Heat capacity and magnetic phase transition of two-dimensional metal-assembled complex (NEt4)[{MnIII(salen)}2FeIII(CN)6].

Heat capacity measurements of the two-dimensional metal-assembled complex, (NEt4)[{MnIII(salen)}2FeIII(CN)6] [Et=ethyl, salen= N,N’-ethylenebis(salicylideneaminato) dianion], were performed in the temperature range between 0.2 and 300 K by adiabatic calorimetry. A ferrimagnetic phase transition was observed at Tc1=7.51 K. Furthermore, another small magnetic phase transition appeared at Tc2=0.78 K. Above Tc1, a heat capacity tail arising from the short-range ordering of the spins characteristic of two-dimensional magnets was found. The magnetic enthalpy and entropy were evaluated to be Δ H=291 J mol-1 and Δ S=27.4 J K-1 mol-1, respectively. The experimental magnetic entropy agrees roughly with Δ S= Rln(5·5·2) (=32.5 J K-1 mol-1; R being the gas constant), which is expected for the metal complex with two Mn(III) ions in high-spin state (spin quantum number S=2) and one Fe(III) ion in low-spin state ( S=1/2). The heat capacity tail above Tc1 became small by grinding and pressing the crystal. This mechanochemical effect would be attributed to the increase of lattice defects and imperfections in the crystal lattice, leading not only to formation of the crystal with a different magnetic phase transition temperature but also to decrease of the magnetic heat capacity and thus the magnetic enthalpy and entropy. [ABSTRACT FROM AUTHOR]