Novel photo-induced deracemization of [Co(acac)3] (acac = acetylacetonate) with a chiral ruthenium(II) complex, Δ-[Ru(menbpy)3]2+ (menbpy = 4,4′-bis{(1R,2S,5R)-(−)-menthoxycarbonyl}-2,2′-bipyridine). Reaction mechanism and significant effects of solvent and anion

Abstract

Deracemization of a racemic mixture of Δ- and Λ-[Co(acac)₃] (acac = acetylacetonate) took place with a chiral ruthenium(II) complex, Δ-[Ru(menbpy)₃]²⁺ (menbpy = 4,4′-bis{(1R,2S,5R)-(−)-menthoxycarbonyl}-2,2′-bipyridine), under visible light (420 < λ < 470 nm) irradiation in the presence of either triethylamine or NaOH. The enantiomeric excess (e.e.) of Λ-[Co(acac)₃] was 28% in acetonitrile–water (7 ∶ 3 v/v; ionic strength (I) = 0.1 mol dm⁻³ (KCl)) without acetylacetone and 38% with acetylacetone (50 mmol dm⁻³) added to the solution. Quenching experiments on Δ-*[Ru(menbpy)₃]²⁺ led to the conclusion that this deracemization occurs through combination of the preferential photo-reduction of Δ-[Co(acac)₃] by Δ-*[Ru(menbpy)₃]²⁺ to afford [Co(acac)₂] and the thermal oxidation of [Co(acac)₂] by Δ-[Ru(menbpy)₃]³⁺ to afford preferentially Λ-[Co(acac)₃]. The selectivity of deracemization remarkably depends on the solvent; the e.e. value is 51% in acetone–water (7 ∶ 3 v/v), 38% in ethanol–water (7 ∶ 3 v/v), and 37% in DMF–water (7 ∶ 3 v/v). The e.e. value significantly decreases to 33% from 51% in acetone–water (7 ∶ 3 v/v) when the KCl concentration is increased to 0.3 mol dm⁻³ from 0.1 mol dm⁻³. When either KF or CH₃CO₂K is added to acetone–water (7 ∶ 3 v/v) instead of KCl the deracemization proceeds much more rapidly without decrease of the e.e. value (≈50%). These counter anion effects are interpreted in terms that the reduction potential of [Co(acac)₃] becomes more negative by addition of either F⁻ or CH₃CO₂⁻.