The synthesis and structure of trimetaphosphimato complexes of hafnium and zirconium

The (trimetaphosphimato)hafnates and -zirconates Na-4(Hf-4(mu(4)-O)(mu-OH)(6)[(PO2NH)(3)](4)).18 H2O [1, Pa (3) over bar, a 22.687(3) Angstrom, Z = 8], Na-4(Hf-4(mu(4)-O)(mu-OH)(6)[(PO2NH)(3)](4)).21 H2O [2, R (3) over bar, a = 14.350(2), c = 50.348(10) Angstrom, Z = 6], Na-4(Zr-4(mu(4)-O) (mu-OH)(6)[(PO2NH)(3)](4)).18 H2O [3, Pa (3) over bar, a = 22.693(3) Angstrom, Z = 8], and Na-4(Zr-4(mu(4)-O)(mu-OH)(6)[(PO2NH)(3)](4)).21 H2O [4, R (3) over bar, a = 14.303(2), c = 50.284(10) Angstrom, Z = 6] were obtained by the stoichiometric reaction of HfOCl2.8 H2O and ZrOCl2.8 H2O with an aqueous solution of Na-3(PO2NH)(3).4 H2O, followed by the diffusion-controlled addition of methanol. During these reactions compounds 1 and 2, or 3 and 4, crystallized simultaneously and the hafnium and zirconium complexes 1 and 3, as well as 2 and 4, were found to be isostructural and isomorphous. The characteristic structural feature which is central to 1, 2, 3, and 4 is the complex tetranuclear anion (M-4(mu(4)-O)(mu-OH)(6)[(PO2NH)(3)](4))(4-) with M = Zr or Hf. The anion consists of five corner-sharing adamantanoid cages. The central cage, {M-4(mu(4)-O)(mu-OH)(6)}(8+), has a tetracoordinated oxygen atom in the middle, and the other cages are formed by the trimetaphosphimate ions acting as tridentate ligands which coordinate to Kf or Zr. The water content of the compounds 1-4 could not be determined unequivocally by chemical analyses since the compounds are always obtained as mixtures of the rhombohedral (2/4) and cubic (1/3) phases. Thermal decomposition of compounds 1-4 starts above 120 degrees C and leads to the formation of HfP2O7 and ZrP2O7/NaZr2(PO4)(3), with the evolution of H2O and NH3. DSC measurements gave no evidence that the compounds 1 and 2, or 3 and 4, might be transformed into each other by heating.