Spin Crossover in 2D and 3D Networks with 4-R-1,2,4-Triazole Ligands: Insights to Multistep Transitions
(2025)
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SpinCrossoverin2Dand3DNetworkswith4-R-124-TriazoleLigandsInsightstoMultistepTransitions.pdf
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- Authors
- Supervisors
- Garcia, Yann
- Abstract
- Spin crossover (SCO) refers to the reversible switching between high-spin (HS) and low-spin (LS) states of transition metal complexes with 3d4-3d7 electronic configurations under external stimuli such as temperature, pressure, light irradiation, or guest molecules. This process is accompanied by pronounced changes in magnetic, optical, and structural properties, endowing SCO compounds with switchable functionality and tunability, and making them highly promising for applications in molecular storage, sensing, molecular electronics, and smart devices. In particular, high-dimensional (2D/3D) coordination polymers have emerged as a major focus in SCO research due to their rigid backbones and strong cooperative effects, often displaying abrupt transitions, wide thermal hysteresis, and even multistep spin transitions. Multistep SCO transitions, characterized by the stabilization of intermediate states, offer unique advantages such as multilevel information storage, multi-threshold sensing, and programmable responses to external stimuli. However, their underlying mechanisms are inherently complex, typically involving the interplay of crystallographic inequivalence, ligand-field variations, supramolecular interactions (hydrogen bonding, π–π stacking, metallophilic contacts), and lattice cooperativity. In this dissertation, classical 1,2,4-triazole derivatives were employed as the central design units to synthesize and characterize a series of 3D and 2D Fe(II) coordination polymers, with emphasis on elucidating the roles of counteranions, lattice solvents, and supramolecular interactions in regulating multistep SCO behavior. In 3D frameworks based on btr (4,4′-bis-1,2,4-triazole), three compounds of [Fe(btr)3]A2 (A = ReO4⁻ (1), PF6⁻ (2), OTf⁻ (3)) were synthesized, each exhibiting distinct SCO behaviors. Compound 1 shows a complete two-step transition arising from crystallographically inequivalent Fe(II) sites; compound 2 displays an incomplete two-step transition, likely due to counteranion disorder and lattice defects; while compound 3 remains locked in the HS state because of excessive framework flexibility and hydrogen-bonding constraints. These results highlight the decisive role of counteranions in modulating cooperativity and structural stability. Furthermore, introduction of the bulky NTf2- anion afforded both hydrated and desolvated phases, [Fe(btr)3](NTf2)2∙H2O (4) and [Fe(btr)3](NTf2)2 (5), representing the first solvent-containing 3D btr framework. The hydrated compound exhibits only partial spin conversion, whereas the desolvated phase undergoes a lattice reorganization and displays a distinct two-step transition with a 12 K thermal hysteresis, underscoring the competition between long-range cooperativity and short-range anticooperativity. In 2D Hofmann-type systems, a novel framework, Fe(MeOPhtrz)2[Au(CN)2]2 (6) (MeOPhtrz = methoxyphenyl-substituted triazole-imine), was constructed for the first time using linear [Au(CN)2]⁻ linkers. Supported by hydrogen bonding, π–π stacking, and Au···π interactions, the resulting anisotropic supramolecular architecture gives rise to an unprecedented three-step SCO transition. Furthermore, two new Hofmann-type frameworks, [Fe(L)2M(CN)4]·nH2O, denoted as 7(Pd)·nH2O and 7(Pt)·nH2O (M = Pd, Pt; n = 2, 1, 0), were synthesized with the ligand (E)-1-(((4H-1,2,4-triazol-4-yl)imino)methyl)naphthalen-2-ol (NapOHtrz). These compounds exhibit guest-dependent SCO behaviors: the hydrated phases display incomplete transitions, partial dehydration induces a reversible two-step transition with wide hysteresis, and complete dehydration results in a sharp, single-step transition near room temperature (subject to further structural and magnetic confirmation). A comprehensive investigation combining magnetic susceptibility, variable-temperature single-crystal and powder X-ray diffraction, 57Fe Mössbauer spectroscopy, differential scanning calorimetry, cryogenic optical microscopy, Raman spectroscopy, and diffuse reflectance measurements has established the synergistic regulation of SCO by counteranions, lattice solvents, and supramolecular interactions. From the perspectives of ligand design, counteranion selection, solvent incorporation, and supramolecular engineering, this work proposes a general mechanistic picture for constructing multistep SCO materials. The results demonstrate that by tuning lattice flexibility, pore chemistry, and host–guest interactions, one can achieve programmable control over spin-state switching within the same family, ranging from incomplete transitions to two-step and even three-step (or wide-hysteresis) behavior. Overall, this dissertation not only expands the structural and property landscape of high-dimensional triazole-based Fe(II) SCO frameworks but also provides new insights into the microscopic mechanisms underlying multistep transitions. The findings show that rational integration of counteranion effects, solvent environments, and supramolecular interactions offers a viable strategy for designing programmable, multi-stimuli-responsive spin-functional materials, thereby laying the foundation for their potential applications in high-density molecular storage, smart sensing, and switchable molecular devices.
- Affiliations
Citations
Li, X. (2025). Spin Crossover in 2D and 3D Networks with 4-R-1,2,4-Triazole Ligands: Insights to Multistep Transitions. https://hdl.handle.net/2078.5/270845