Characterization of [ReO(Tetramethylthiourea)4](PF6)3 in Solution: Electrochemical,NMR and Ligand Substitution Studies

The complex [ReO(Me₄tu)₄]³⁺, with Me₄tu = tetramethylthiourea, is characterized in nonaqueous media. Its structure is studied in acetonitrile solution by ¹H and ¹³C NMR and its electrochemical behavior in the same solvent is analyzed by means of cyclic voltamperometric measurements. Heteronuclear correlation and variable temperature NMR experiments suggest that the complex ion shows a similar structure in solution and in the solid state. At low temperatures (below 0°C) free rotation of the dimethylamine groups around the thiocarbonyl carbon-nitrogen bond is restricted and the dissolved complex adopts the rigid structure observed in the solid state. Cyclic voltamperometric results suggest that the redox behavior of this compound can be explained through an E_rC_iE_r mechanism (a chemical reaction coupled between two electron-transfer reactions). An initial one electron reduction of the complex from Re(V) to Re(IV) followed by an irreversible chemical reaction, leads to a new electroactive species of Re(IV)*, which is reduced to Re(III). The ability of [ReO(Me₄tu)₄](PF₆)₃ to serve as a precursor for other Re(V) complexes by ligand substitution is established, in acetone solution, with different kinds of incoming ligands: diethyldithiocarbamate (Et₂dtc), ethylenediamine (en) and pyridine (py). The previously known complexes [Re^v ₂O₃(Et₂dtc)₄], [Re^VO₂(en)₂](PF₆) and [Re^VO₂(py)₄](PF₆) are easily synthesized under mild conditions with high yields.