Chiral discrimination in diasteroisomeric systems of cadmium with 1,10-phenanthroline and S-amino acidates

Abstract

Δ,Λ-[Cd(phen)2(S-aa)]+NO−3 diastereomeric systems (where phen = 1,10-phenanthroline and S-aa = S-alaninate, S-serinate, S-threoninate, S-prolinate, S-valinate, S-leucinate and S-phenylalaninate) have been prepared in the form of methanolic solutions and characterized by spectral (UV, CD, ORD) and conductance measurements. The CD spectra of these systems in the β′ band region of the diimine ligand exhibit residuary exciton splittings, whose intensities and sign patterns suggest the following decreasing order of [Λ]/[Δ] diastereoisomeric equilibrium constants: S-prol > S-phenala > S-val > 1 > S-ala > S-leu > S-ser > S-threo. The same order is inferred from the ORD data over the range 400–600 nm. Comparison with the selectivities observed for some zinc analogues and the analysis of molecular models suggest that in the cadmium systems the chiral discriminations are governed mainly by a balancing between intramolecular steric repulsion, differential solvation and discriminating ion association. Solvent effects on the optical activity of some cadmium and zinc systems in methanol/n-propanol and methanol/benzene mixtures have also been studied. The corresponding results are discussed in terms of the contributions of the above discriminating forces.