzahl von Parametern vorgestellt, welche die lokale Komplexkonzentration an der Oberfläche beeinflussen wie etwa die Wahl der Liganden, die Konzentration im Volumen, die Temperatur und die Natur des IL‐Lösungsmittels. Als experimentelle Methoden wurden vor allem winkelaufgelöste Röntgenphotoelektronenspektroskopie (Englisch: Angle‐Resolved X‐Ray‐Photoelectron Spectroscopy, ARXPS) und Oberflächenspannungsmessungen mit der Methode des hängenden Tropfens (Englisch: Pendant Drop Method) unter ultrareinen Vakuumbedingungen eingesetzt. Die hier zusammengefassten Ergebnisse bilden die Grundlage für die Weiterentwicklung von maßgeschneiderten katalytischen Anwendungen mit großer innerer Oberfläche, z.B. in der Katalyse mit geträgerten ionischen Flüssigphasen (Englisch: Supported Ionic Liquid Phase, SILP); bei solchen Systemen hat das Design der Grenzfläche einen erheblichen Anteil an der katalytischen Effizienz.

We investigated the surface tension and surface composition of binary ionic liquid (IL) mixtures of ILs sharing the same cation. As model system, binary mixtures of 1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc], molar volume: 154.4 cm3·mol-1 at 293 K) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2C1Im][Tf2N], molar volume: 256.2 cm3·mol-1 at 293 K), with very different surface tensions, and with anions of very different chemical structure and size were studied over the whole composition range. The surface tension was obtained by pendant-drop (PD) measurements in the presence of 0.1 MPa argon between 294 and 323 K, and the surface composition was determined by angle-resolved photoelectron spectroscopy (ARXPS) in ultrahigh vacuum at 293 K. The ARXPS results reveal a strong preferential enrichment of [C2C1Im][Tf2N] at the vacuum-liquid interface of the binary mixtures, which is more pronounced at lower [C2C1Im][Tf2N] bulk contents. This microscopic behavior is reflected in the macroscopic surface tensions, which are significantly lower than calculated assuming a linear mixing behavior based on the bulk composition. A previously developed prediction model to correlate the surface tension with the molar surface composition yields deviations of more than 5% from the measured values, which we attribute to the strongly different sizes of the anions. By accounting for the surface areas occupied by the ILs, we present an improved new model which describes the experimental data very well within 1.4%.

The interaction of metal complexes with ionic liquids, with a particular focus on the stability and surface concentration of the metal centers, is crucial in applications involving catalysts based on supported ionic liquids. In this study, we synthesized the complexes [Ru(tpy)(bpy)Cl][PF6] and [Ru(tpy)(dcb)Cl][PF6] (tpy = 2,2',2''-terpyridine, bpy = 2,2'-bipyridine, dcb = 4,4'-dicarboxy-2,2'-bipyridine) and we prepared solutions using the ionic liquids (ILs) 1-ethyl-3-methylimidazolium acetate [C2C1Im][OAc] and 1-butyl-3-methylimidazolium hexafluorophosphate [C4C1Im][PF6]. The chemical environment of the Ru(II) metal center and the interfacial behavior of the complexes in the different IL solutions were determined using angle-resolved X-ray photoelectron spectroscopy (ARXPS). In [C4C1Im][PF6], [Ru(tpy)(bpy)Cl][PF6] maintains its chemical structure, while in [C2C1Im][OAc], partial changes in the chemical environment of the Ru center are indicated by XPS, likely due to ligand exchange. The presence of carboxylic acid functional groups in the bipyridyl ligand seems to inhibit this ligand exchange. The investigated complexes do not exhibit surface activity but are depleted from the IL/gas interface. These findings hold significance for the design of new supported ionic liquid phase catalysts based on Ru complexes.

