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 (ImhedpH<sub>4</sub>) and cobalt or manganese salts under hydrothermal conditions result in three new metal diphosphonates: β-Co<sub>3</sub>(ImhedpH)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>·2H<sub>2</sub>O (<b>1</b>), Co<sub>3</sub>(ImhedpH)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub> (<b>2</b>), and Mn(ImhedpH<sub>2</sub>)·H<sub>2</sub>O (<b>3</b>). In compound <b>1</b>, the columns made up of {Co1<sub>2</sub>O<sub>2</sub>} dimers and {PO<sub>3</sub>C} tetrahedra through corner-sharing are cross-linked through {Co2O<sub>6</sub>} octahedra, forming an inorganic layer. Neighboring layers are pillared by coordinated imidazole groups of ImhedpH<sup>−</sup> 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 <b>2</b> shows a layer structure, in which Co<sub>3</sub>(ImhedpH)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub> trimer units are connected through the corner-sharing of {Co1O<sub>5</sub>} trigonal bipyramids and {PO<sub>3</sub>C} tetrahedra, forming an inorganic layer containing 20-member rings composed of six Co atoms, two μ<sub>3</sub>-O1 units, and four O−P−O units. The noncoordinated imidazole groups protrude from two sides of the layer. Compound <b>3</b> shows a ladder structure, where the Mn(II) ions are bridged by ImhedpH<sub>2</sub><sup>2−</sup> ligands through double O−P−O units to form a single chain, and two such chains are further fused together by sharing edges of {MnO<sub>5</sub>} trigonal bipyramids. The magnetic properties of <b>1</b>−<b>3</b> have been studied. Ferrimagnetism and field-induced magnetic transition from ferrimagnetism to a fully polarized state are observed in <b>1</b>. Compounds <b>2</b> and <b>3</b> reveal dominant antiferromagnetic interactions between metal centers, and two-step field-induced magnetic phase transitions are found in <b>2</b>.