Structural−Electronic Correlation in the First-Order Phase Transition of [FeH₂L^2-Me](ClO₄)₂ (H₂L^2-Me = Bis[((2-methylimidazol-4-yl)methylidene)-3- aminopropyl]ethylenediamine)

Abstract

The synthesis and detailed study of the new mononuclear spin crossover complex [Fe(II)H2L(2-Me)](ClO4)2 (where H2L(2-Me) = bis[((2-methylimidazol-4-yl)methylidene)-3-aminopropyl]ethylenediamine) are reported. Variable-temperature magnetic susceptibility measurements show the occurrence of a steep spin crossover centered at 171.5 K with a hysteresis loop of ca. 5 K width (T(/2)(increasing) = 174 K and T(1/2)(decreasing) = 169 K, for increasing and decreasing temperatures, respectively). The crystal structure has been resolved for the high-spin (HS) and low-spin (LS) states at 200 and 123 K, respectively, revealing a crystallographic phase transition that occurs concomitantly to the spin crossover: at 200 K, the complex crystallizes in the monoclinic system, space group P2(1)/n, while the space group is P2(1) at 123 K. The mean Fe-N distances are shortened by 0.2 A, but the thermal spin crossover is accompanied by significant structural changes: the rearrangement of the central atom C12 of a six-membered chelate ring of [Fe(II)H2L(2-Me)]2+ to two positions (C12A and C12B) and, consequently, the lack of an inversion center at 123 K (P2(1) space group). Both HS and LS supramolecular structures involve all possible hydrogen bonds between imidazole and amine NH functions, and perchlorate anions; however, the HS supramolecular structure is a one-dimensional (1D) network, and the LS phase may better be described as a two-dimensional (2D) extended structure of A and B molecules. The structural phase transition of [FeH2L(2-Me)](ClO4)2 seems to trigger the steep and hysteretic spin crossover. Discontinuities in the temperature dependence of the Mössbauer parameters (isomer shift and quadrupole splitting) at the spin crossover temperature confirmed the occurrence of a structural phase transition. The experimental enthalpy and entropy variations were determined by differential scanning calorimetry (DSC) as 7.5 +/- 0.4 kJ/mol and 45 +/- 3 J K(-1) mol(-1), respectively. The regular solution theory was applied to the experimental data, yielding an interaction parameter of Gamma = 3.36 kJ/mol, which is larger than 2RT(1/2), which fulfills the condition for observing hysteresis.

Nanoscale Self‐Hosting of Molecular Spin‐States in the Intermediate Phase of a Spin‐Crossover Material

“…Synthesis and characterization : Complex 1 was prepared through anaerobic reaction of freshly synthesized H 2 L 2‐Me Schiff‐base ligand21 with Fe II Cl 2 ⋅ 4 H 2 O in ethanol, followed by anion metathesis20 with NaAsF 6 . In this way, 1 was anaerobically obtained exclusively as orange single crystals, which were deposited from the filtered reaction medium.…”

Influence of Hydrogen Bonding on the Hysteresis Width in Iron(II) Spin‐Crossover Complexes

“…[12] A structural phase transition and the associated modification of the hydrogen-bond network are discussed as being responsible for the step ST with hysteresis of the compound FeH 4 L4. [15] Significant differences in the transition temperature of [FeH 2 L5(py) 2 ]·py in the solid state (T 1/2 Ͼ 350 K) and in solution (T 1/2 ≈ 200 K, both gradual spin transitions) were also associated with the hydrogen-bond network observed in the X-ray structure. [11] Such observations are not limited to monomeric complexes.…”

“…Several examples demonstrate an influence of the presence or absence of hydrogen bonds on both the transition temperature [10][11][12][13] as well as on cooperative effects. [9,[13][14][15] For some of the examples, an influence of the hydrogen bonds on the electron density of donor atoms and by this on the ligand field strength is discussed. [10][11][12][13] To date, however, no consistent model for the explanation of the different effects is available.…”

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

“…5 K wide and simultaneous crystallographic phase transition between the space groups P2 1 /n (HS) and P2 1 (LS) [50].…”