Lone-Pair-Induced Chirality in Polyoxotungstate Structures: Tin(II) Derivatives of A-Type XW9O34n- (X = P, Si). Interaction with Amino Acids

Abstract

Tin(II) tungstophosphate and tungstosilicate derivatives [SnII3(α-PW9O34)2]12- (1), [SnII3(α-SiW9O34)2]14- (2), and [SnII3(β-SiW9O34)2]14- (3) have been prepared by reaction of tin(II) sulfate with Na8H[A,β-PW9O34]·24H2O, Na10[α-SiW9O34]·xH2O, and Na10[β-SiW9O34]·26H2O, respectively. Oxidation of 1 with Br2 yields [(HOSnIVOH)3(PW9O34)2]12- (1‘). The new heteropolyanions were characterized by elemental analysis, infrared spectroscopy, multinuclear NMR, and X-ray crystallography. Crystals of Cs11H[SnII3(PW9O34)2]·14H2O (anion 1) are triclinic, space group P1̄, with lattice constants a = 12.904(3) Å, b = 18.059(3) Å, c = 21.618(3) Å, α = 111.68(2)°, β = 94.04(2)°, γ = 101.41(1)°, V = 4531.5(14) Å3, Z = 2. Crystals of Cs12.5H1.5[SnII3(α-SiW9O34)2]·24.5H2O (anion 2) are triclinic, space group P1̄, with lattice constants a = 13.101(5) Å, b = 18.140(7) Å, c = 21.367(8) Å, α = 111.54(4)°, β = 92.22(4)°, γ = 102.98(2)°, V = 4561(3) Å3, Z = 2. Both 1 and 2 have structures in which three SnII cations are sandwiched between XW9 units. In 1 the coordination of each Sn is trigonal bipyramidal with the “equatorial” unshared electron pair directed toward the center of the anion. The inequality of the equatorial (2.11 Å) and axial (2.33 Å) Sn−O bond lengths results in one PW9 group being rotated by 7° with respect to the other PW9. The overall anion symmetry is D3. Anion 2 has a more pronounced (10°) rotation of the SiW9 groups, which results in “tetrahedral” Sn coordination, again with the unshared electron pairs directed internally (overall anion symmetry C3). The greater rotation in the structure of 2 may be a consequence of electrostatic repulsions between the more highly charged SiW9O3410- anions. All three anions display one-line 119Sn-NMR and two-line 183W-NMR (intensity 1:2) spectra, although the lines are broad (60−100 Hz) for 2 and 3 suggesting a slow fluxional process, probably racemization via an internal twist. In contrast, the two W-NMR lines of 1 are narrow and the more intense line shows two-bond coupling to P (1.7 Hz). This line is split into two by the addition of the chiral amino acids lysine, isoleucine, and histidine, but is unaffected by achiral glycine. Crystals of a lysine complex have been isolated, but have in all cases so far proved to be disordered.