Enantioselective resolutions and circular dichroism studies of lanthanide-containing Keggin-type [Ln(PW11O39)2](11-) polyoxometalates

Enantiopure crystals of K(1.3)Na(3.2)H(6.5)l-Pr(PW(11)O(39))(2)]·8.3l-proline·21.5H(2)O (1), K(1.3)Na(3.2)H(6.5)d-Pr(PW(11)O(39))(2)]·8.3d-proline·17H(2)O (2), and K(1.3)Na(3.2)H(6.5)l-Er(PW(11)O(39))(2)]·8.3l-proline·22.5H(2)O (3) were successfully obtained by using l- and d-proline (pro) as chiral auxiliary agents. In these crystals, l- and d-Ln(PW(11)O(39))(2)](11-) anions are attached by two l- and d-pro molecules, respectively, through a O···N hydrogen-bonding interaction between the square-antiprismatic LnO(8) center and amino-N atoms. The l- and d-Pr(PW(11)O(39))(2)](11-) anions in aqueous solutions exhibited a couple of mirror-imaged CD spectra due to (3)H(4/2)→(3)P(0,1,2) and (1)D(2) transitions in the stereogenic Pr(3+) center. Chirality inductions by l- and d-pro from a racemic solution of Er(PW(11)O(39))(2)](11-) was demonstrated by means of CD spectroscopy.