Synthesis and Characterization of Technetiumtetrakis(acetonitrile)bis(triphenylphosphine) Cationic Complexes

A new route to low-valent technetium complexes containing multiple
acetonitrile ligands has been developed. The reduction of TcCl4(PPh3)2 with zinc metal dust in acetonitrile results in the formation
of [Tc(CH3CN)4(PPh3)2][Zn2Cl6]1/2. The hexafluorophosphate salt
of the analogous Tc(II) cation can be prepared via chemical
oxidation of the Tc(I) species, and the Tc(I) cation can be
regenerated via chemical reduction. The compounds have been
characterized in the solid state via single-crystal X-ray crystallography, and in solution via a combination of spectroscopic
techniques and cyclic voltammetry. The structural parameters found
in the two complexes are similar to each other; however, the
difference in oxidation state is reflected, as expected, in the
spectroscopic results. The electrochemical data, obtained from
cyclic voltammograms of Tc(CH3CN)4(PPh3)2](PF6)n (n = 1,2),
mirror the synthetic results in that both compounds possess a
reversible redox couple at −0.55 V versus ferrocene, which has
been assigned to the Tc(II)/Tc(I) couple.