1D-polyoxometalate-based composite compounds -: Design, synthesis, crystal structures, and properties of [{Ln(NMP)6}(PMo12O40)]n (Ln = La, Ce, Pr; NMP = N-methyl-2-pyrrolidone)

The structures of three 1:1 composite compounds prepared with the polyoxometalate (POM) [PMo12O40](3-) and the cations [Ln(NMP)(6)](3+) [Ln = La (1), Ce (3), Pr (2); NMP = N-methyl-2-pyrrolidone] exhibit two types of zig-zag chains with alternating cations and anions through Mo-O-t-Ln-O-t-Mo links in the crystal. The compounds were characterized by IR, UV, and ESR spectroscopy, single-crystal X-ray structural analysis, and by a study of their thermal properties. In all the compounds, the La3+, Pr3+, and Ce3+ centers are eight-coordinate with the oxygen atoms in bicapped trigonal-prismatic geometries. The variation of the average Ln-O separations along the La, Ce, and Pr series is consistent with the effects of, the lanthanide contraction. The results of the single-crystal X-ray diffraction analyses and the IR spectroscopic studies are in agreement and both show the metal cation units are coordinatively bonded to the Keggin clusters. The UV spectra of the title compounds suggest that their structures are entirely disrupted in dilute solution. Low-temperature ESR spectra indicate that thermal electron delocalization occurs among the Mo atoms in the three compounds. The results of cyclic voltammetry (CV) show that compounds 1, 2, and 3 all undergo five two-electron reversible reductions and that the [PMo12O40](3-) anions are the active centers for electrochemical redox reactions in solution, while the corresponding cations have only a small effect on the electrochemical properties. Studies of magnetic properties show that I is diamagnetic, while 2 and 3 exhibit antiferromagnetic Pr-Pr or Ce-Ce exchange interactions. In addition to the results of CV, the average bond lengths and the magnetic properties of compound 3 indicate that the oxidation state of cerium is in in the compound of formula [{Ce(NMP)(6)}(PMo12O40)]. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.