Toward tunable immobilized molecular catalysts: Functionalizing the methylene bridge of bis(N-heterocyclic carbene) ligands

A new immobilization mode for methylene-bridged bis(NHC) (NHC=N-heterocyclic carbene) ligand systems is presented, allowing fine-tuning of the steric and electronic properties of the bidentate ligand. For example, four hydroxymethyl-functionalized imidazolium salts (1 a-c and 2 a) and three bis(NHC) Pd complexes 3 a, 3 b, and 4 a are described. The chloro-functionalized bis(NHC) Pd complex 4 a was obtained quantitatively by conversion of the hydroxyl substituent of complex 3 a into a chloro substituent by employing thionyl chloride. All three bis(NHC) complexes 3 a, 3 b, and 4 a were characterized by NMR spectroscopy, elemental analysis, mass spectrometry, and single-crystal X-ray diffraction. Two different synthetic routes were applied to immobilize the bis(NHC) Pd complex 3 a on polystyrene. The obtained heterogeneous catalyst 5 b was utilized for Suzuki-Miyaura cross-coupling reactions and could be recycled without significant activity loss in four runs. Furthermore, the water-soluble homogeneous catalyst 3 a itself could be employed for Suzuki-Miyaura cross-coupling reactions in water.