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 AgO 2 CR, where R = CH 3, CF 3, and CF 2 CF 3 . 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.