The use of homogeneous catalysts dissolved in ionic liquids (ILs) is an established field of research. Thin IL films containing dissolved catalyst complexes can be immobilized on solid porous supports, thereby creating a heterogenized catalyst material. Aiming at the deliberate positioning of such supported ionic liquid phase (SILP) catalyst, we carried out investigations of two very similar Pt-complexes: depending on the ligand periphery, the first one is homogeneously dissolved in the IL while the second one strongly enriches at the gas/IL interface. To study these different locations within thick IL films of approximately 1 mm thickness, we investigated the hydrogenation of ethene in a continuous pool-reactor setup. The two complexes dissolved in the IL [C4C1Im][PF6] showed different activity which can be attributed to their different locations. At 313 K and 0.62 MPa total pressure, the surface-enriched complex was approximately two times more active. However, under these conditions the formation of Pt particles could be observed, with the surface-enriched complex exhibiting a stronger tendency for particle formation compared to the one homogeneously distributed in the IL, as derived from XPS and light-scattering measurements. Tune it up or down! The positioning of platinum complexes can be deliberately adjusted in thick films of ionic liquid based on appropriate ligand design. Catalytic activity in ethene hydrogenation resembles the different positions due to easier access of surface enriched complexes. image

The so-called buoy-effect, that is, the targeted surface enrichment of a Pt catalyst dissolved in ionic liquids (ILs), is achieved by attaching perfluorinated alkyl chains to the ligand system, which drags the metal complex toward the interface. Using angle-resolved X-ray photoelectron spectroscopy, it is demonstrated how this surface enrichment can be tailored by variation of the solvent IL. In [C(n)C(1)Im][PF6] ILs (n = 2, 4, 8), the surface is fully saturated with the complex at 10%(mol) bulk content, while in [C(4)C(1)Im][Tf2N] only at 20%(mol) saturation is observed. At low catalyst concentrations of 1%(mol), where saturation is not yet reached, the enrichment increases with decreasing length of the IL alkyl chain. As a general rule, the degree of surface enrichment decreases with the decrease in surface tension of the solvent IL, that is, in the order [C(2)C(1)Im][PF6] > [C(4)C(1)Im][PF6] > [C(8)C(1)Im][PF6] > [C(4)C(1)Im][Tf2N]. In ILs with very low surface tension, enrichment is even suppressed. These results reveal the surface tension of the solvent IL as rational parameter for tailoring the interfacial structure of IL-based catalyst systems, such as supported ionic liquid phase (SILP) catalysis, where the nature of the IL/gas interface is expected to strongly influence the performance of the process.

The cover feature shows a ruthenium(II) polypyridine complex dissolved in 1-ethyl-3-methylimidazolium acetate under X-ray irradiation. The long hydrophobic alkyl chains of the complex act as buoys, pulling it towards the liquid/vacuum interface, as proven by ARXPS measurements. More details can be found in the Research Article by Hans-Peter Steinrück, Federico J. Williams, and co-workers (DOI: 10.1002/open.202400092).

Controlling the local concentration of metal complexes at the surface of ionic liquids (ILs) is a highly sought-after objective due to its pivotal implications in supported ionic liquid phase (SILP) catalysis. Equally important is to avoid per- and polyfluorinated substances due to environmental concerns. Herein, we investigate the surface enrichment of Ru polypyridyl complexes with fluorine-free alkylic side groups of varying lengths and shapes, using the hydrophilic IL [C2C1Im][OAc] as solvent. Additional charged carboxylate groups are included into the polypyridyl ligands to increase the solubility of the complex in the IL. When the ligand system is functionalized with long and hydrophobic alkyl side chains, the complex predominantly localizes at the IL/vacuum interface, as deduced from angle-resolved X-ray photoelectron spectroscopy. Conversely, in the presence of short or more bulky substituents, no surface enrichment is observed. This buoy-like behaviour with fluorine-free side groups is explored for 0.05 %mol to 1 %mol solutions. Intriguingly, surface saturation occurs at approximately 0.5 %mol, which is beneficial to the efficient operation of catalytic systems featuring high surface areas, such as SILP catalysts.

