Exchange and Other Reactions Associated with Zinc(II) Dithiocarbamate Oxidation and Reduction Processes Observed at Mercury and Platinum Electrodes in Dichloromethane

The electrochemical behavior of zinc(II) dithiocarbamate complexes (Zn(RR^′dtc)₂) has been investigated at both platinum and mercury electrodes and compared with that of the analogous cadmium complexes (Cd(RR^′dtc)₂). Oxidation at mercury electrodes in the presence of M(RR^′dtc)₂(M = Zn, Cd) consists of three reversible processes. The first of these involves an exchange reaction between metal(II) dithiocarbamate and electrode mercury: M(RR^′dtc)₂+ Hg → Hg(RR^′dtc)₂+ M²⁺+ 2e⁻(M = Zn, Cd). This reaction is mediated by formation of a bimetallic cation [MHg(R^′dtc]²⁻, small amounts of which were detected on the synthetic time scale associated with bulk electrolysis experiments at a mercury-pool electrode. The remaining two oxidation processes arise from formation of Hg(RR^′dtc)₂, which is in equilibrium with the bimetallic complex at the electrode surface. At platinum electrodes, oxidation processes are observed at much more positive potentials than at mercury electrodes and yield the thiuram disulfide complexes [MR₂R^′₂tds]²⁺. However, for Zn(RR^′dtc)₂, the more positive oxidation potential (E_p(Zn) = 1.6 V vs E_p(Cd) = 1.3 V vs Ag/AgCl) leads to further oxidation to give an unidentified product. The oxidation products formed at platinum electrodes in bulk electrolysis experiments interact strongly with electrode mercury to give polarographic responses that are similar to those for the oxidation products formed at mercury electrodes, demonstrating the considerable lability of the zinc- (and cadmium-) mercury dithiocarbamate interactions. Reduction processes for Zn(RR^′dtc)₂are less affected by the choice of electrode material than are the oxidation processes. At both platinum and mercury electrodes, the major reduction process occurs at very negative potentials (approximately −2 V vs Ag/AgCl) and yields elemental zinc or zinc amalgam, respectively, and free dithiocarbamate. At mercury electrodes, a minor additional reduction pathway involving exchange between Zn(RR^′dtc)₂and electrode mercury was noted. The analogous process for Cd(RR^′dtc)₂provides the major route for reduction of the cadmium complexes at mercury electrodes. Surprisingly, no reduction of Cd(RR^′dtc)₂is observed at platinum electrodes.