Chiral Polyoxotungstates. 1. Stereoselective Interaction of Amino Acids with Enantiomers of [Ce<sup>III</sup>(α<sub>1</sub>-P<sub>2</sub>W<sub>17</sub>O<sub>61</sub>)(H<sub>2</sub>O)<i><sub>x</sub></i>]<sup>7-</sup>. The Structure of dl-[Ce<sub>2</sub>(H<sub>2</sub>O)<sub>8</sub>(P<sub>2</sub>W<sub>17</sub>O<sub>61</sub>)<sub>2</sub>]<sup>14-</sup>
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The ammonium salt of the 1:1complex (1) of Ce(III) with α1-[P2W17O61]10- was prepared and characterized by elemental analysis, vibrational and NMR spectroscopy (31P, 183W), cyclic voltammetry, and single-crystal X-ray analysis (P1̄; a = 15.8523(9) Å, b = 17.4382(10) Å, c = 29.3322(16) Å, α = 99.617(1)°, β = 105.450 (1)°, γ = 101.132(1)°, V = 7460.9(7) Å3, Z = 2). The anion consists of a centrosymmetric head-to-head dimer, [{Ce(H2O)4(P2W17O61)}2],- with each 9-coordinate Ce cation linked to four oxygens of one tungstophosphate anion and to one oxygen of the other anion. On the basis of P NMR spectroscopy, a monomer−dimer equilibrium exists in solution with K = 20 ± 4 M-1 at 22 °C. Addition of chiral amino acids to aqueous solutions of 1 results in splitting of the 31P NMR signals as a result of diastereomer formation. No such splitting is observed with glycine or dl-proline, or when chiral amino acids are added to the corresponding complex of the achiral α2-isomer of [P2W17O61]10-. From analysis of the 31P NMR spectra, formation constants of the two diastereomeric adducts of 1 with l-proline are 7.3 ± 1.3 and 9.8 ± 1.4 M-1.
