New Iron(II) Spin Crossover Coordination Polymers [Fe(μ-atrz)₃]X₂·2H₂O (X = ClO₄^¯, BF₄^¯) and [Fe(μ-atrz)(μ-pyz)(NCS)₂]·4H₂O with an Interesting Solvent Effect

A potential bridging triazole-based ligand, atrz (<i>trans</i>-4,4′-azo-1,2,4-triazole), is chosen to serve as building sticks and incorporated with a spin crossover metal center to form a metal organic framework. Coordination polymers of iron­(II) with the formula [Fe­(μ-atrz)₃]­X₂·2H₂O (where X = ClO₄^– (<b>1</b>·2H₂O) and BF₄^– (<b>2</b>·2H₂O)) in a 3D framework and [Fe­(μ-atrz)­(μ-pyz)­(NCS)₂]·4H₂O (<b>3</b>·4H₂O) in a 2D layer structure were synthesized and structurally characterized. The magnetic measurements of <b>1</b>·2H₂O and <b>2</b>·2H₂O reveal spin transitions near room temperature; that of <b>3</b> exhibits an abrupt spin transition at ∼200 K with a wide thermal hysteresis, and the spin transition behavior of these polymers are apparently correlated with the water content of the sample. Crystal structures have been determined both at high spin and at low spin states for <b>1</b>·2H₂O, <b>2</b>·2H₂O, and <b>3</b>·4H₂O. Each iron­(II) center in <b>1</b>·2H₂O and <b>2</b>·2H₂O is octahedrally coordinated with six μ-atrz ligands, which in turn links the other Fe center forming a strong three-dimensional (3D) network; counteranion and water molecules are located in the voids of the lattice. The FeN₆ octahedron of <b>3</b>·4H₂O is formed with two atrz, two pyrazine (pyz) ligands, and two NCS^– ligands, where the ligands atrz and pyz are bridged between iron centers forming a 2D layer polymer. A zigzag chain of water molecules is found between the layers, and there is a distinct correlation between the thermal hysteresis with the amount of water molecules the exist in the crystal