Solution study of chiral 1,2-diol Yb3+ chelates through near-IR circular dichroism

Ytterbium is efficiently chelated by 1,2-diols in a variety of very different solvents. We found a set of solution equilibria among three complexes of minimum formula [Yb·(diol)], [Yb·(diol)2] and [Yb·(diol)3]. By extensive near-Infrared circular dichroism (NIR CD) investigation, we determined fingerprint CD spectra of the di- and tri-chelated species and the relative formation constants. Since NIR CD is sensitive only to chiral Yb-species, NIR CD can be very useful for studying complex mixtures, such as those used in enantioselective catalysis and involving a chiral non-racemic diol as an auxiliary and an ytterbium salt. The chiroptical spectra show completely conserved features through very different diol structures, demonstrating the identity of the coordination polyhedron around Yb(III) and giving rise to a new empirical method for assigning 1,2-diols absolute configuration.