Complexes of ZrIV and HfIV with monolacunary Keggin- and Dawson-type anions

The reactions of the tetranuclear hydroxo complexes [M-4(mu(2)-OH)(8)(H2O)(16)](8+) (M = Zr or Hf) with the lacunary Keggin-type ([alpha- PW11O39](7-)) and Dawson-type ([alpha(2)-P2W17O61](10-)) phosphotungstates in aqueous solutions produce the sandwich polyoxometalate complexes [M(alpha-PW11O39)(2)](10-) (M = Zr(1) or Hf (2)) and [M(alpha(2)-P2W17O61)(2)](16-) (M = Zr (3) and Hf (4)). The complexes were isolated and structurally characterized as salts with potassium and dimethylammonium cations. The zirconium and hafnium atoms have a square antiprismatic coordination environment (coordination number is 8). In all complexes, the mutual arrangement of the ligands corresponds to the syn isomer. Hafnium complexes 2 and 4 are the first structurally characterized polyoxometalate complexes of this metal. The structures of the resulting compounds were confirmed also by P-31 NMR spectroscopy in solution.