Structure and magnetism of VSB-2,-3, and-4 or Ni-4(O3P-(CH2)-PO3)(2)centerdot(H2O)(n) (n=3, 2, 0), the first ferromagnetic nickel(II) diphosphonates: Increase of dimensionality and multiple coordination changes during a quasi topotactic dehydration

[Ni-4(O3P-CH2-PO3)(2).(H2O)(3), or VSB-2 was prepared in a pure form under hydrothermal conditions (4-6 days, 453 K, autogenous pressure). It dehydrat es in two steps, giving successively the dihydrate VSB-3 at 275 degrees C a nd the anhydrous methyl diphosphonate VSB-4 at 350 degrees C. The structure s of the three solids were solved ab initio from X-ray powder data. The thr ee compounds are monoclinic (space group Cc for VSB-2 with a = 19.177(3) An gstrom, b = 8.0930(9) Angstrom, c = 9.1824(8) Angstrom, beta = 102.387(9)de grees, and V = 1391.9(2) Angstrom(3), space group C2/c for VSB-3 and VSB-4 with a = 18.683(4) Angstrom, b = 8.152(2) Angstrom, c = 8.936(2) Angstrom, beta = 107.65(1)degrees, V = 1297.0(3) Angstrom(3); and a = 17.647(2) Angst rom, b = 8.0727(6) Angstrom, c = 8.7741(7) Angstrom, beta = 106.339(7)degre es, V = 1199.5(1) Angstrom(3), respectively). The layered VSB-2 is built fr om sheets of trimeric edge sharing units of Ni2+ octahedra onto which Ni oc tahedra and diphosphonate groups are grafted. During the first loss (-1 H2O ), the migration of a nickel cation toward a tetrahedral site leads to the connection of the layers and renders the dihydrate VSB-3 three-dimensional. In the totally dehydrated compound VSB-4, stable up to 575 degrees C, the trimers consist of a central octahedron with two edge-shared square pyramid s. The Ni coordinations follow the unique sequence: 4 oct (VSB-8) --> 3 oct + tetra (VSB-3) --> 1 oct + 2 SQ + tetra (VSB-4) which has drastic effects on the magnetic properties discussed in this paper. Below 4 K, VSB-2 and V SB-4 are indeed canted ferromagnets, whereas VSB-3 is a pure ferromagnet. T hese open framework phases do not exhibit any porosity.