Synthesis of a new chiral copper(I) complex and its application to stereoselective photoreduction of [Co(edta)]- (H4edta = ethylenedinitrilotetraacetic acid)

Abstract

A chiral bipyridine derivative, 4,4′-6,6′-tetramethyl-5,5′-bis[(S)-(-) -1-phenylethylcarbamoyl]-2,2′-bipyridine (L), was newly synthesized. Using its copper(I) complex, [CuL(PPh ₃ ) ₂ ] ⁺ , [Co(edta)] ^- (H ₄ edta = ethylenedinitrilotetraacetic acid) was stereoselectively photoreduced upon irradiation with near-UV light (360–400 nm) corresponding to the metal-to-ligand charge transfer absorption band of the copper(I) complex, where the Λ enantiomer of [Co(edta)] ^- was preferentially reduced. The stereoselectivity of 42% enantiomeric excess was observed at 10% conversion, where the solvent was EtOH–water (75:25 v/v). This [Co(edta)] ^- photoreduction by [CuL(PPh ₃ ) ₂ ] ⁺ proceeds through both static and dynamic quenching mechanisms. Quenching experiments with the optical isomers, Δ- and Λ-[Co(edta)] ^- , clearly indicated that quenching takes place with little stereoselectivity, but charge separation and/or a reverse electron transfer occur stereoselectively.