Immobilization of N-Heterocyclic Carbenes: Functionalization of Mono- and Bis-(NHC) Compounds

N-heterocyclic carbenes (NHCs), rivalling phosphines as viable spectator ligands in transition metal catalysis, rank among the most useful tools in organic and organometallic chemistry. In this context, the development of reusable immobilized NHC catalysts is one area of promise. This dissertation mainly focuses on the functionalization of NHC compounds towards the preparation of novel immobilized homogeneous catalysts, both with solid and "liquid" support (water-soluble catalysts).

The first part focuses on a new immobilization mode for methylene-bridged bis(NHC) ligand systems. The hydroxymethyl-functionalized moiety provides a versatile platform to immobilize the prominent bis(NHC) motif while preserving the possibility to introduce different wingtip substituents. Exemplarily, four hydroxymethyl-functionalized imidazolium salts and five bis(NHC) metal complexes (Pd, Pt and Ir) were synthesized. With the obtained ligands and complexes, polymer-supported and water-soluble catalysts have been successfully prepared,

respectively, and both were applied in Suzuki-Miyaura cross-coupling reactions. As an extension of the first part, the following work comprises the synthesis and characterization of three new hydroxymethyl-bridged, dinuclear bis(NHC) Ag(I), Cu(I) and Au(I) complexes. The differences and structural properties of all compounds are

compared to their previously reported non-functionalized analogues by various spectroscopic methods and X-ray diffraction studies. Further, DFT calculations were conducted to gain better understanding of this ligand system and the resulting structural arrangement of its coinage metal complexes.

The last part of this work describes the preparation of water-soluble catalysts, which is another way to achieve easy separation and reuse of the catalyst. Four sulfonated water-soluble PEPPSI-Pd-NHC complexes were prepared in a straightforward two-step synthesis. Their activities have been examined in Suzuki-Miyaura cross-coupling reactions in water under air. Detailed investigations of the most effective Pd-NHC complex include the catalytic activity on a broad variety of substrates and reusability.