Bistable Hofmann-Type Fe^II Spin-Crossover Two-Dimensional Polymers of 4-Alkyldisulfanylpyridine for Prospective Grafting of Monolayers on Metallic Surfaces

Abstract

Aiming at investigating the suitability of Hofmann-type two-dimensional (2D) coordination polymers {Fe II (L ax ) 2 [M II (CN) 4 ]} to be processed as single monolayers and probed as spin crossover (SCO) junctions in spintronic devices, the synthesis and characterization of the M II derivatives (M II = Pd and Pt) with sulfur-rich axial ligands (L ax = 4-methyl- and 4-ethyl-disulfanylpyridine) have been conducted. The thermal dependence of the magnetic and calorimetric properties confirmed the occurrence of strong cooperative SCO behavior in the temperature interval of 100–225 K, featuring hysteresis loops 44 and 32.5 K/21 K wide for Pt II -methyl and Pt II /Pd II -ethyl derivatives, while the Pd II -methyl derivative undergoes a much less cooperative multistep SCO. Excluding Pt II -methyl, the remaining compounds display light-induced excited spin-state trapping at 10 K with T LIESST temperatures in the range of 50–70 K. Single-crystal studies performed in the temperature interval 100–250 K confirmed the layered structure and the occurrence of complete transformation between the high- and low-spin states of the Fe II center for the four compounds. Strong positional disorder seems to be the source of elastic frustration driving the multistep SCO observed for the Pd II -methyl derivative. It is expected that the peripheral disulfanyl groups will favor anchoring and growing of the monolayer on gold substrates and optimal electron transport in the device.

Two-Dimensional Coordination Polymer Showing Spin-Crossover Behavior with a 64 K Wide Hysteresis Loop

“…In the third cycle, compound 1·4CHCl 3 remained in the HS state between 250 and 160 K, completing one-step abrupt spin transition as the temperature decreased and reaching the fully populated LS state at 60 K, thus a clear occurrence of temperature-induced one-step complete SCO from HS to LS − with T 1/2 ↓ equal to 143 K is concluded. When the temperature rises, compound 1·4CHCl 3 remains in the LS state until 200 K, then there is an abrupt rise in the curve to the HS state before 225 K with T 1/2 ↑ being 207 K. The resulting W hys value is 64 K, unexpected among the largest values for 2D SCO compounds. − The hysteretic transition is further confirmed by the differential scanning calorimetry (DSC) data (Figure S2). The exothermic and endothermic peaks for compound 1·4CHCl 3 roughly locate at 141 and 199 K, respectively, which result in a lower W hys value (approximately 58 K) than that obtained from magnetic measurements.…”

“…The second method has been well documented in Fe­(II)-1,2,4-triazole SCO CPs − and Hofmann-type SCO materials. , In addition, we notice that 2D SCO CPs formed with rigid bis-monodentate ligands have the potential to show relatively large W hys values in a unconventional way . For example, [Fe­(NCS) 2 (4,4′-bis-1,2,4-triazole) 2 ]·H 2 O showed SCO behavior with a W hys value of 21 K, which had been attributed to the rotation of the triazole plane. , However, to our knowledge, all 2D SCO Fe­(II) CPs exhibited W hys values that were up to date less than 50 K, no matter how to achieve by means of temperature variation, solvent induction, light irradiation, pressure, and downsizing, etc. − In this paper, we report a 2D CP that shows one-step complete SCO behavior with unexpectedly large W hys value of 64 K, and additionally this CP also shows temperature-induced excited spin-state trapping effect (TIESST) with the T TIESST value equal to 91 K.…”

Iron(II) Mediated Supramolecular Architectures with Schiff Bases and Their Spin-Crossover Properties

“…The supramolecular complexes in hysteresis spin switching can exist in two stable electronic states. The bi-stability of SCO supramolecular complexes, which shows a relatively large thermal hysteresis (ΔT ≥ 45 K) centered around room temperature (RT), is desired for various applications—especially for obtaining molecule-based switching and memory devices [ 34 , 35 ].…”

Modulation of the spin transition in 2D Hofmann frameworks via π⋯π stacking between the axial 2,5-dipyridyl-1,3,4-oxadiazoles

“…5–11 Among them, Fe 2+ Hofmann-type SCO frameworks are one of the most notable systems. 6,10–35 These frameworks generally contain neutral bimetallic layers consisting of Fe 2+ ions and [M(CN) 4 ] 2− anions (M = Pt 2+ , Pd 2+ , Ni 2+ ) in the equatorial plane, which allow a large number of linear N-donor ligands to coordinate axially to the Fe 2+ centers in order to produce 2D/3D Hofmann SCO materials. 10 If the linear exo -bidentate ligands are used as pillars, 3D porous Hofmann SCO frameworks can be constructed, 11 while 2D Hofmann SCO networks will be built if the axial ligands are merely monodentate.…”

“…12 Since the monodentate ligand is always more readily available than the bidentate one, the 2D Hofmann SCO networks can be more easily constructed, which will provide a good opportunity for exploring the influence of intermolecular interactions (hydrogen bonding, π⋯π stacking and interdigitated effect) on the SCO behavior. 12–35 So far, most of the monodentate axial ligands used in 2D Hofmann SCO networks are substituted pyridine derivatives, owing to their cheap and easy large-scale preparation. Moreover, pyridine derivatives modified with small substituent groups (–F, –Cl, –Br, –I, –NH 2 , etc. )…”