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J. Phys. Chem. B 2007, 111, 12508-12517
Spin Crossover Phenomenon Accompanying Order-Disorder Phase Transition in the Ligand of [FeII(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† Yuji Miyazaki,*,‡ Tadahiro Nakamoto,‡ Satoaki Ikeuchi,‡ Kazuya Saito,§ Akira Inaba,‡ Michio Sorai,‡ Takeo Tojo,⊥ Tooru Atake,⊥ Galina S. Matouzenko,*,| Samir Zein,| and Serguei A. Borshch*,| Research Center for Molecular Thermodynamics, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan, Department of Chemistry, Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan, Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan, and Laboratoire de Chimie (UMR CNRS and ENS-Lyon No. 5182), EÄ cole Normale Supe´ rieure de Lyon, 69364 Lyon cedex 07, France ReceiVed: April 19, 2007; In Final Form: July 20, 2007
The spin crossover phenomenon of the recently described spin crossover complex [FeII(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 Ttrs ) 185.61 K. The transition enthalpy and entropy amounted to ∆trsH ) 15.44 kJ mol-1 and ∆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 ln 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 ln 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.
1. Introduction Octahedral transition metal complexes with d4, d5, d6, and d7 electronic configurations exist either in the high-spin (HS) or low-spin (LS) ground state, depending on the ligand field strength. When the ligand field strength is close to the electron pairing energy, the spin state transition can occur between the two spin states by variation of temperature, pressure, light irradiation, and so on. Such phenomena are called “spin crossover” phenomena and have been extensively investigated.1-16 Focusing on temperature-induced spin crossover phenomena, they are usually classified into two types on the basis of the temperature dependence of the HS fraction: abrupt (or discontinuous) and gradual (or continuous) types. In the abrupt type, the spin state conversion between the HS and LS states takes place within a narrow temperature range (
