Mixed-ligand benzaldehyde thiosemicarbazone complexes of palladium containing triphenylphosphine as ancillary ligand : Synthesis, structure and catalytic application in C-N coupling reactions

From reaction of Na-2[PdCl4] with 4-R-benzaldehyde thiosemicarbazone (HL-R, R = OCH3, CH3, H, Cl and NO2) and 2-picolinic acid in 1 : 1 : 1 mole ratio in refluxing ethanol a group of complexes of type [{Pd(NS-R)Cl}(n)] is formed as minor product, along with another group of complexes of type [{Pd(CNS-R)}(n)] as the major product (where NS-R and CNS-R denote the NS- and CNS-coordinated thiosemicarbazone respectively). Reaction of triphenylphosphine with the oligomeric [{Pd(NS-R)Cl}(n)] species affords mononuclear complexes of type [Pd(NS-R)(PPh3)Cl] via splitting of the sulfur-bridge. Crystal structures of [Pd(NS-CH3)(PPh3)Cl] and [Pd(NS-NO2)(PPh3)Cl] complexes have been determined. In these [Pd(NS-R)(PPh3)Cl] complexes, the thiosemicarbazones are coordinated to the metal center, via dissociation of the acidic proton, as monoanionic bidentate NS-donors forming five-membered chelate rings and, the triphenylphosphine is trans to the imine-nitrogen and the chloride is trans to the coordinated sulfur. The [Pd(NS-R)(PPh3)Cl] complexes have shown notable efficiency in catalyzing Buchwald type C-N coupling reactions.