Co-ligand effects in the catalytic activity of Pd(II)–NHC complexes

Three cis-chelating di-N-heterocyclic carbene palladium(II) complexes [PdX2(diNHC)] (X = I, 1; X = SCN, 2; X = CF3CO2, 3) bearing different anionic co-ligands were synthesized and fully characterized. A comparison of their catalytic activities in the Mizoroki–Heck reaction and conjugate addition of arylboronic acids to cyclic enones revealed increasing efficiency in the order SCN− < I− < CF3CO2−. The di(trifluoroacetato) complex 3 showed the best activity in both transformations highlighting the importance of co-ligands effects in catalysis. In addition, the molecular structure of an unusual poly-heteronuclear complex salt 4 is reported, which has been isolated as a byproduct in the synthesis of complex 3.