Stereoselective photosensitized decomposition of tris(oxalato)cobaltate(III) and tris(acetylacetonato)cobalt(III) by tris(2,2′-bipyridine)ruthenium(II)

Abstract

The photosensitized decomposition of [Co(ox)₃]^3–(H₂ox = oxalic acid) and [Co(acac)₃](Hacac = acetylacetone) proceeded stereoselectively in the presence of optically active [Ru(bipy)₃]²⁺(bipy = 2,2′-bipyridine). Homochiral (Δ-Δ or Λ-Λ) preference for [Co(ox)₃]^3– contrary to heterochiral (Δ-Λ) preference for [Co(acac)₃] was found in aqueous solution according to the circular dichroism appearing upon decomposition of the racemic complex, and by the decomposition rate of each optical isomer. For the [Co(ox)₃]^3– complex the selectivity varied with the addition of an organic solvent such as methanol and acetonitrile following a decrease in the decomposition rate. The factors resulting in the different stereoselectivity between the similar tris-chelate complexes containing planar bidentate ligands have been investigated. Quenching rate constants for Δ-*[Ru(bipy)₃]²⁺ obtained from emission lifetimes demonstrated the homochiral preference in the quenching process with both [Co(ox)₃]^3– and [Co(acac)₃] in aqueous solution. Chromatographic experiments using a SP-sephadex C-25 column exchanged with Δ-[Ru(bipy)₃]²⁺ demonstrated the higher stability of the Δ-Δ compared with the Δ-Λ pair. It is concluded that overall selectivity is controlled by the relative and competitive rates between the back electron transfer and decomposition processes.