Cooperative high-temperature spin crossover accompanied by a highly anisotropic structural distortion

Spin transitions are a spectacular example of molecular switching that can provoke extreme electronic and structural reorganizations in coordination compounds. A new 3D cyanoheterometallic framework, [Fe(pz)(Au(CN)2)2], has been synthesized in which a highly cooperative spin crossover has been observed at 367 and 349 K in heating and cooling modes, respectively. Mössbauer spectroscopy revealed a complete transition between the diamagnetic and paramagnetic states of the iron centres. The low-spin-to-high-spin transition induced a drastic structural distortion involving a large one-directional expansion (ca. 10.6%) and contraction (ca. 9.6%) of the lattice. Negative thermal expansion along the c axis was detected below and above the transition temperature.</p

Crystal structure of high-spin tetraaquabis(2-chloropyrazine-κN4)iron(II) bis(4-methylbenzenesulfonate)

The title salt, [FeII(C4H3ClN2)2(H2O)4](C7H7O3S)2, contains a complex cation with point group symmetry 2/m. The high-spin FeII cation is hexacoordinated by four symmetry-related water and two N-bound 2-chloropyrazine molecules in a trans arrangement, forming a distorted FeN2O4 octahedron. The three-dimensional supramolecular structure is supported by intermolecular O—H...O hydrogen bonds between the complex cations and tosylate anions, and additional π–π interactions between benzene and pyrazine rings. The methyl H atoms of the tosylate anion are equally disordered over two positions