Sulfur-containing alkenes-A new class of chelating ligands: Synthesis, coordination to palladium, and structure of the resulting complexes

Stable 1,2-disulfanylalkene palladium complexes [(RS-CH=CR'-SR)PdCl2] were synthesized in 85-94% yield by reaction of palladium(II) chloride with sulfur-containing ligands RS-CH=C(R')-SR (analogs of dithiolate ligands). The structure of the complexes was studied by NMR spectroscopy and quantum-chemical methods. The binding energy in palladium complexes with bis(arylsulfanyl)- and bis(alkylsulfanyl)alkenes was estimated (DFT) at 50 and 56 kcal/mol, respectively. Variation of substituents on the sulfur atoms is a convenient tool for fine tuning of the ligand properties and controlling the strength of the complex. The bite angle of the ligands does not depend on the substituent nature and is 88-89 degrees, which is typical of square-planar complexes. According to the bite angle, the examined ligands are analogs of well known bidentate phosphine ligands, but the former are more labile since the corresponding binding energy is lower by 36 kcal/mol.