Enantioselective Resolutions and Circular Dichroism Studies of Lanthanide-Containing Keggin-Type [Ln(PW11O39)2]11– Polyoxometalates

Enantiopure crystals of K1.3Na3.2H6.5[l-Pr(PW11O39)2]·8.3l-proline·21.5H2O (1), K1.3Na3.2H6.5[d-Pr(PW11O39)2]·8.3d-proline·17H2O (2), and K1.3Na3.2H6.5[l-Er(PW11O39)2]·8.3l-proline·22.5H2O (3) were successfully obtained by using l- and d-proline (pro) as chiral auxiliary agents. In these crystals, l- and d-[Ln(PW11O39)2]11– anions are attached by two l- and d-pro molecules, respectively, through a O···N hydrogen-bonding interaction between the square-antiprismatic LnO8 center and amino-N atoms. The l- and d-[Pr(PW11O39)2]11– anions in aqueous solutions exhibited a couple of mirror-imaged CD spectra due to 3H4/2→3P0,1,2 and 1D2 transitions in the stereogenic Pr3+ center. Chirality inductions by l- and d-pro from a racemic solution of [Er(PW11O39)2]11– was demonstrated by means of CD spectroscopy.