Nonchelate and chelate complexes of palladium(II) with N-heterocyclic carbene ligands of amido functionality

The imidazolium ligand precursors [(LHH2)-H-1-H-1]Cl and [(LH2H2)-H-2-H-1]Cl (H-1 = NHCO, H-2 = NCHN) for the potentially bidentate and pincer-type amido-NHC ligands were synthesized in 66-78% yields. Selective deprotonation of H-2 in these salts with pyridine in the presence of palladium chloride resulted in the monodentate palladium(II) complexes Pd((LH1)-H-1)(py)Cl-2 and Pd((LH21)-H-2)(py)Cl-2. 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 PdL21 and PdL2(py), respectively. Intriguingly, in certain cases, both the cis and the trans isomers of PdL21 were formed and isolated in pure forms. A theoretical study indicates that the trans-PdL21 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((LH1)-H-1)(py)Cl-2 > PdL2(py) > PdL21.