Synthesis and Characterization of a Dinuclear Iron(II) Spin Crossover Complex with Wide Hysteresis

Abstract

The magnetic properties and results from X-ray structure analysis for a new pair of iron(II) spin-crossover complexes [FeL1(meim) 2](meim) ( 1(meim)) and [Fe 2L2(meim) 4](meim) 4 ( 2(meim) 4), with L1 being a tetradentate N 2O 2 (2-) coordinating Schiff-base-like ligand [([3,3']-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentane-dionato)(2-)N,N',O (2),O (2)'], L2 being an octadentate, dinucleating N 2O 2 (2-) coordinating Schiff-base-like ligand [3,3',3'',3''']-[1,2,4,5-phenylenetetra(iminomethylidyne)]tetra(2,4-pentanedionato)(2-) N, N', N'', N''', O (2), O (2) ', O (2) '', O (2) '''], and meim being N-methylimidazole, are discussed in this work. Crystalline samples of both complexes show a cooperative spin transition with an approximately 2-K-wide thermal hysteresis loop in the case of 1(meim) ( T 1/2 increase = 179 K and T 1/2 decrease = 177 K) and an approximately 21-K-wide thermal hysteresis loop in the case of dinuclear complex 2(meim) 4 ( T 1/2 increase= 199 K and T 1/2 decrease= 178 K). For a separately prepared powder sample of 2, a gradual spin transition with T 1/2 = 229 K is observed that was additionally followed by Mossbauer spectroscopy. The results from X-ray structure analysis give a deeper insight into the molecule packing in the crystal and, by this, help to explain the increase of cooperative interactions during the spin transition when going from the mononuclear to the dinuclear complex. Both compounds crystallize in the triclinic space group P1, and the X-ray structure was analyzed before and after the spin transition. The change of the spin state at the iron center is accompanied by a change of the O-Fe-O angle, the so-called bite of the equatorial ligand, from about 109 degrees in the high-spin state to 89 degrees in the low-spin state. The cooperative interactions responsible for the thermal hysteresis loop are due to elastic interactions between the complex molecules in both cases. However, due to the higher symmetry of the dinucleating ligand in 2(meim) 4, a 3D network of short contacts is formed, while for mononuclear complex 1(meim), a 2D layer of linked molecules is observed. The spin transition was additionally followed in solution using (1)H NMR spectroscopy for both complexes. In both cases, a gradual spin transition is observed, and the increase of cooperative interactions when going from the mononuclear to the dinuclear system is solely attributed to the extended network of intermolecular contacts.

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

“…Discontinuous spin transitions were recently associated with order/disorder transitions of counter ions 25 or additional ligand molecules 26 in the crystal packing. The order/disorder transition at a certain temperature facilitated a significant increase in the number of intermolecular contacts below this Hydrogen atoms were omitted for clarity.…”

High-Temperature Wide Thermal Hysteresis of an Iron(II) Dinuclear Double Helicate

“…1.5·cm 3 ·K mol −1 at 379 K. Then, the χMT value further decreases gradually to reach ca. 0.5 cm 3 ·K·mol −1 at 300 K. The critical temperatures of the warming (Tc↑) and cooling (Tc↓) modes in the abrupt spin transition area are 485 and 401 K, respectively, indicating the occurrence of ΔT = 84 K, which is the widest thermal hysteresis loop in the dinuclear system reported so far [26][27][28][29]33,34,48]. It is also noteworthy that the Tc↑ of 1·AsF6 is considerably higher than that of the related ethylene-bridged helicate [Fe II 2(L2 C2 )2](PF6)4·5H2O·MeCN (Tc↑ = 437 K) [34] by about 48 K. …”

“…In one of these studies, we reported the SCO iron(II) dinuclear double helicate for the first time [34]. This compound, with the formula [Fe II 2 (L2 C2 ) 2 ](PF 6 ) 4 ·5H 2 O·MeCN, has two bis-tridentate type ligand strands (L2 C2 ) in which two 1,2,3-triazolimine moieties of the ligand strand are bridged by an ethylene chain (Scheme 1), and shows an anomalous two-step SCO with 11 K-wide hysteresis in a second step centered at 432 K, which is the highest hysteretic transition temperature in the dinuclear SCO system reported so far [26][27][28][29]33,48].…”

“…arising from the combination of the SCO metal center and other metal centers [2,[19][20][21][22][23][24]. In particular, dinuclear species have received much attention since they are a simple model for revealing intra-and intermolecular interactions for enhancing cooperativity [25][26][27][28][29][30][31][32][33][34]. From a more practical point of view, the importance of studying the reproducible nature and scan rate dependence of the hysteresis loop and high-temperature SCO with sufficient thermal stability has recently been pointed out in order to define the limiting characteristics of SCO materials [33,[35][36][37][38][39].…”

First principles calculation of a large variation in dielectric tensor through the spin crossover in the CsFe[Cr(CN)6] Prussian blue analogue

“…↓ is strongly lattice-dependent. It is mediated and controlled by the elastic interactions between SCO-active centers in the crystal structure [11][12][13][14], and should be controllable by a process of rational materials design and synthesis [15][16][17].…”