Synthesis, Characterization, and Crystal Structures of Two Divalent Metal Diphosphonates with a Layered and a 3D Network Structure

Reactions of N-methyliminobis(methylenephosphonic acid), CH3N(CH2PO3H2)2 (H4L), with divalent metal acetates
under different conditions result in metal diphosphonates with different structures. Mn(H3L)2·2H2O (complex 1) with
a layer structure was prepared by a layering technique. It is triclinic, P1̄, with a = 9.224(3) Å, b = 9.780(3) Å,
c = 10.554(3) Å, α = 82.009(6)°, β = 74.356(6)°, γ = 89.853(6)°, Z = 2. The Mn(II) ion is octahedrally coordinated
by six phosphonate oxygen atoms from four ligands, two of them in a bidentate and two in a unidentate fashion.
Each MnO6 octahedron is further linked to four neighboring MnO6 octahedra through four bridging phosphonate
groups, resulting in a two-dimensional metal phosphonate (002) layer. These layers are held together by strong
hydrogen bonds between uncoordinated phosphonate oxygen atoms. The zinc complex Zn3(HL)2 (complex 2) was
synthesized by hydrothermal reactions (4 days, 438 K, autogenous pressure). It is monoclinic, P21/n with a =
7.7788(9) Å, b = 17.025(2) Å, c = 13.041(2) Å, β = 94.597(2)°, Z = 4. The structure of complex 2 features a
3D network built from ZnO4 tetrahedra linked together by bridging phosphonate groups. Each zinc cation is
tetrahedrally coordinated by four phosphonate oxygen atoms from four ligands, each of which connects with six
zinc atoms, resulting in voids of various sizes. Magnetic measurements for the manganese complex shows an
antiferromagnetic interaction at low temperature. The effect of the extent of deprotonation of phosphonic acids on
the type of complex formed is discussed.