Mechanochemical Effect in the Iron(III) Spin Crossover Complex [Fe(3-MeO-salenEt)₂]PF₆ as Studied by Heat Capacity Calorimetry

Magnetic and thermal properties of the iron(III) spin crossover complex [Fe(3MeO-salenEt)₂]PF₆ are very sensitive to mechanochemical perturbations. Heat capacities for unperturbed and differently perturbed samples were precisely determined by adiabatic calorimetry at temperatures in the 10−300 K range. The unperturbed compound shows a cooperative spin crossover transition at 162.31 K, presenting a hysteresis of 2.8 K. The anomalous enthalpy and entropy contents of the transition were evaluated to be Δ_trs H = 5.94 kJ mol^-1 and Δ_trs S = 36.7 J K^-1 mol^-1, respectively. By mechanochemical treatments, (1) the phase transition temperature was lowered by 1.14 K, (2) the enthalpy and entropy gains at the phase transition due to the spin crossover phenomenon were diminished to Δ_trs H = 4.94 kJ mol^-1 and Δ_trs S = 31.1 J K^-1 mol^-1, and (3) the lattice heat capacities were larger than those of the unperturbed sample over the whole temperature range. In spite of different mechanical perturbations (grinding with a mortar and pestle and grinding in a ball-mill), two sets of heat capacity measurements provided basically the same results. The mechanochemical perturbation exerts its effect more strongly on the low-spin state than on the high-spin state. It shows a substantial increase of the number of iron(III) ions in the high-spin state below the transition temperature. The heat capacities of the diamagnetic cobalt(III) analogue [Co(3MeO-salenEt)₂]PF₆ also were measured. The lattice heat capacity of the iron compounds has been estimated from either the measurements on the cobalt complex using a corresponding states law or the effective frequency distribution method. These estimations have been used for the evaluation of the transition anomaly.