Large Thermal Hysteresis for Iron(II) Spin Crossover Complexes with N-(Pyrid-4-yl)isonicotinamide

Abstract

A new series of iron(II) 1D coordination polymers with the general formula [FeL1(pina)]·xsolvent with L1 being a tetradentate N2O2(2-) coordinating Schiff-base-like ligand [([3,3']-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentanedionato)(2-)-N,N',O(2),O(2)'], and pina being a bridging axial ligand N-(pyrid-4-yl)isonicotinamide, are discussed. The X-ray crystal structure of [FeL1(pina)]·2MeOH was solved for the low-spin state. The compound crystallizes in the monoclinic space group P21/c, and the analysis of the crystal packing reveals the formation of a hydrogen bond network where additional methanol molecules are included. Different magnetic properties are observed for the seven samples analyzed, depending on the nature of the included solvent molecules. The widest hysteresis loop is observed for a fine crystalline sample of composition [FeL1(pina)]·xH2O/MeOH. The 88 K wide thermal hysteresis loop (T1/2↑ = 328 K and T1/2↓ = 240 K) is centered around room temperature and can be repeated without of a loss of the spin transition properties. For the single crystals of [FeL1(pina)]·2MeOH, a 51 K wide hysteresis loop is observed (T1/2↑ = 296 K and T1/2↓ = 245 K) that is also stable for several cycles. For a powder sample of [FeL1(pina)]·0.5H2O·0.5MeOH a cooperative spin transition with a 46 K wide hysteresis loop around room temperature is observed (T1/2↑ = 321 K and T1/2↓ = 275 K). This compound was further investigated using Mössbauer spectroscopy and DSC. Both methods reveal that, in the cooling mode, the spin transition is accompanied by a phase transition while in the heating mode a loss of the included methanol is observed that leads to a loss of the spin transition properties. These results show that the pina ligand was used successfully in a crystal-engineering-like approach to generate 1D coordination polymers and improve their spin crossover properties.

Solvent Influence on the Magnetic Properties of Iron(II) Spin‐Crossover Coordination Compounds with 4,4′‐Dipyridylethyne as Linker

“…As has been shown previously, [8][9][10][11][12][13][14][15][16] the solvent has a significant influence on the SCO behaviour of this type of compound. Depending on the solvent used and preparation method, different iron(II) complexes were obtained.…”

“…The solvent dependence of SCO behaviour has already been described for this type of compound but with N-(4-pyridyl)isonicotinamide as the axial ligand. In contrast to this linker, which has an amide group that can easily form hydrogen bonds with solvent molecules or neighbouring complexes, [15] bpey is a ligand that has no strong hydrogen-bond donor or acceptor atoms. Similar complexes with 4,4′-bipyridine as axial ligand show hysteretic SCO behaviour, but have no solvent included in the crystal packing.…”

“…These regions were determined by using Bragg's law (sin θ = λ/2d) and the results obtained for similar 1D chain compounds have already demonstrated that even changes in the Fe-Fe distances in the chain due to the spin transition can be explained (larger θ → smaller d and vice versa). [15] The strong diffraction peaks in the region between 7 and 10°most likely arise from iron centres with a high electron density. Additionally, for most of the 1D coordination polymers synthesised by our group, the polymer chains are parallel, supporting a diffraction pattern dominated by inter-chain iron-iron distances.…”

“…[7] For the complexes investigated by our group, this is most commonly observed for coordination polymers, for which different solvents used in the synthesis often lead to different magnetic properties. [8][9][10][11][12][13][14][15][16] In contrast, such changes are rare for mononuclear complexes. [17] So far, how-method but in different solvents led to the same compound exhibiting the same three-step spin crossover independent of the magnetic properties of the solvated species.…”

High-Temperature Wide Thermal Hysteresis of an Iron(II) Dinuclear Double Helicate

“…There is a need for SCO materials which are capable of abrupt and complete HS-LS interconversion at around room temperature (RT) with wide thermal hysteresis width (∆T) for potential applications in molecular memories, switches, and display devices. As such, various cooperative SCO compounds have been produced with supramolecular assembly via intermolecular hydrogen bonding [8][9][10] and π-π stacking [11][12][13], or coordination polymeric architecture by using bridging ligands [14][15][16][17][18]. Multinuclear clusters have also been widely studied in the hope of developing multistable compounds with more accessible spin states towards denser information storage, or hybrid SCO materials with multifunctional properties (e.g., SCO with charge transfer, magnetic coupling, luminescence, etc.)…”