Low-temperature heat capacity and thermochemical study of [Re2(Pro)6(H2O)4](ClO4)6 (Re = Nd, Gd; Pro = proline) crystals

The heat capacities of two solid complexes of rare-earth compounds with proline [Re2(Pro)6(H2O)4](ClO4)6 (Re=Nd, Gd; Pro=proline, C5H9O2N), were measured with a high-precision automatic adiabatic calorimeter over the temperature range from 78 to 370 K. The heat capacity curves of the two complexes are smooth and similar in magnitude to each other except for a melting transition of [Gd2(Pro)6(H2O)4](ClO4)6 that occurred in the temperature range of 340–346 K with a peak temperature of 342.48±0.01 K. The molar enthalpy and entropy of fusion of [Gd2(Pro)6(H2O)4](ClO4)6 were determined to be 26.104±0.031 k J mol−1 and 76.23±0.08 J K−1 mol−1, respectively, on the basis of the heat capacity measurements. Thermal decompositions of the two complexes were studied through the thermogravimetry (TG) experiments and possible mechanisms of the decompositions were suggested according to the TG analysis. The molar combustion energies of the two complexes were determined to be 17010±1.6 and 15630±1.5 J mol−1 for [Nd2(Pro)6(H2O)4](ClO4)6 and [Gd2(Pro)6(H2O)4](ClO4)6, respectively, by means of oxygen-bomb calorimetric measurements.