| Polyoxometalates (POMs) are robust, early transition metal oxide anions that are presently widely used in catalysis, predominantly in heterogeneous catalysis. My research focuses on the lanthanide chemistry of two families of polyoxometalates. (1) The alpha-PW9O34 9- polyoxometalate, derived from the parent, Keggin POM, and (2) the alpha1 and alpha2 isomers of P 2W17O6110-, derived from the parent Wells-Dawson POM.; The reaction products of PW9O349- with Eu(III) in aqueous solution, identified by multinuclear NMR spectroscopy and X-ray crystallography, are highly dependent on the counter cation, pH, and stoichiometry. To unambiguously assign the species in our solution studies, we optimized reaction conditions of Eu(III) and PW 9O349- to isolate the four nano-sized compounds that are observed in the speciation studies. The solid-state crystal structures not only are consistent with the solution species, but also reveal the unique abilities of the countercations to influence speciation and structure.; The alpha1 and alpha2-P2W17 O6110- isomers are distinguished by the position of the defect in the belt or cap, respectively. The basicity of the four oxygen atoms in the vacancy offers unique possibilities to tailor synthetic chemistry. The stability constants, K1, for the 1:1 Ln: alpha 1P2W17O6110- complexes, determined by a competitive method monitoring 31P NMR with EDTA as the reference competing ligand, increase significantly as the lanthanide series is traversed, reflecting the high basicity and lack of flexibility of the alpha1 site. The 1:2 Ln: alpha1P2W17 O6110- complexes are much less stable than the 1:1 Ln: alpha1P2W17O61 10- complexes, as shown by the stability constant, K2, and are countercation dependent. The log K1/log K2 ratios suggest that the 1:1 complexes can be isolated cleanly compared to the alpha 2-P2W17O6110- analogs and indeed, we find this is the case. We used this information to define the synthesis of hybrid inorganic-organic Ln alpha1P 2W17O6110- complexes, which led to the production of Ln POMs that react with organic ligands, some that highly sensitize the luminescence of the lanthanide ion. The solution behavior is very different for the alpha2-P2W17O 6110- isomer that possesses a vacancy in the "cap" position. In this family, the polyoxometalates appear to associate to form 1:2 Ln: alpha2P2W17O61 10- complexes and 2:2 Ln: alpha2P2W 17O6110- complexes that are dimers in the solid state. The 2:2 Ln: alpha2P2W17O 6110- complexes show structural differences across the Ln series that is a function of lanthanide ion size. (Abstract shortened by UMI.)... |
