Structural Insights into Hysteretic Spin‐Crossover in a Set of Iron(II)‐2,6‐bis(1 H ‐Pyrazol‐1‐yl)Pyridine) Complexes

Spieker

Heinrich

et al. 2022

Preprint

Self Cite

An iron(II) complex (1·CH3CN) composed of a tridentate all nitrogen coordinating ligand—ethyl 2,6-bis(1H-pyrazol-1-yl)isonicotinate (BPP-COOEt)—shows bistable spin-state switching characteristic with thermal hysteresis width (ΔT1/2) = 44 K and switching temperature (T1/2) = 298 K in the first cycle. Crystal structures of the LS and HS forms of the complex reveal that spin-state switching induces a pronounced angular distortion, creating an energy barrier separating the LS and HS states. Traversing the barrier requires substantial molecular rearrangement in the presence of constraints imposed by the crystal lattice, rendering the spin-state switching of 1·CH3CN hysteretic in the solid-state. The rare observation of bistable SCO with T1/2 centred at room temperature (RT) renders the complex an ideal model system to study reversible spin-state tuning under ambient conditions.

Section: •Ch3cnmentioning

confidence: 62%

Section: Introductionmentioning

confidence: 70%

Section: •Ch3cnmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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An iron(II) complex shows bistable spin-state switching characteristic with T1/2 centred at room temperature

Spieker

Heinrich

et al. 2022

Preprint

Self Cite

“…First-row transition metal complexes featuring d 4 -d 7 electronic configuration, especially iron(II) complexes, that show reversible interconversion between the low-and high-spin states-termed as spin-crossover (SCO)-are a class of switchable materials that can undergo abrupt and hysteretic -bistable-spin-state switching suitable for device applications. [1][2][3][4] For practical device applications, hysteretic switching needs to be realized in the thin film state. Preparation of thin films of SCO complexes requires solution processing or vacuum sublimation.…”

Section: Introductionmentioning

confidence: 99%

Bistable spin-state switching characteristic of a charge-neutral iron(II) complex

Spieker

Heinrich

et al. 2022

Preprint

Self Cite

Spin-crossover (SCO) complexes that show abrupt and hysteretic spin-state switching characteristics—termed as bistable spin-state switching—are proposed suitable to realize molecule-based switching and memory elements. For realistic applications, spin-state switching needs to be demonstrated in the thin film state, requiring vacuum sublimation of SCO complexes to fabricate clean and impurity-free thin films. Charge-neutral iron(II) complexes are a class of SCO complexes that are reported to undergo sublimation, and their spin-state switching characteristics in the thin film state have been studied. However, hysteretic SCO in the thin film state is a scarcely observed phenomenon, requiring the development of iron(II) charge-neutral complexes that can undergo bistable spin-state switching in the bulk and thin film states. Herein, we report a new iron(II) charge-neutral complex—[Fe(H2Bpz2)24,4'-Br2-bpy] (H2Bpz2 = di¬hydro¬bis¬(pyrazol-1-yl)borate; 4,4'-Br2-bpy = 4,4'-dibromo-2,2'-bipyridine)—that undergoes abrupt and hysteretic spin-state switching in the bulk-state with T1/2 = 113 K and ΔT1/2 = 13 K at a scan rate of 0.25 K/min. The HS-to-LS switching of the complex is scan-rate-dependent, whereas the LS-to-HS switching is scan-rate-independent. Moreover, a reverse-SCO phenomenon was observed upon heating the sample in the 78 K-105 K temperature range at a scan rate of 3 K/minute. However, the reverse SCO was not observed when the complex was studied at scan rates of 1 and 0.5 K/min. Such observations indicate the presence of a kinetically trapped HS-fraction (frozen-in effect) during the HS-to-LS switching, when the sample was studied at the scan rate of 3 K/min. Crucially, the complex can be sublimed; efforts are on to elucidate the nature of SCO in the thin film state. Overall, a simple and easy to prepare sublimable complex—[Fe(H2Bpz2)24,4’-Br2-bpy]—shows bistable spin-state switching characteristics that can be leveraged to fabricate spin-state switchable thin film architectures.

Structural Insights into Hysteretic Spin‐Crossover in a Set of Iron(II)‐2,6‐bis(1H‐Pyrazol‐1‐yl)Pyridine) Complexes

Suryadevara

Mizuno

et al. 2022

Chemistry A European J

Self Cite

Bistable spin-crossover (SCO) complexes that undergo abrupt and hysteretic (ΔT 1/2 ) spin-state switching are desirable for molecule-based switching and memory applications. In this study, we report on structural facets governing hysteretic SCO in a set of iron(II)-2,6-bis62 K -is observed for 1, whereas 2 undergoes above-room-temperature lattice-solvent content-dependent SCO -T 1/2 = 331 K; ΔT 1/2 = 43 K. Variable-temperature single-crystal X-ray diffraction studies of the complexes revealed pronounced molecular reorganizations -from the Jahn-Teller-distorted HS state to the less distorted LS state -and conformation switching of the ethyl group of the COOEt substituent upon SCO. Consequently, we propose that the large structural reorganizations rendered SCO hysteretic in 1 and 2. Such insights shedding light on the molecular origin of thermal hysteresis might enable the design of technologically relevant molecule-based switching and memory elements.