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

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.