Heat capacities and phase transitions of the molecule-based mixed-valence complex NBu4[FeIIFeIII(ox)3] and the mixed-metal complex NBu4[ZnIIFeIII(ox)3]

Heat capacities of two molecule-based materials NBu4[(FeFeIII)-Fe-II(ox)(3)] and NBu4[(ZnFeIII)-Fe-II(ox)(3)] (Bu = n-butyl ox = oxalate) were measured by adiabatic calorimetery. In case of ferrimagnetic NBu4[(FeFeIII)-Fe-II(ox)(3)] two phase transitions were detected at 43.3 K (T-2) and 217 K (T-1) along with a small heat capacity anomaly at 16.3K (T-3) The heat capacity anomaly at 43.3 K is due to the ferrimagnetic ordering, whereas that at 217 K has been assigned to an order-disorder phase transition due to increasing conformational change of the organic cation. The small heat capacity anomaly at T-3 seems to arise from a spin glass like state at low temperatures. The enthalpy and entropy gains from the heat capacity anomalies at T-1: T-2 and T-3 are 2.29 kJ mol(-1) and 11.47 J K-1 mol(-1); 1.37 kJ mol(-1) and 22.65 J K-1 mol(-1); 0.068 kJ mol(-1) and 4.89 J K-1 mol(-1); respectively. The sum of the entropies (=27.54 J K-1 mol(-1)) corresponding to the heat capacity anomalies at T-2 and T-3 is very close to the expected magnetic entropy R ln 30 (=28.27 J K-1 mol(-1)) for the spin multiplicity of high spin Fe-III and Fe-II ions. No phase transition around 38 K has been detected, as predicted from previous magnetic measurements. In case of paramagnetic NBu4 [Zn-II Fe-III(ox)(3)] a heat capacity anomaly due to a similar order-disorder phase transition was observed at 227 K accompanied with the enthalpy and entropy gains of 2.91 kJ mol(-1) and 13.32 J K-1 mol(-1), respectively. A model has been applied to realize the high temperature order-disorder phase transition.