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

Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata-700 032, India

E-mail : samaresh_b@hotmail.com

Manuscript received online 24 July 2013, accepted 30 September 2013

From reaction of Na₂ [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.