We present the preparation and investigation of a fluorine-free surface-active bis-N-heterocyclic carbene (NHC) platinum(II) complex - trans-[Pt(mPEG3C8Im)2Cl2] - for interface-enhanced supported ionic liquid phase (SILP) catalysis within a group of (mPEGn)-substituted ionic liquids (ILs) ([(mPEGn)2Im][A] ILs). The complex was characterized by means of single-crystal X-ray diffraction (scXRD) analysis and multinuclear (1H, 13C, 195Pt) NMR spectroscopy, indicating the presence of two almost equimolar syn-anti-rotamers of the square-planar complex in solution. Angle-resolved X-ray photoelectron spectroscopy (ARXPS) revealed pronounced interface-accumulation of trans-[Pt(mPEG3C8Im)2Cl2] in IL solutions of [(mPEG2)2Im][A] (A-=I- and PF6 -).

The targeted enrichment of a Pt complex with an ionic liquid (IL)-derived ligand system in IL solution is demonstrated by using angle-resolved X-ray photoelectron spectroscopy. When the ligand system is complemented with fluorinated side chains, the complex accumulates strongly at the IL/gas interface, while in an equivalent solution of a complex without these substituents no such effect could be observed. This buoy-like behavior induces strong population of the complex at the outermost molecular layer close to surface saturation, which was studied over a range from 5 to 30 %(mol). The surface enrichment was found to be most efficient at the lowest concentration, which is particularly favorable for catalytic applications such as supported ionic-liquid-phase (SILP) catalysis.

Invited for the cover of this issue are the groups of Hans-Peter Steinrück and Peter Wasserscheid at the Friedrich-Alexander-Universität Erlangen-Nürnberg. The image depicts two Pt catalysts dissolved in an ionic liquid. For one of them, fluorinated side chains in the ligand system act as buoys leading to pronounced enrichment of the complex at the gas/IL interface, as is evidenced by strongly enhanced Pt signals in angle-resolved photoelectron spectroscopy. For the complex without fluorinated side chains, no such effect is observed. Read the full text of the article at 10.1002/chem.202203325.

The front cover artwork is provided by the groups of Prof. Hans-Peter Steinrück and Prof. Peter Wasserscheid at the Friedrich-Alexander-Universität (FAU) Erlangen-Nürnberg. The image shows substitution of volatile acetonitrile ligands by a nitrile-functionalized imidazolium cation. The formed cationic complex and the counter ions exhibit a specific preferential orientation at the ionic liquid/gas interface, which can be studied by angle-resolved XPS. Read the full text of the Research Article at 10.1002/cphc.202200391.

We present an ARXPS study on the surface composition and interfacial behavior of commercial [Rh(COD)2][TfO] in [C2C1Im][TfO], [C4C1Im][TfO], [C8C1Im][TfO], and [C2C1Im][EtOSO3]. The complex was found to be non-intact in a solution of these ILs through the loss of COD ligands, accompanied by the depletion of the metal center from the IL/vacuum interface. Increasing the chain length of the aliphatic substituent on the imidazolium cation of the [TfO]−-based ILs led to a more pronounced depletion from the interface, due to the higher surface affinity of the solvent cations with the longer alkyl chains. The loss of COD ligands offered facile in situ ligand substitution with surface-active TPPTS to afford a moderate increase in the surface concentration of Rh. We propose the formation of a Schrock−Osborn-type catalyst [Rh(COD)(TPPTS)2][TfO]. Information on the surface composition and targeted design of the gas/IL interface is highly relevant for applications in IL-based catalytic systems, such as in supported ionic liquid phase (SILP) catalysis.

