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Physical Properties of 2D Spin-Crossover Solids from an Electro-Elastic Description: Effect of Shape, Size, and Spin-Distortion Interactions
Published in Jean-Claude Levy, Magnetic Structures of 2D and 3D Nanoparticles, 2018
Kamel Boukheddaden, Ahmed Slimani, Mouhamadou Sy, Franc¸ois Varret, Hassane Oubouchou, Rachid Traiche
[Fe(NCSe)(py)22(m-bpypz)], where py = pyridine and bpypz = 3,5-bis(2-pyridyl)-pyrazolate, hereafter abbreviated Fe(NCSe), is an archetype of cooperative spin crossover compounds, that is, it exhibits a first-order transition between low-temperature LS and HS states. The molecular structure of the Fe(NCSe) is presented in Fig. 9.1. The molecule is constituted of two metal centers connected by bridging ligands. Upon conversion from LS to HS, the Fe-ligands distances increase by ∼10% and the distance between the two iron centers within the binuclear unit by 5.5%, which shows that even at the intramolecular level, the structure absorbs a part of the Fe-ligand expansion. The occurrence of first-order transition with hysteresis is assigned to the presence of elastic interactions between the spin-crossover molecular units. The switching between spin states can be triggered by several means such as temperature and pressure variations, irradiation in the visible range, pulsed magnetic field, ligand photo-isomerization [1–10]. It can be monitored by various techniques due the large changes in the physical properties (structure, volume, color, magnetization, dielectric moment, specific heat, etc.) and spectroscopic properties (optical, Raman, Mössbauer) under the effect of the spin transformation at the molecular and lattice levels. Detailed and comprehensive information can be found in some recent reviews [1]. The transformation from the diamagnetic LS state to the paramagnetic HS state of Fe(NCSe) is illustrated in Fig. 9.2, with a color change consistent with the reported optical properties. The structural changes have been analyzed in detail by X-ray diffraction on single crystals showing a volume increase of ∼10% upon the LS to HS transformation. However, a fine analysis showed that the change in the cell parameters upon transition was highly anisotropic. Indeed, while b and c increase at the transition from LS to HS, a parameter shrinks.
Iron(II) complex of 2-(1H-pyrazol-1-yl)pyridine-4-carboxylic acid (ppCOOH) suitable for surface deposition
Published in Journal of Coordination Chemistry, 2018
Víctor García-López, Mario Palacios-Corella, Miguel Clemente-León, Eugenio Coronado
Spin-crossover (SCO) complexes are promising building blocks for spintronic and high-density memory devices as they contain a transition-metal ion that can be reversibly switched between two distinct spin states, low-spin (LS) and high-spin (HS) with external stimuli, such as temperature, light, pressure, magnetic, or electric field [1]. In the last years there is increasing interest in charge transport properties of SCO compounds for their incorporation into devices [2–4]. In this context, the deposition of SCO molecules onto surfaces has stimulated research focused mostly on ultrahigh vacuum deposition of thermally evaporable neutral SCO complexes [3,5–8]. Solution-based approaches, such as self-assembly methodologies, have been less explored. In previous work, we studied the self-assembled monolayers (SAM) of iron(II) complexes of the tridentate ligand bppCOOH (see Scheme 1) [9], which belongs to the well-known family of [FeII(bpp)2]2+ (bpp = 2,6-bis(pyrazol-1-yl)pyridine) SCO complexes [10]. The carboxylic acid group of this ligand enables the deposition of the complex on SiO2 and Al2O3 substrates [11] and the binding to metal ions leading to a nonanuclear complex containing six iron(II) complexes and an iron(III) trimer [12].
