Unlocking the Fluorine-Free Buoy Effect: Surface-Enriched Ruthenium Polypyridine Complexes in Ionic Liquids.

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.

Exploring the interfacial behavior of ruthenium complexes in ionic liquids: implications for supported ionic liquid phase catalysts.

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.

Understanding the Buoy Effect of Surface-Enriched Pt Complexes in Ionic Liquids: A Combined ARXPS and Pendant Drop Study.

Recently, we demonstrated that Pt catalyst complexes dissolved in the ionic liquid (IL) [C4 C1 Im][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 previous 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 ultraclean vacuum conditions. This combination allows for connecting the microscopic information on the IL/gas interface derived 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 IL-based catalyst systems with large contact areas to the surrounding reactant/product phase, such as in supported IL phase (SILP) catalysis.

Formation and Surface Behavior of Pt and Pd Complexes with Ligand Systems Derived from Nitrile-functionalized Ionic Liquids Studied by XPS.

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.

The Buoy Effect: Surface Enrichment of a Pt Complex in IL Solution by Ligand Design.

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 Buoy Effect: Surface Enrichment of a Pt Complex in IL Solution by Ligand Design.

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.

Formation and Surface Behavior of Pt and Pd Complexes with Ligand Systems Derived from Nitrile-functionalized Ionic Liquids Studied by XPS.

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.

Molecular Stacking on Graphene.

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.