The Arrangement of Two N-Heterocyclic Carbene Moieties in Palladium Pincer Complexes Affects Their Catalytic Activity towards Suzuki-Miyaura Cross-Coupling Reactions in Water

Palladium complexes with pincer ligands containing one pyridine and two N-heterocyclic carbene units with acetyl-protected D-glucopyranosyl groups in C-C-N and C-N-C arrangements were synthesized. The complexes form diastereomers due to the twisted pincer ligands and chiral D-glucopyranosyl units. The diastereomers of the C-C-N complex are in equilibrium in solution, whereas only one of the diastereomers of the C-N-C complex forms kinetically. Deprotection of the acetyl groups in the ligands afforded water-soluble complexes with one of the hydroxide groups in the D-glucopyranosyl groups coordinated to the metal ion. In CD3OD, the deprotected complex with the C-N-C ligand gradually decomposed, whereas that with the C-C-N ligand was stable at room temperature. The complexes catalyzed Suzuki-Miyaura cross-coupling reactions in water with turnover numbers of 75,000 for the C-C-N and 8,900 for C-N-C complexes. The C-C-N complex was not deactivated by the addition of metallic Hg meaning that the active species is the complex itself or its derivatives having the pincer ligand. On the other hand, the C-N-C complex exhibited no catalytic activity for the coupling reaction in the presence of metallic Hg, meaning that the active species are heterogeneous catalysts, such as Pd nanoparticles.