Mixed dicarboxylato - Bis(carbene) complexes of palladium(II): Synthesis, structures, trans - Cis isomerism, and catalytic activity

Abstract

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, CF 3, 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. © 2006 American Chemical Society.