title: Task-Specific Ionic Liquids and Ionic Liquid Crystals and Their Transition Metal Complexes for Interface-Enhanced Catalysis
Creators: name: Seidl, Vera
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: Because of their attractive and swiftly tunable properties, ILs provide a broad range of application in catalytic reactions. The usage of ILs as thin films, e.g., in SILP particles, drastically reduces the amount of required IL, which leads to lower costs and facilitates the handling of the catalyst. Moreover, for catalysis in IL thin films, reduced mass transfer limitations result in an enhanced activity compared to a biphasic IL system. However, and despite the aforementioned advantageous progress, catalysis of reagents with poor solubility in ILs is still challenging. Within interface-enhanced catalysis, the positioning of the catalyst at the IL interface – either at the liquid/gas or liquid/solid interface – is suggested to minimize kinetic limitations, in particular for poorly soluble reagents. Moreover, the anisotropic chemical environment at both interfaces can modify the catalyst's reactivity compared to the same catalyst in an isotropic IL environment. Thus, the development of interface-enhanced SILP catalysis is a new and promising approach for increased efficiency in catalysis, combining the advantages of both concepts. This thesis aims in one part for the development of suitable ILs, which can be used as thin films in the interface-enhanced SILP catalysis. Therefore, and based on the experience of previous ILs by Meyer and co-workers, imidazolium-based ILs with highly polar methylated polyethylene glycol (mPEG) substituents are developed. These bis -mPEGn -ILs shall provide improved solubility of reactants for catalysis due to possible interactions with small molecules or (in)organic cations. For instance, the ability to conduct protons is supposed to facilitate photo- or electrochemical proton reduction at the liquid/solid interface. In addition, the good dissolution behavior of mPEG-based ILs should ensure a good solubility of the proposed catalyst. For the optimal usage in catalysis, these ILs are characterized with regard to various properties including the thermo- and electrochemical as well as surface behavior. The selectivity in the target catalytic reactions can be controlled by temperature, if the applied ILs provide liquid crystalline behavior. Regardless, such ILC-SILP materials are only scarcely described in literature, since many expenses need to be met by both the catalyst and the ILC, such as a mesophase in a suitable temperature range, but also the type of mesophase can play a crucial role. Based on previous results by Meyer and co-workers on structurally similar 1,3-didodecyl-imidazolium and imidazolinium salts, the impact of a C(2) -substitution at the cationic core of these salts on the formation of a mesophase is investigated. For this purpose, the 1,3-didodecylimidazolinium salt is exemplarily functionalized with two different alkyl moieties at the C(2)-position, while the residual molecular structure remains identical. For catalysis at or in close contact with the liquid/solid interface, new N-polyaryl and N '-mPEGn - substituted imidazolium salts are developed. Similar to the bis -mPEGn -IL, also the mPEG moiety in the asymmetric IL should facilitate the mass transport to the catalytically active center and guarantee the solubility in the "bulk" IL. On the other hand, the polyarene functionality can act as a molecular wire for electron transport from the solid support to the catalytically active center and, therefore, also allows for photo- or electrochemical catalytic application. For an improved interaction with the solid support, an appropriate linker functionality attached to the IL's phenyl moiety will be required and developed. Moreover, these TSILs are used as precursors for transition metal NHC complexes, providing potentially enhanced reactivity for photo- and electrochemical catalysis. With the aim of finding a suitable catalyst for various catalytic reactions, especially for proton reduction, different transition metals are used for the synthesis of such bis-NHC complexes, in order to subsequently investigate their magnetic and electrochemical properties. To further improve the possibilities of chemical and structural analyses of surface-immobilized ILs and their corresponding transition metal complexes, a structural motif is developed. In this homogeneous model system, an asymmetrically substituted imidazolium salt or its metal complex is coordinated to a molecular titanium precursor via a linker moiety. This provides (A) a molecular example for ease of characterization and (B) a mimic of a potentially photo- or electroactive surface (e.g., TiO2).