Characterization of Tris(Diselenocarbamato)Cobalt(III) and Pentakis(Diselenocarbamato)Dicobalt(III) Complexes by Electrochemical, Cobalt-59 N.M.R. and Mass-Spectrometric Techniques. Comparisons With Dithiocarbamate Analogs

Abstract

A series of Co(RR′dsc)₃ and [Co₂(RR′dsc)₅]⁺ complexes (R, R′ = two alkyl groups or one heterocyclic group; dsc = NCSe₂) have been synthesized and their redox behaviour, chemical reactivity and spectroscopic properties compared with the corresponding dithiocarbamate (RR′dtc) complexes.

Electrochemical oxidation of Co(RR′dsc)₃ in dichloromethane at platinum electrodes occurs at potentials about 0.34 V less positive than for Co(RR′dsc)₃. The formally cobalt(IV) complexes [Co(RR′dsc)₃]⁺ can be identified as a product which is then converted into [Co₂(RR′dsc)₅]⁺ via dimerization and an internal redox reaction. Despite the enhanced thermodynamic stability implied by the redox potentials, [Co(RR′dsc)₃]⁺ has similar kinetic stability to the analogous dithiocarbamate complexes. Co(RR′dsc)₃ is reduced at fairly negative potentials on both platinum and mercury electrodes with extremely rapid loss of [RR′dsc]^-. [Co(RR′dsc)₃]^- is therefore thermodynamically and kinetically more unstable than [Co(RR′dtc)₃]^- . The [Co₂(RR′dsc)₅]⁺ complexes are also more readily oxidized and harder to reduce than the sulfur analogues. Oxidation of [Co₂(RR′dsc)₅]⁺ produces [Co₂(RR′dsc)5]²⁺ at low temperatures and fast scan rates, but no stable reduced form of the dimer is accessible on the voltammetric time scale examined. The reduction process for the dimer is consistent with the reaction [Co₂(RR′dsc)₅]⁺ +e^- → Co(RR′dsc)₃+ Co(RR dsc)₂. Electrochemical oxidation data obtained at mercury electrodes for the diselenocarbamate complexes are complicated by adsorption but are similar to that found at platinum electrodes. This contrasts with the dithiocarbamates where a mercury electrode specific pathway is observed. Cobalt-59 n.m.r. spectroscopy in dichloromethane shows the non- equivalence of the two cobalt atoms in [Co₂(RR′dsc)₅]⁺. The chemical shifts for Co(RR′dsc)₃ complexes exhibit similar substituent effects to the dithiocarbamates in cobalt-59 n.m.r. measurements as was the case in oxidative electrochemistry. Cobalt-59 n.m.r. spectroscopy and mass spectrometry demonstrate that exchange, substitution and redox reactions can lead to the formation of mixed ligand diselenocarbamate complexes and mixed dithiocarbamate/diselenocarbamate complexes for both the cobalt(III) monomers and dimers.