Modification of magnetocaloric properties upon a change in the spin state of iron(III) in tetrapyrrole paramagnets

Highlights

• Magnetocaloric behavior of chloride iron(III) 5,10,15,20-tetraphenylporphin and dimethylformamide iron(III) mesoporphyrin IX were studied.

• The paramagnets display the positive a room-temperature magnetocaloric effect.

• MCE values are associated with the Fe(III) spin state and the exchange between a paramagnetic ligand and central ion.

Abstract

Porphyrin complexes of 3d-metals with an open electronic shell are ranked as molecular materials with both electronic functionality and paramagnetic behavior due to exhibiting a positive magnetocaloric effect (MCE) at the temperature close to the room. We have determined MCE, heat, and an enthalpy/entropy change during magnetization of chloride ligated pentacoordinated iron(III) 5,10,15,20-tetraphenylporphin, (Cl)FeTPP and dimethylformamide ligated sixcoordinated iron(III) mesoporphyrin IX, [(DMF)₂FeMP]⁺Cl^- at 278–338 K in magnetic fields from 0 to 1.0 T by the direct microcalorimetric method. The specific heat capacity in solid (Cl)FeTPP/[(DMF)₂FeMP]⁺Cl^- was directly determined depending on the temperature in zero magnetic fields using DSC. To improve understanding of the correlation between magnetic properties of the iron(III) complexes and its spin state, we have compared the magnetic behavior of paramagnets studied with those for manganese(III) porphyrins. Both the iron(III) spin state and the exchange (ferromagnetic or antiferromagnetic depending on functional substitution in a complex) between a paramagnetic ligand and the central ion are reflected in the magnetocaloric behavior of iron(III) porphyrins studied.

Introduction

Porphyrin complexes of 3d-metals including iron with an open electronic shell are ranked as molecular materials with both electronic functionality [1], [2] and paramagnetic behavior [3], [4]. The μ_eff of 5.96 at the room temperature (4.2 μ_B at 2 K) observed for high-spin chloride ligated iron(III) porphyrins with the square-pyramidal coordination center and a planar macrocycle [1] indicates the large zero-field splitting of the ⁶A₁ ground state, which causes magnetic properties of these complexes. The zero-field splitting value, D 6.33(8) cm⁻¹ was reported for tetragonal polycrystalline chloride ligated iron(III) 5,10,15,20-tetraphenylporphin [5]. Depending on substitution in macrocycle meso/β positions, the anomalous difference in the magnetic behavior of iron(III) porphyrins was established. The strong ferromagnetic coupling between the spins of iron and π-radical of oxidized porphyrin or the antiferromagnetic coupling between the ring radicals was observed [6]. A spin-crossover between the intermediate and low-spin states when the temperature is lowered was revealed in iron(III) porphyrins with two axial pyridine/4-CN-pyridine molecules instead of the chloride anion and with the specific set of substituents [7]. There is the transition from the high-spin S = 5/2 state to the intermediate-spin S = 3/2 one, the pure intermediate-spin S = 3/2 state or the high-spin S = 5/2 state in the saddle-shaped sixcoordinated complex in the case of other types of substitution in macrocycle meso-positions or periphery [8]. In a whole, highly distorted sixcoordinated iron(III) porphyrins with ruffled or saddled structures exhibit spin-crossover behavior [7], [9], [10]. Such complexes in which the varying temperature, pressure, and exposure to light can lead to changes of the spin state of transition metal atoms are of particular interest as materials for displays and memory systems [11], [12], [13]. As for iron(III) porphyrins, the possibility of optical control of the spin carrier magnetic state may provide a way to opto-magnetic materials for sensing, solar energy conversion, and information technologies [8].

At present, the study of the magnetocaloric effect is the actual task for energy transformations, applications in refrigeration, heat pumping, and energy generation [14]. Works on this topic appear more often in the world literature [14], [15], [16], [17]. A change of internal entropy ∆S of a magnetic material by magnetization under adiabatic condition was most often obtained from magnetic characteristics, for example from magnetization, from which the MCE is then calculated. Many works were completed in the range up to 100 K [18]. There are comparatively few works devoted to the direct method of obtaining the MCE in the range of room temperatures [19].

We address the issue to explore how the magnetocaloric properties like a magnetocaloric effect (MCE) and the thermodynamic parameters during magnetization of iron(III) porphyrins (Fig. 1) change if the transition from the withdrawing substitution in a macrocycle to opposite substitution is performed, i.e., we have studied the possibility of controlling the magnetocaloric behavior of paramagnets based on d-metal porphyrins by modifying the molecular structure. This has been done by performing direct measurements of MCE and specific heat capacity of chloride ligated pentacoordinated iron(III) 5,10,15,20-tetraphenylporphin, (Cl)FeTPP and dimethylformamide ligated sixcoordinated iron(III) mesoporphyrin IX dimethyl ether, [(DMF)₂FeMP]⁺Cl^- using the microcalorimetric method described in the article [2]. Their samples were taken over the temperature range of 273–328 K and in magnetic fields from 0 to 1.0 T. The specific heat capacity in solid (Cl)FeTPP/[(DMF)₂FeMP]⁺Cl^- in zero fields have been directly determined by differential scanning calorimetry. Thermodynamic parameters of the iron(III) complexes magnetization namely heat and the enthalpy/entropy change was determined. To improve understanding of the correlation between magnetocaloric properties and the electronic structure of d-metal porphyrins, we have compared the magnetocaloric behavior of the paramagnets studied with that for (X)MnP where X is Cl, Br, or AcO studied earlier [20], [21], [22], [23], [24], [25].