Recently, we demonstrated that Pt catalyst complexes dissolved in the ionic liquid (IL) [C4C1Im][PF6] can be deliberately enriched at the IL surface by introducing perfluorinated substituents, which act like buoys dragging the metal complex towards the surface. Herein, we extend our angle-resolved X-ray photoelectron spectroscopy (ARXPS) studies at complex concentrations between 30 and 5%mol down to 1%mol and present complementary surface tension pendant drop (PD) measurements under ultra clean vacuum conditions. This combination allows for connec-ting the microscopic information on the IL/gas interface from ARXPS with the macroscopic property surface tension. The surface enrichment of the Pt complexes is found to be most pronounced at 1%mol. It also displays a strong temperature dependence, which was not observed for 5%mol and above, where the surface is already saturated with the complex. The surface enrichment deduced from ARXPS is also reflected by the pronounced decrease in surface tension with increasing concentration of the catalyst. We furthermore observe by ARXPS and PD a much stronger surface affinity of the buoy-complex as compared to the free ligands in solution. Our results are highly interesting for an optimum design of ionic catalyst solutions contact areas with a surrounding reactant/product phase, such as in SILP catalysis.

Abstract The sequential vertical polyfunctionalization of 2D addend‐patterned graphene is still elusive. Here, we report a practical realization of this goal via a “molecular building blocks” approach, which is based on a combination of a lithography‐assisted reductive functionalization approach and a post‐functionalization step to sequentially and controllably link the molecular building blocks ethylpyridine, cis‐dichlorobis(2,2′‐bipyridyl)ruthenium, and triphenylphosphine (4‐methylbenzenethiol, respectively) on selected lattice regions of a graphene matrix. The assembled 2D hetero‐architectures are unambiguously characterized by various spectroscopic and microscopic measurements, revealing the stepwise stacking of the molecular building blocks on the graphene surface. Our method overcomes the current limitation of a one‐layer‐only binding to the graphene surface and opens the door for a vertical growth in the z‐direction.

We studied the formation and surface behavior of Pt(II) and Pd(II) complexes with ligand systems derived from two nitrile-functionalized ionic liquids (ILs) in solution using angle-resolved X-ray photoelectron spectroscopy (ARXPS). These ligand systems enabled a high solubility of the metal complexes in IL solution. The complexes were prepared by simple ligand substitution under vacuum conditions in defined excess of the coordinating ILs, [C3 CNC1 Im][Tf2 N] and [C1 CNC1 Pip][Tf2 N], to immediately yield solutions of the final products. The ILs differ in the cationic head group and the chain length of the functionalized substituent. Our XPS measurements on the neat ILs gave insights in the electronic properties of the coordinating substituents revealing differences in donation capability and stability of the complexes. Investigations on the composition of the outermost surface layers using ARXPS revealed no surface affinity of the nitrile-functionalized chains in the neat ILs. Solutions of the formed complexes in the nitrile ILs showed homogeneous distribution of the solute at the surface with the heterocyclic moieties preferentially orientated towards the vacuum, while the metal centers are rather located further away from the IL/vacuum interface.

In comparison to the clean conditions of ultrahigh vacuum (UHV), we investigate the influence of ambient conditions on the potential screening (PS) at the interfaces of Au and Pt electrodes with the ionic liquids [C8C1Im][Tf2N] and [C8C1Im]Cl. Our study is based on a proof-of-principle experiment that shows that PS measurements performed by XPS yield the same results as measurements using a specific 3-electrode sample holder setup, both in UHV. Based on this, we compare PS measurements under ultraclean conditions by XPS with PS measurements with a 3-electrode setup in N2 and in air. We demonstrate that there is indeed an influence of the ambient gas atmosphere, which depends on applied voltage and combination of IL and electrode material. For Pt electrodes, there is a pronounced influence of the ambient conditions on the PS for both [C8C1Im][Tf2N] and [C8C1Im]Cl, while for Au electrodes hardly any influence is seen. We attribute the observed effects mainly to water affecting the electrical double layers at the IL/electrode interfaces. In addition, we observe that reaction products formed under faradaic conditions can lead to a contamination of the 3rd electrode (reference electrode), yielding time-dependent deviations from the true behavior.