Mixed-ligand benzaldehyde thiosemicarbazone complexes of palladium containing N,O-donor ancillary ligands. Syntheses, structures, and catalytic application in C–C and C–N coupling reactions

Abstract

The reaction of a 1 : 1 mixture of 4-R-benzaldehyde thiosemicarbazone [denoted in general as HL–R; where H stands for the dissociable acidic proton and R (R = OCH₃, CH₃, H, Cl and NO₂) for the substituent] and 1-nitroso-2-naphthol (abbreviated as Hnn), with an equivalent quantity of Na₂[PdCl₄] in ethanolic medium affords a group of mixed-ligand complexes of the type [Pd(L–R)(nn)]. A similar reaction of a mixture of HL–R and quinolin-8-ol (Hq) with Na₂[PdCl₄] affords another family of mixed-ligand complexes of the type [Pd(L–R)(q)]. Crystal structures of [Pd(L–Cl)(nn)], [Pd(L–CH₃)(q)] and [Pd(L–Cl)(q)] have been determined. In all the complexes the thiosemicarbazones are coordinated to the metal center, via dissociation of the acidic proton, as monoanionic bidentate N,S-donors forming five-membered chelate rings. In the [Pd(L–R)(nn)] complexes, the 1-nitroso-2-naphtholate anion is coordinated as a N,O-donor forming a five-membered chelate ring. Similarly in the [Pd(L–R)(q)] complexes, the quinolin-8-olate anion is bound to the metal center in the N,O-mode forming a five-membered chelate ring. All the [Pd(L–R)(nn)] and [Pd(L–R)(q)] complexes show characteristic ¹H NMR signals, and in dichloromethane solution they all display intense absorptions in the visible and ultraviolet regions. Catalytic activities of the [Pd(L–R)(nn)] and [Pd(L–R)(q)] complexes have been examined towards some C–C and C–N coupling reactions, where both were found to show notable catalytic efficiency.