Thermally‐Induced Spin‐Crossover in Fe_1‐xT_x(pyrazine)[Fe(CN)₅NO] with T=Co, Ni – Effects of Iron Atom Dilution

Abstract

2D ferrous nitroprusside with the pyrazine molecule as a pillar between adjacent layers shows a thermally induced spincrossover behavior. This supposes that the energy gap between the high and low spin states is the order of kT in the temperature region where such a spin transition is observed. If a fraction of the iron atoms involved in that transition is progressively replaced by a second metal, to form a solid solution (alloy), the thermal effect could be modified due to the iron atom dilution, but such behavior also depends on the bonding properties of the ligands for the iron atom and of the NO and CN interaction in the interlayer region. This hypothesis was the idea that motivated the preparation of the titled systems of solid solutions and their study from magnetic and Mössbauer data recorded at different temperatures in the range of 5-300 K, complemented with XRD, Raman, and IR measurements. Co and Ni were considered to form the solid solutions under study because they form isostructural solids in the parent 3D coordination polymers. For both metals, the formed solids show a spin-crossover effect for all the compositions range but from a certain degree of dilution for the iron atom, above 75 % substitution by the second metal, the thermal hysteresis almost disappears. This suggests that from that dilution degree, the spin transition involves structural changes at the local coordination environment for the iron atom but not for the entire solid framework. This hypothesis is properly supported by the recorded Raman and Mössbauer spectra. The presence of Co and Ni atoms in the solid solutions results in higher stability for the entropy-driven high spin state. This is appreciated as lower values for T# HS , 1/2 , and T" LS , 1/2 corresponding to the temperatures where the subscript 1/2 indicates the temperature where 50 % of the metal centers have changed their spin state.

The Nature of the Atypical Kinetic Effects Observed for the Thermally Induced Spin Transition in Ferrous Nitroprussides with Short Organic Pillars

“…This has been illustrated for pyrazine as a pillar molecule in the series of solid solutions, Fe 1-x T x (Pyz)[Fe(CN) 5 NO] with T = Co, Ni. [21] The repulsive interaction effect in the recorded magnetic data for these solid solutions was found to be analogous to the above discussed for x = 0.0 (Figures S10 and S11). The repulsive interaction between adjacent layers, due to the CN-ON interaction, and its effect on the recorded magnetic data using different scan rate values, are independent of the iron atom dilution degree.…”

“…Ferrous nitroprussides can be prepared as solid solutions. This has been illustrated for pyrazine as a pillar molecule in the series of solid solutions, Fe 1‐x T x (Pyz)[Fe(CN) 5 NO] with T=Co, Ni [21] . The repulsive interaction effect in the recorded magnetic data for these solid solutions was found to be analogous to the above discussed for x=0.0 (Figures S10 and S11).…”

“…The TG curves and IR spectra are available in Figures S1 to S6 in Supplementary Information. The solid solutions Fe 1‐x T x (Pyz)[Fe(CN) 5 NO] with T=Co, Ni, were obtained by the same procedure [21] . The reference material, Fe(Pyz)[Pt(CN) 4 ], was prepared by the precipitation reaction from diluted solutions of Fe(SO 4 )(NH 4 ) 2 (SO 4 ) ⋅ 6H 2 O (Mohr salt), Pyrazine, and K 2 [Pt(CN) 4 ] in 1 : 1 : 1 molar ratio, aged for a week, and then separated, washed and dried as above indicated.…”

Variation of the Cooperativity in Diluted Hofmann-Based Spin-Crossover Coordination Solids {Fe1–xMx(pz)[Pd(CN)4]}

“…The variation of the cooperativity in SCO solids is an important focus of research and is particularly appealing in terms of controlling abruptness, hysteresis width, shape, and degree of completeness of the thermal SCO behavior and can be investigated in combination with the structural changes around the metal centers that propagates across the entire solid. ,, One of the most potent chemical tools to explore such variation is to dilute the SCO solid with diamagnetic and paramagnetic dopants (i.e., metal ions) that, on one side, control the strength of the interactions among the SCO centers ,,,,− and, on the other side, impact the HS → LS thermal equilibrium (thus, Δ E 0 HL between two spin manifolds) and the shape, width, completeness, and abruptness of the thermal hysteresis. Therefore, the influence of the metal dilution in the cooperative SCO solids is often considered significant.…”

“…In this regard, previous reports establish that the dilution with inert diamagnetic Zn-ions induces the so-called “ negative chemical pressure ”. Consequently, the transition temperature moves toward lower temperature with increasing Zn-ion concentration and becomes more gradual (i.e., exhibited by reduced hysteresis width that eventually vanishes) and incomplete at higher Zn concentration. ,,,− Most metal dilution studies are limited mainly to molecule-based mononuclear cooperative SCO solids ,,− ,− but rarely on the network-based structure reported so far. , In addition, systematic studies of the effect of different paramagnetic and diamagnetic ions (at various concentrations) on the variation of the cooperativity in network-based SCO solids are also absent in the literature. To accomplish such an objective, we herein report a combination of experimental and theoretical studies on the variation of the thermal spin-relaxation behavior (thus probing the variation of the cooperativity) in a model guest-free Hofmann-based cooperative SCO solid {Fe 1– x M x (pz)­[Pd­(CN) 4 ]} via metal dilution with the paramagnetic ions, M = Ni­(II), Co­(II), and Mn­(II) and diamagnetic ion, M = Zn­(II).…”