FULL PAPER DOI:10.1002/ejic.201402005 Star-Shaped Quaternary Alkylammonium Polyhedral Oligomeric Silsesquioxane Ionic Liquids Sundar Manickam, [a] Paola Cardiano, [a] Placido G. Mineo, [b] and Sandra Lo Schiavo* [a] Keywords: Ionic liquids / Polyammonium salts / Ion exchange / Cage compounds / Silicon / Silsesquioxanes New star-shaped quaternary ammonium (QA) polyhedral oligomeric silsesquioxane (POSS) salts based on octakis- (tetraalkylammonium) POSS cations bearing alkyl chains of different length (methyl, n-butyl, and n-octyl) and differently polarizable anions [iodide, bis(trifluoromethylsulfonyl)imide, and ibuprofen] have been synthesized and characterized. Differential scanning calorimetry (DSC) and thermogravime- tric analysis (TGA) investigations show that the nature of the cation plays a key role in determining the thermal behavior of QA POSS salts featuring the longest alkyl chains, whereas Introduction Polyhedral oligomeric silsesquioxanes (POSSs) represent a class of intriguing materials, which, thanks to their nano- meter size, shape, and versatile reactivity, have been largely exploited as building blocks at the molecular scale for nano- technology applications and the development of high-per- forming hybrid nanomaterials. POSS units are cube-shaped species with diameters between 1 and 3 nm, characterized by an inorganic inner siloxane core (SiO 1.5 ) x surrounded by organic substituents, which may be inert or confer the POSS nanoparticles with specific chemical reactivity and/or physicochemical properties. [1–4] These features make POSSs versatile nanofillers, which, unlike conventional ones, offer advantages in terms of their monodisperse size, uniform distribution, low density, and synthetically controlled func- tionality. Many literature reports show that the incorpora- tion of POSS units into polymeric matrices provides dra- matic improvements in thermal stability, oxidation resis- tance, surface hardening, mechanical, electric, and optical properties as well as flammability reduction. This explains why POSS-based materials have been the subject of innov- ative research in the last decade, which has resulted in appli- cations in aerospace, protective coatings, microelectronics, [a] Dipartimento di Scienze Chimiche, University of Messina, Viale F. Stagno D’Alcontres 31, S. Agata, 98166 Messina, Italy E-mail: sloschiavo@unime.it www.unime.it [b] Dipartimento di Scienze Chimiche, University of Catania, Viale A. Doria 6, 95125 Catania, Italy Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201402005. Eur. J. Inorg. Chem. 2014, 2704–2710 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2704 the influence exerted by anions becomes relevant for those bearing short and medium alkyl chains. All the QA POSS salts display amorphous behavior and, with the exception of octakis(trimethylpropylammonium)- and octakis(tri-n-butyl- propylammonium)octasilsesquioxane iodide, may be classi- fied as ionic liquids. Dynamic contact-angle studies reveal their hydrophilic attitude owing to the preferential orienta- tion of the tetraalkylammonium groups at the film–water interface. catalysis, organic light-emitting diodes (OLEDs), and zeo- lite-like materials. POSS nanotechnology has also been ex- ploited to provide access to a large variety of biocompatible hybrid materials (polymeric or not) for biomedical applica- tions and used for the development of cardiovascular im- plants, tissue engineering products, coatings for quantum dot nanocrystals, and so on. [2] Further, the POSS hydro- phobic Si–O “spherical” core, when combined with suitable functionalized substituents, leads to the formation of am- phiphilic materials, which have been utilized for drug/gene delivery and the formation of hydrogels. [2,3] The current interest in materials based on ionic liquid (ILs) has led to the employment of the POSS silica core as a scaffold for the construction of hybrid nanostructured ILs. In recent years, ILs, that is, salts that melt below 100 °C, have emerged as a new class of functional materials with unique and synthetically tailored properties, such as low vapor pressure, wide liquid range, excellent chemical and thermal stability, good electrochemical stability, high ionic conductivity, dispersant capabilities, tunable dielectric constants, and biocompatibility; as a result, they have found applications in many chemical and industrial fields. [5–8] Their immobilization on suitable substrates such as poly- meric networks and nanostructured scaffolds (nano- tubes, [9,10] graphene, [11] etc.) resulted in significant improve- ments in their processing capabilities, stability, and struc- ture control, which allowed some drawbacks arising from their intrinsic liquid nature to be overcome. [12–14] Tanaka et al. have demonstratedthat the star-shaped oc- tacarboxy–POSS-based IL featuring imidazolium groups as countercations, namely, [POSS(COO – ) 8 ][B-mim + ] 8 (B-mim