Phase behaviour and magnetocaloric effect of poly(propylene imine) Iron(III) dendromesogen of the third generation
Published in Liquid Crystals, 2021
V.V. Korolev, M.S. Gruzdev, A.G. Ramazanova, O.V. Balmasova, U.V. Chervonova
The temperature dependence of the MCE has an extreme appearance (Figure 13). In the temperature range of 340–350 K, a sharp maximum is observed at 345 К. The MCE has a maximum value at magnetic phase transition temperatures [27]. The presence of extreme temperature dependence of the MCE of the sample at 345 К indicates a magnetic phase transition. Thus, the temperature dependences of the MCE of iron (III) containing complex of the third-generation dendrimer (Figure 13) were obtained by us for the first time. This conclusion is in a good agreement with the EPR and Mossbauer spectroscopy data presented in [33]. According to [33] complex of poly(propylene imine) dendrimer of the third generation with iron(III) exhibits spin-crossover properties, i.e. under the external influence, the transition occurs between high spin and low-spin states as noted above. Based on the temperature dependence of the specific heat capacity it was established that the supramolecular aggregation of complex molecules in Colh mesophase under temperature during the ‘crystal – mesophase’ transition is determined by the organic part of the molecule (Figure 10). The ordering of the iron coordination sites due to the columnar packing of macromolecules leads to an increase in the magnetocaloric effect of the studied complex. Thereby, the crystal – mesophase structural transition correlates with the magnetic phase transition which is represented as a sharp maximum in the temperature dependencies of the MCE and specific heat capacity of the complex (Figure 10, Figure 13). The correlation between the magnetic phase transition and the ‘crystal – mesophase’ transition of the 3-K2.10-(FeCl3)9 complex was discovered for the first time by us. Thus, as a result, the problem of creating a mesogenic complex of a 3d element with variable spin properties was solved.
Review: Downsizing effect on 2-D and 3-D spin crossover metal-organic frameworks
Published in Journal of Coordination Chemistry, 2019
Christina D. Polyzou, Vassilis Tangoulis
The cooperative nature of spin conversion leads in some cases to hysteretic behavior in microscale as a result of the effectiveness of the intermolecular interactions. The first attempt to correlate in detail cooperativity with structure was made with [Fe(phen)2(NSC)2] (phen: 1,10-phenanthroline) [3], where the determining role in the cooperative character of spin-transition was the interactions between aromatic phen belonging to adjacent complexes. Nevertheless, due to the difficulties of handling and controlling the intermolecular interactions, the alternative path of the synthesis of coordination polymers was chosen in order to achieve cooperativity in SCO compounds. During the 80s and 90s, a series of publications were reported related to construction of 1-D and 2-D coordination polymers with small and rigid triazole ligands. The 1-D polymers [Fe(NH2trz)3](NO3)2 and [Fe(Htrz)2(trz)](BF4) (NH2trz: 4-amino-1,2,4-triazole, Htrz: H-1,2,4-triazole and trz: its deprotonated form) exhibited hysteresis widths around 35 K and 38 K, respectively [4], in much higher transition temperatures than in conventional Fe(II) spin crossover complexes. In the case of 2-D polymer [Fe(btr)2(NCS)2].H2O (btr: 4,4′-bis-1,2,4-triazole) the hysteresis width was 21 K but the transition temperatures were lower than the previous ones [5]. Two years later, a new 2-D bimetallic coordination polymer [Fe(py)2Ni(CN)4] (py: pyridine) was reported exhibiting SCO with a respectable thermal hysteresis width (ca. 10 K) at relatively high temperatures [6], while in 1999 the first 3-D spin crossover coordination polymer with a triazole derivative and formula [Fe(btr)3(ClO4)2] was reported by Tuchagues and coworkers exhibiting a two-step spin-transition but with a very small hysteresis width (3 K) [7]. A few years later, Real and his research group introduced the first 3-D bimetallic cyano-bridged coordination polymer [Fe(pz)MII(CN)4].nH2O (pz: pyrazine, M = Ni, Pd, Pt) with cooperative spin crossover behavior and hysteresis widths between 20 and 33 K close to room temperature [8]. The latter compound opened the way for encapsulation of aromatic organic molecules and for the construction of new 3-D bimetallic FeII/MII coordination polymers with various rigid azo-organic ligands exhibiting cooperativity with one- or multi-step hysteretic behavior.