N-Heterocyclic carbeneligands bearing hydrophilic and/or hydrophobic chains: Rh(i) and Pd(ii) complexes and their catalytic activity

Abstract

A series of novel imidazolium salts bearing hydrophilic tetraethylene glycol (TEG) and/or hydrophobic long-chain alkyl (n-C₁₂) functionalities, which are precursors for desired N-heterocyclic carbene (NHC) ligands, were synthesized and characterized. Rh(I)–NHC complexes were prepared in good yields by the silver carbene transfer method with NHC–Ag species derived from the imidazolium salts. The molecular structure of the Rh(I)–NHC complex having n-C₁₂ chains has been determined by a single-crystal X-ray diffraction study and the complex is found to possess extended alkyl chains with anti conformations in the solid state. Hydrosilylation with the Rh(I) complexes and Suzuki–Miyaura coupling reactions with the Pd(II) complexes with these NHC ligands were carried out. In the latter case, the TEG moiety enhances catalytic activity considerably.