Polyoxometalates with supporting phosphate ligand:: synthesis and characterization of α-[SiW11O39M(H2PO4)]n-

A novel series of Keggin species with a covalently bonding support-ligand, formulated alpha-[SiW11O39M(H2PO4)](n-) (M=Co-II, Mn-II, Ni-II or Cr-III), have been synthesized and characterized by elemental analyses, IR, CV, NMR and X-ray powder diffraction measurements. The structure is reminiscent of an ancient Chinese three-legged wine vessel, and is a possible model for POM-nucleotide interaction. An interesting property is the different thermal behaviors for this series in comparison with those for corresponding 3d metal monosubstituted Keggin polyoxometalates. This can be attributed largely to the pendant surface ligand and H bonding interaction. Compared with alpha-PW11M, the orders of line broadening of P-31 NMR signals are the same for the two series of complexes alpha-PW11M and alpha-SiW11M-PO4, but both contact and dipolar interactions might make a significant contribution to the chemical shift for phosphorus for the latter.