Iron (II) isothiocyanate complexes with substituted pyrazines: Experimental and theoretical views on their electronic structure

Spin Crossover in 3D Metal Centers Binding Halide-Containing Ligands: Magnetism, Structure and Computational Studies

The capability of a given substance to change its spin state by the action of a stimulus, such as a change in temperature, is by itself a very challenging property. Its interest is increased by the potential applications and the need to find sustainable functional materials. 3D transition metal complexes, mainly with octahedral geometry, display this property when coordinated to particular sets of ligands. The prediction of this behavior has been attempted by many authors. It is, however, made very difficult because spin crossover (SCO), as it is called, occurs most often in the solid state, where besides complexes, counter ions, and solvents are also present in many cases. Intermolecular interactions definitely play a major role in SCO. In this review, we decided to analyze SCO in mono- and binuclear transition metal complexes containing halogens as ligands or as substituents of the ligands. The aim was to try and find trends in the properties which might be correlated to halogen substitution patterns. Besides a revision of the properties, we analyzed structures and other information. We also tried to build a simple model to run Density Functional Theory (DFT) calculations and calculate several parameters hoping to find correlations between calculated indices and SCO data. Although there are many experimental studies and single-crystal X-ray diffraction structures, there are only few examples with the F, Cl, Br and series. When their intermolecular interactions were not very different, T1/2 (temperature with 50% high spin and 50% low spin states) usually increased with the calculated ligand field parameter (Δoct) within a given family. A way to predict SCO remains elusive.

Crystal structure of the tetraaquabis(thiocyanato-κN)cobalt(II)–caffeine–water (1/2/4) co-crystal

In the structure of the title compound [systematic name: tetraaquabis(thiocyanato-κN)cobalt(II)–1,3,7-trimethyl-1,2,3,6-tetrahydro-7H-purine-2,6-dione–water (1/2/4)], [Co(NCS)2(H2O)4]·2C8H10N4O2·4H2O, the cobalt(II) cation lies on an inversion centre and is coordinated in a slightly distorted octahedral geometry by the oxygen atoms of four water molecules and two N atoms of twotrans-arranged thiocyanate anions. In the crystal, the complex molecules interact with the caffeine molecules through O—H...N, O—H...O and C—H...S hydrogen bonds and π–π interactions [centroid-to-centroid distance = 3.4715 (5) Å], forming layers parallel to theabplane, which are further connected into a three-dimensional network by O—H...O and O—H...S hydrogen bonds involving the non-coordinating water molecules.

High temperature spin crossover in [Fe(pyrazine){Ag(CN)2}2] and its solvate

Thermally induced spin transition at 370 K has been observed in a Hofmann-clathrate-like metal–organic framework.

Crystal structure of high-spin tetra-aqua-bis-(2-chloro-pyrazine-κN (4))iron(II) bis-(4-methyl-benzene-sulfonate)

The title salt, [Fe(II)(C4H3ClN2)2(H2O)4](C7H7O3S)2, contains a complex cation with point group symmetry 2/m. The high-spin Fe(II) cation is hexa-coordinated by four symmetry-related water and two N-bound 2-chloro-pyrazine mol-ecules in a trans arrangement, forming a distorted FeN2O4 octa-hedron. The three-dimensional supra-molecular structure is supported by inter-molecular O-H⋯O hydrogen bonds between the complex cations and tosyl-ate anions, and additional π-π inter-actions between benzene and pyrazine rings. The methyl H atoms of the tosyl-ate anion are equally disordered over two positions.

Crystal structure of the co-crystal fac-tri-aqua-tris(thio-cyanato-κN)iron(III)-2,3-di-methyl-pyrazine (1/3)

In the crystal of the title compound, [Fe(NCS)3(H2O)3]·3C6H8N2, the Fe(III) cation is located on a threefold rotation axis and is coordinated by three N atoms of the thiocyanate anions and three water mol-ecules in a fac arrangement, forming a slightly distorted N3O3 octa-hedron. Stabilization within the crystal structure is provided by O-H⋯N hydrogen bonds; the H atoms from coordinating water mol-ecules act as donors to the N atoms of guest 2,3-di-methyl-pyrazine mol-ecules, leading to a three-dimensional supra-molecular framework.