Spin crossover phenomenon accompanying order-disorder phase transition in the ligand of [Fe^II(DAPP)(abpt)](ClO₄)₂ compound (DAPP = Bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) and its successive self-grinding effect

The spin crossover phenomenon of the recently described spin crossover complex [Fe^II(DAPP)(abpt)](ClO₄)₂ [DAPP = bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-amino-3,5-bis(pyridin-2-yl)- 1,2,4-triazole] accompanying an order-disorder phase transition of the ligand was investigated by adiabatic heat capacity calorimetry, far-IR, IR, and Raman spectroscopies, and normal vibrational mode calculation. A large heat capacity peak due to the spin crossover transition was observed at T_trsH = 185.61 K. The transition enthalpy and entropy amounted to Δ_trsH = 15.44 kJ mol^-1 and Δ_trsS = 83.74 J K^-1 mol^-1, respectively. The transition entropy is larger than the expected value 60.66 J K^-1 mol^-1, which is contributed from the spin multiplicity (R In 5; R: the gas constant), disordering of the carbon atom of the six-membered metallocycle in the DAPP ligand, and one of the two Perchlorate anions (2R In 2), and change of the normal vibrational modes between the high-spin (HS) and low-spin (LS) states (35.75 J K^-1 mol^-1). The remaining entropy would be ascribed to changes of the lattice vibrations and molecular librations between the HS and LS states. Furthermore, [Fe(DAPP)(abpt)](ClO₄)₂ crystals disintegrated and became smaller crystallites whenever they experienced the phase transition. This may be regarded as a successive self-grinding effect, evidenced by adiabatic calorimetry, DSC, magnetic susceptibility, and microscope observation. The relationship between the crystal size and the physical quantities is discussed.