Influence of ionic liquid composition on surface enrichment of fluorine-free Ru complexes

Tailoring the surface composition of ionic liquids (ILs) is a key strategy for enhancing the performance of supported ionic liquid phase (SILP) catalysts. Here, we investigate the interfacial behavior of fluorine-free Ru polypyridyl complexes functionalized with <i>n</i>-nonyl side chains (Ru-C₉) dissolved in two ILs with contrasting physicochemical properties, such as hydrophobicity and surface tension, namely [C₄C₁Im][PF₆] and [C₄C₁Im][OAc]. Using angle-resolved X-ray photoelectron spectroscopy (ARXPS), we show that Ru-C₉ complexes undergo pronounced surface enrichment in [C₄C₁Im][PF₆], adopting a buoy-like orientation at the IL/vacuum interface. Surface saturation is achieved at a bulk concentration of 0.12%_mol, a regime relevant for efficient SILP catalysis. In contrast, no interfacial enrichment is observed in [C₄C₁Im][OAc], highlighting the critical role of IL composition, particularly anion identity and surface tension, in governing the surface distribution of metal complexes. These findings provide molecular-level insights into the design of IL-based catalytic systems with optimized interfacial properties.