N-heterocyclic carbenes of late transition metals in catalysis - Anion influence and green chemistry applications

This work focuses on the application of N-heterocyclic carbenes (NHC) with late transition metals, mainly iridium(I), rhodium(I) and palladium(II) complexes, in catalysis. Neutral and cationic iridium(I) complexes of the general formula [Ir(I)CyNHC(COD)]A were synthesized with a range of anions A. Their catalytic activity in transfer hydrogenation relates to their anions' pKa values and can be explained by hard-soft acid-base (HSAB) theory. Neutral and cationic NHC-palladium(II) complexes with a monodendate phthalimido-functionalized NHC ligand [Pd(II)NHC(Cl)(X)]Y were applied in Suzuki-Miyaura cross-coupling reaction under aqueous aerobic condition. Changing the second ligand from strong sigma-donating NHC, to labile phosphane, to labile pyridine and to weakly coordinating acetonitrile, significantly affects the catalytic activity. Hydroxyl-functionalized bis-NHC of rhodium(I) and palladium(II) complexes of the general formula [Rh(I)bis-NHC(COD)]PF6 and [Pd(II)bis-NHC]Br2 were applied as homogenous catalysts in transfer hydrogenation and Suzuki-Miyaura coupling reaction. The bis-NHC palladium(II) complex was further immobilized via the hydroxyl-functionality at the bridge on polymer resin and applied as heterogeneous catalyst in Suzuki-Miyaura coupling reaction.