Complete and incomplete spin transitions in 1D chain iron(ii) compounds

Abstract

The synthesis and characterisation of two new octahedral iron(II) SCO coordination polymers [FeL1(bimm)] (1) and [FeL2(bppa)bimm = bis(1H-imidazol-1-yl)methane and bppa = 1,3-bis(pyridine-4-yl)propane) is presented.Results from X-ray structure analysis at different temperatures revealed in the case of 1 that the transition from a gradual to a cooperative SCO with a 5 K wide hysteresis is due to an increase of the short intermolecular contacts, which exceed a certain threshold for the cooperative effect. In the case of compound 2 an incomplete spin transition with a 4 K wide hysteresis was observed. The low temperature w M T product remains constant at a value typical for a mixed HS/LS state in stepwise spin transitions. A quantitative correlation between the cooperative effects of 12 monomer and polymer iron(II) SCO complexes and their structural properties derived from X-ray structure analysis, the so-called crystal contact index, CCI, is introduced.

Solvent Influence on the Magnetic Properties of Iron(II) Spin‐Crossover Coordination Compounds with 4,4′‐Dipyridylethyne as Linker

“…As has been shown previously, [8][9][10][11][12][13][14][15][16] the solvent has a significant influence on the SCO behaviour of this type of compound. Depending on the solvent used and preparation method, different iron(II) complexes were obtained.…”

“…[7] For the complexes investigated by our group, this is most commonly observed for coordination polymers, for which different solvents used in the synthesis often lead to different magnetic properties. [8][9][10][11][12][13][14][15][16] In contrast, such changes are rare for mononuclear complexes. [17] So far, how-method but in different solvents led to the same compound exhibiting the same three-step spin crossover independent of the magnetic properties of the solvated species.…”

“…The OH groups of the coordinating ethanol molecules in the asymmetric unit are donors for hydrogen bonds with a carbonyl oxygen of a neighbouring molecule (O17-H17A···O60, O57-H57A···O55 and O64-H64···O15). Through these hydrogen bonds, the dimers form zig-zag chains along the vector [1][2][3][4][5][6][7][8][9][10][11]. The details of the hydrogen bonds are summarised in Table 4 and the molecular packing of the compound in the crystal is shown in Figure 3.…”

Complete Two‐Step Spin‐Transition in a 1D Chain Iron(II) Complex with a 110‐K Wide Intermediate Plateau

“…The averaged metal-ligand distances around the inner coordination sphere of Fe1 (Fe1-N eq : 2.09 Å, Fe1-O eq : 2.01 Å, Fe1-N ax : 2.25 Å) indicate a distorted, axially elongated octahedral environment, typical for HS iron(II) of this ligand type. [9,17,18,20,23] The averaged metal-ligand distances in the [FeN 4 O 2 ] coordination core of Fe2 (Fe2-N eq : 1.90 Å, Fe2-O eq : 1.93 Å, Fe2-N ax : 1.99 Å) are about 10 % shorter than those for the HS state, as discussed in the literature for iron(II) SCO complexes. [2] Especially, the pronounced reduction of the Fe-N ax distance (0.26 Å) in the direction of a more regular octahedral environment is indicative of LS iron(II).…”

“…Systematic investigations on a series of 1D chain compounds with Schiff base like equatorial ligands used in our group lead to different types of spin transition. [16,17] Two different reasons were identified to be responsible for the step in the transition curve: inequivalent iron sites in the HS state or, if a single iron site is observed in the HS state, a change in symmetry leading to a phase with two iron sites (HS, LS) in the region of the plateau. An analysis of the intermolecular interactions revealed that a pronounced zigzag structure of the 1D chain combined with restraining interactions between the 1D chains (several short contacts) favour such a phase transition and lead to steps in the transition curve.…”