Structures and Spin States of Bis(tridentate)-Type Mononuclear and Triple Helicate Dinuclear Iron(II) Complexes of Imidazole-4-carbaldehyde azine

Abstract

Mononuclear [Fe(H(2)L(R))(2)](2+) and dinuclear [Fe(2)(H(2)L(R))(3)](4+) (R = H, 2-Me, 5-Me) complexes containing the new imidazole-4-carbaldehyde azine ligand (H(2)L(H)) and its derivatives (H(2)L(2-Me) and H(2)L(5-Me)) prepared from the condensation reaction of 4-formylimidazole or 2-methyl- or 5-methyl-4-formylimidazole with hydrazine (2:1) were prepared, and their magnetostructural relationships were studied. In the mononuclear complexes, H(2)L(R) acts as an unsymmetrical tridentate ligand with two imidazole nitrogen atoms and one azine nitrogen atom, while in the dinuclear complexes, H(2)L(R) acts as a dinucleating ligand employing four nitrogen atoms to form a triple helicate. At room temperature, [Fe(2)(H(2)L(H))(3)](ClO(4))(4) and [Fe(2)(H(2)L(2-Me))(3)](ClO(4))(4) were in the high-spin (HS) and low-spin (LS) states, respectively. The results are in accordance with the ligand field strength of H(2)L(2-Me) with electron-donating methyl groups being stronger than H(2)L(H), with the order of the ligand field strengths being H(2)L(2-Me) > H(2)L(H). However, in the mononuclear [Fe(H(2)L(H))(2)](ClO(4))(2) and [Fe(H(2)L(2-Me))(2)](ClO(4))(2) complexes, a different order of ligand field strengths, H(2)L(H) > H(2)L(2-Me), was observed because [Fe(H(2)L(H))(2)](ClO(4))(2) was in the LS state while [Fe(H(2)L(2-Me))(2)](ClO(4))(2) was in the HS state at room temperature. X-ray structural studies revealed that the interligand steric repulsion between a methyl group of an H(2)L(2-Me) ligand and the other ligand in [Fe(H(2)L(2-Me))(2)](ClO(4))(2) is responsible for the observed change in the spin state. Two kinds of crystals, needles and blocks, were isolated for [Fe(2)(H(2)L(H))(3)](BF(4))(4), and both exhibited a sharp spin transition, [LS-HS] <--> [HS-HS]. The spin transition of the block crystals is more abrupt with a hysteresis, T(c) upward arrow = 190 K and T(c) downward arrow = 183 K with DeltaT = 7 K.

Guest‐, Light‐ and Thermally‐Modulated Spin Crossover in [FeII2] Supramolecular Helicates

“…These species are amenable to rational design; the right choice of metals and polydentate ligands allows the prediction and formation of helicates with different numbers of metals and strands . In addition, with the right choice of ligand donors, it is possible to chemically tune the crystal field around the metals (e.g., Fe II ) of the supramolecular assembly in order to facilitate the occurrence of SCO . Some of the reported examples show evidence that the spin‐active centers can be brought from the LS to a metastable HS state in response to light irradiation by the LIESST (light‐induced excited spin‐state trapping) effect .…”

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](PF6)4·5H2O·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].…”

Synthesis and reactivity of a 4His enzyme model complex

“…This behavior is consistent with low symmetry at the iron center. 52-54 The data was simulated using the program julX 21 which gave a very good fit using g = 2.24 and D = −0.80 cm −1 which is typical for distorted octahedral geometry. 55 …”