Mixed Dicarboxylato−Bis(carbene) Complexes of Palladium(II): Synthesis, Structures, Trans−Cis Isomerism, and Catalytic Activity

Mixed dicarboxylato−bis(carbene) complexes of palladium(II) have been prepared by reacting cis-diiodo-bis(N,N‘-dimethylbenzimidazolin-2-ylidene)palladium(II) (cis-A) with AgO2CR, where R = CH3,
CF3, and CF2CF3. In this manner, cis-diacetato-bis(N,N‘-dimethylbenzimidazolin-2-ylidene)palladium(II)
(1), cis-di(trifluoroacetato)-bis(N,N‘-dimethylbenzimidazolin-2-ylidene)palladium(II) (2), and cis-di(pentafluoropropionato)-bis(N,N‘-dimethylbenzimidazolin-2-ylidene)palladium(II) (3) were obtained. Complexes
1−3 were fully characterized by multinuclear NMR spectroscopies as well as ESI mass spectrometry.
X-ray crystal structure analyses of the first mixed carboxylato/benzimidazolin-2-ylidene complexes reveal
mononuclear species with a square-planar palladium(II) center coordinated by two monodentate carbene
and two monodentate carboxylato ligands in a cis arrangement. The cis configuration was found to be
thermodynamically favored in this type of complexes. The results also show that the introduction of
N-heterocyclic carbene ligands stabilizes the palladium−carboxylate moiety effectively, thus preventing
both reductive decomposition and autoionization processes. A preliminary catalytic study revealed that
complexes 1−3 are highly active in the Mizoroki−Heck coupling of aryl bromides and activated aryl
chlorides.