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

Abstract

Three new spin crossover compounds [FeL1(azpy)] (1), [FeL1(azpy)]·toluene [1(tol)] and [FeL2(azpy)] (2), where L1 and L2 are the tetradentate N2O22– coordinating Schiff base like ligands {diethyl (E,E)‐2,2′‐[1,2‐phenyl‐bis(iminomethylidyne)]bis[3‐oxobutanoate]‐(2‐)‐N,N′,O3,O3′} and {dimethyl(E,E)‐2,2′‐[1,2‐phenyl‐bis(iminomethylidyne)]bis[3‐oxobutanoate]‐(2‐)‐N,N′,O3,O3′}, respectively, and azpy = 4,4′‐azopyridine are presented. All compounds have been investigated by using T‐dependent susceptibility measurements, and different types of spin transitions are observed. In the case of 1 and 2, intermediate plateaus (γHS ≈ 0.5) with widths of 110 and 75 K, respectively, were observed, while for 1(tol) a gradual spin transition is obtained. Upon heating of 1(tol) above 390 K, a loss of the included solvent molecule toluene is observed, which results in a two‐step spin transition of the tempered sample similar to that of 1. Mössbauer spectra of 1, 1(tol) and the tempered sample 1(temp) reveal two different iron(II) sites in the low‐spin (LS) state for all three samples. T‐dependent Mössbauer spectra indicate that the inequivalent iron centres are one but not the only reason for the wide plateau of 1. Results from X‐ray structure analysis of 1 in the region of the plateau show the presence of two inequivalent iron centres, of which one is in the high‐spin (HS) state and one in the low‐spin state. The 1D chains of alternating HS and LS iron centres are arranged in a parallel manner within one layer; between the different layers, the direction of the chains rotates by 90° relative to the previous one. This cross‐linked arrangement with an interlocking of the layers is most likely the reason for the appearance of the wide plateau.

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.…”

“…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.…”

“…[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.…”

Spiral Assembly of the 1D Chain Sheet of Fe(NCBH3)2(bpa)2·(biphenyl) (bpa = 1,2-bis(4-pyridyl)ethane) and its Stepwise Spin-Crossover Phenomenon

“…Recently, a two-step SCO was observed for several systems involving crystallographically unique metal center whatever the temperature. The occurrence of two-step spin crossover has been attributed to random disorder of the HS/LS species at the intermediate state in [30], to the special cross-linked arrangement of the 1D chains in [31], and to the interchain interactions in [32]. By comparing the present results in spiral assembly with those in 1D assembly, it is thought that the spiral assembly having a 3 1 screw axis causes the stepwise spin-crossover phenomenon.…”

Symmetry Breaking in Iron(II) Spin-Crossover Molecular Crystals

“…The crystal structure was solved in the IP, at 200 K, where the crystal is orthorhombic, space group P212121, and displays a perfect ordering HS-LS along the chains (Figure 12). In the LS state, the crystal changes to monoclinic P21 (130 K) but, unfortunately, complete analysis of the structure could not be performed due to twinning during the phase transition [66]. (Figure 12).…”