Effect of ion coordination on the conformational and electronic structure of 3,4-Bis(alkylthio)thiophenes

A joint experimental and theoretical approach is used to study the coordination of soft transition metal ions with 3,4-bis(alkylthio)thiophenes. The synthesis of [PdCl₂{3,4-bis(3-methylbutylthio)thiophene}], [PtCl₂{3,4-bis(3-methylbutylthio)thiophene}] and [PdCl₂{3′,4′-bis[2-(S)-methylbutylthio]- 2,2′:5′,2″-terthiophene}] by exchange of the benzonitrile ligands in [PdCl₂(PhCN)₂] and [PtCl₂(PhCN)₂] for the thiophene derivatives with thioether side chains is described. Changes in the conformational and electronic structure of the molecules were studied by X-ray diffraction, cyclic voltammetry, and optical and NMR spectroscopy; these experimental data are interpreted with the help of quantum mechanical calculations based on density functional theory. The complexes crystallize in a mixture of diastereoisomers, these stereoisomers are in equilibrium in solution, as observed by variable-temperature NMR spectroscopy. The complexation diminishes the electron-donor ability of the sulfur to the thiophene ring, as observed from the blueshift of the absorption maximum and from the absence of an oxidation wave in the positive potential regime for monomeric 3,4-bis(alkylthio) thiophenes. In this way it is possible to tune the properties of thiophene derivatives by ion coordination.