New palladium(II) complexes containing 3-mercapto-1,2,4-triazole ligands: synthesis, characterization, crystal structure, and density functional theory calculations

Abstract

Two new palladium(II) complexes of 4-amino-3-methyl-1,2,4-triazol-5-thione (H3L) have been synthesized and characterized. The reaction of H3L, palladium(II) chloride, and sodium acetate in a molar ratio 2:1:2 gave the homoleptic binuclear palladium(II) complex [Pd2(H2L)4]·2DMSO (1), while its treatment with palladium(II) chloride, sodium acetate, and triphenylphosphane in a molar ratio 4:4:7:4 gave the tetranuclear palladium(II) complex [Pd4(H2L)2(HL)(L)(PPh3)4]Cl·7MeOH·3H2O (2) with terminal phosphane ligands. All complexes have been characterized by a combination of IR, 1H and 31P NMR spectroscopy, ESI(+) mass spectrometry, and elemental analysis as well as X-ray diffraction studies. On the basis of determined molecular structures, each triazole moiety in the binuclear complex is singly deprotonated, whereas the centrosymmetric tetranuclear complex contains three different 3-mercapto-1,2,4-triazolate moieties, namely singly, twofold, and threefold deprotonated in its solid state. In the binuclear complex, the triazolate ligands act as N,S-bidentate bridging ones and coordinate with the metal centers in a trans-(2,2) fashion. In the tetranuclear complex, the four triazolate ligands act as bridging agents between four metal centers through their thiol sulfur atoms and their endocyclic nitrogen atoms in a head-to-head fashion. In addition, each deprotonated NH2 group of the ligand acts as bridging agent between two adjacent palladium metals. According to density functional theory calculations, the structural constraints imposed by the bridging deprotonated NH2 groups are responsible for the observed close Pd–Pd distance in this complex. No indication for metallophilic interactions was found.