Stereoselective Formation and Chemical Properties of Chiral Polynuclear Complexes Depending on Oxidation Numbers

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The UV-vis spectroelectrochemical properties of S-bridged tricobalt(III) complexes [Co^<III>{Co^<III>(aet or L-cys)_3}_2]^<3+ or 3-> and dicobalt(III) complexes [Co^<III>{Co^<III>(aet or L-cys)_3}(dien)]^<3+ or 0> (aet = 2-aminoethanethiolate, L-cys = L-cysteinate, dien = diethylenetriamine) were investigated using a thin-layer quartz cell with a platinum mesh working electrode. The absorption spectroelectrochemical measurements for the tricobalt(III) complexes indicated that a two-electron reduction proceeds through pseudo two-steps, Co^<III>Co^<III>Co^<III>→Co^<III>Co^<II>Co^<III>→Co^<III>Co^<II>Co^<II> for the aet complex, while through pseudo one-step for the L-cys complex. The CD spectroelectrochemical measurements demonstrated that the optically active isomer (ΔΔ form) of the aet tricobalt(III) complex loses chirality during the one-electron reduction, whereas the Δ_<LLL>Δ_<LLL> isomer of the L-cys tricobalt(III) complex is reduced with an inversion of configuration, to form the Λ_<LLL>Λ_<LLL> isomer. The Λ_<LLL>Λ_<LLL> isomer is reduced with a retention of configuration. The aet and L-cys dicobalt(III) complexes were converted to the corresponding tricobalt(III) complexes by electrochemical reduction, accompanied by the release of the Co(II) ion and dien ligand. It was also elucidated that this conversion occurred with a retention of configuration.Similar properties and reactivities have been studied for the polynuclear complexes with various metal ions and organic ligands.