Magnetic Properties of Metal Diphosphonate Compounds with One-Dimensional Chain Structures

Two new phosphonate compounds (NH₄)₂M₂(hedpH)₂ (M = Co, 1; Fe, 2), where hedp is 1-hydroxyethylidenediphosphonate [CH₃C(OH)(PO₃)₂], have been synthesized under hydrothermal conditions at 140 °C. Crystal data:  1, triclinic, P1̄, a = 5.4635(13) Å, b = 7.7205(18) Å, c = 9.976(2) Å, α = 71.198(4)°, β = 82.385(4)°, γ = 87.920(5)°, V = 394.82(16) ų, Z = 1; 2, triclinic, P1̄, a = 5.5191(12) Å, b = 7.7183(16) Å, c = 10.051(2) Å, α = 71.551(4)°, β = 82.552(4)°, γ = 87.627(4)°, V = 402.74(15) ų, Z = 1. The compounds are isomorphous; all contain anionic double chains of {M₂(hedpH)₂}_n^2n-, composed of {MO₆} octahedra and {CPO₃} tetrahedra. The double chain can be viewed as a ladder within which dimers of {M₂(μ−O)₂} (rung) are separated by O−P−O bridges (rail). Very strong hydrogen bonds exist between the neighboring chains, hence forming a layer in the ab plane. The NH₄⁺ counterions occupy the cavities within the layer. These layers are further connected by hydrogen bonds to form a three-dimensional network. The temperature-dependent magnetic susceptibility measurements show dominant antiferromagnetic interactions in both compounds, mediated through the μ−O and/or O−P−O bridges between the metal centers. The field dependent magnetization reveals that both experience field-induced magnetic transitions at very low temperatures. The critical fields at 1.9 K are ca. 35 kOe for 1 and 40 kOe for 2, respectively.