Nonchelate and Chelate Complexes of Palladium(II) with N-Heterocyclic Carbene Ligands of Amido Functionality

The imidazolium ligand precursors [L1H1H2]Cl and [L2H12H2]Cl (H1 = NHCO, H2 = NCHN) for the potentially bidentate and pincer-type amido-NHC ligands were synthesized in 66−78% yields. Selective deprotonation of H2 in these salts with pyridine in the presence of palladium chloride resulted in the monodentate palladium(II) complexes Pd(L1H1)(py)Cl2 and Pd(L2H12)(py)Cl2. The use of K2CO3 in pyridine or DMF led to the double and triple deprotonations of the ligand precursors, giving the bis-bidentate and pincer-type palladium(II) complexes PdL12 and PdL2(py), respectively. Intriguingly, in certain cases, both the cis and the trans isomers of PdL12 were formed and isolated in pure forms. A theoretical study indicates that the trans-PdL12 is thermodynamically more stable than the cis isomer (ca. 5.8 kcal mol-1). All the new complexes are characterized by NMR (1D and 2D) and single-crystal X-ray diffraction studies. A systematic study of the new complexes in Suzuki coupling reactions revealed the following order of activities: Pd(L1H1)(py)Cl2 > PdL2(py) > PdL12.