Cobalt and Manganese Diphosphonates with One-, Two-, and Three-Dimensional Structures and Field-Induced Magnetic Transitions

Reactions of 2-(1-Imidazole)-1-hydroxyl-1,1′-ethylidenediphosphonic acid (ImhedpH₄) and cobalt or manganese salts under hydrothermal conditions result in three new metal diphosphonates: β-Co₃(ImhedpH)₂(H₂O)₄·2H₂O (1), Co₃(ImhedpH)₂(H₂O)₄ (2), and Mn(ImhedpH₂)·H₂O (3). In compound 1, the columns made up of {Co1₂O₂} dimers and {PO₃C} tetrahedra through corner-sharing are cross-linked through {Co2O₆} octahedra, forming an inorganic layer. Neighboring layers are pillared by coordinated imidazole groups of ImhedpH⁻ ligands, leading to a three-dimensional open framework containing two kinds of channels with sizes of 8.256 × 9.851 Å and 8.030 × 4.745 Å (van der Waals radii not accounted for). Compound 2 shows a layer structure, in which Co₃(ImhedpH)₂(H₂O)₄ trimer units are connected through the corner-sharing of {Co1O₅} trigonal bipyramids and {PO₃C} tetrahedra, forming an inorganic layer containing 20-member rings composed of six Co atoms, two μ₃-O1 units, and four O−P−O units. The noncoordinated imidazole groups protrude from two sides of the layer. Compound 3 shows a ladder structure, where the Mn(II) ions are bridged by ImhedpH₂²⁻ ligands through double O−P−O units to form a single chain, and two such chains are further fused together by sharing edges of {MnO₅} trigonal bipyramids. The magnetic properties of 1−3 have been studied. Ferrimagnetism and field-induced magnetic transition from ferrimagnetism to a fully polarized state are observed in 1. Compounds 2 and 3 reveal dominant antiferromagnetic interactions between metal centers, and two-step field-induced magnetic phase transitions are found in 2.