Efficient Air‐Stable Organometallic Low‐Molecular‐Mass Gelators for Ionic Liquids: Synthesis, Aggregation and Application of Pyridine‐Bridged Bis(benzimidazolylidene)–Palladium Complexes

Abstract

Beyond catalysis: Novel pincer-type pyridine-bridged bis(benzimidazolylidene)–palladium complexes have been synthesised and shown to efficiently gelate a variety of solvents and customary ionic liquids. π-Stacking, van der Waals interactions, hydrogen and metal–metal bonding are responsible for 3D gel networks formed by the metallogelators. Novel pincer-type, pyridine-bridged bis(benzimidazolylidene)–palladium complexes 5–7 were synthesised from cheap commercial precursors under microwave assistance. Although simple in structure, carbene complexes 5 a,b are efficient low-molecular-mass metallogelators. They gelate not only a broad variety of protic and aprotic organic solvents, but also different types of customary ionic liquids (such as imidazolium, pyridinium, pyrazolidinium, piperidinium and ammonium salts) at concentrations as low as 0.5 mg mL⁻¹. The morphologies of the resulting 3D gel networks composed from long and thin fibres were studied by TEM and light microscopy for a selection of organic and ionic liquids. The achiral gelators are able to induce the formation of helical fibres. The thermal stability of the gel samples increases with the gelator concentration as demonstrated by thermoreversible DSC studies. Temperature-dependent NMR and X-ray diffraction studies, as well as comparisons with pincer complex analogues bearing shorter alkyl chains, suggest that the 3D networks responsible for gelation are based on non-covalent interactions, such as π-stacking, van der Waals interactions, and hydrogen and metal–metal bonding. Ionic liquids and gels obtained from them and 5 a,b display comparable high conductivities, which characterises pyridine-bridged bis(benzimidazolylidene)–palladium pincer complexes as air-stable metallo gelators that efficiently immobilise ionic liquids in low gelator concentration indicating—beyond catalysis—their potential applications in electrochemical devices.