ELECTROCHEMISTRY AND SPECTROELECTROCHEMISTRY OF POLYNUCLEAR RHENIUM(III) CHLORIDE CLUSTER COMPLEXES AND THEIR ONE-ELECTRON-REDUCTION PRODUCTS IN A BASIC ROOM-TEMPERATURE CHLOROALUMINATE MOLTEN-SALT

The dimeric, metal-metal-bonded rhenium(III) complex [Re2Cl8]2− is stable in the basic aluminum chloride-1-methyl-3-ethyl-imidazolium chloride (AlCl3-MeEtimCl) molten salt and can be reduced to [Re2Cl8]3− at a glassy-carbon electrode in a reversible electrode reaction with a voltammetric half-wave potential, E1/2, of ca. −0.58 V in the 49.0/51.0 mol % melt versus the A13+/Al couple in the 66.7/33.3 mol % melt at 40°C. It is possible to electrochemically generate stable, bulk solutions containing [Re2Cl8]3−, provided that oxygen is rigorously excluded from the electrochemical cell. The average Stokes-Einstein products for [Re2Cl8]2− and [Re2Cl8] are 2.0 × 10−10 and 1.2 × 10−10 g cm s−2 K−1, respectively. The addition of Re3Cl9 to basic AlCl3–MeEtimCl produces the trimeric, metal-metal-bonded rhenium(III) complex [Re3Cl12]3−. This species can also be reduced at a glassy-carbon electrode in a one-electron, reversible electrode reaction with an E1/2 of about −0.34 V in this same basic melt. However, E1/2 for this reaction is dependent upon the pCl of the melt, indicating the loss of chloride ion from the coordination sphere of the cluster during reduction to form a species of the type [Re3Cl12_x](4−x)− for which x may be 1. It is possible to electrochemically generate stable solutions of this reduced species in the absence of oxygen. The average Stokes-Einstein products for [Re3Cl12]3− and its one-electron-reduction product are 1.3 × 10−10 and 1.0 × 10−10 g cm s−2 K−1, respectively. Both [Re2Cl8]2− and [Re3Cl12]3− exhibit an additional multielectron voltammetric reduction process at potentials negative of their one-electron reduction waves. In both cases, bulk electrolysis experiments conducted at potentials negative of these very large waves destroyed the parent clusters. In basic melt at temperatures of ca. 175° C or more, each [Re3Cl12]3− ion is irreversibly converted to one [Re2Cl8]2− ion. Absorption spectroscopic data for [Re2Cl8]2−, [Re3C12]3−, and their one-electron-reduction products are reported. © 1990, American Chemical Society. All rights reserved.