| Summary: | 碩士 === 國立臺灣大學 === 化學研究所 === 94 === A polymorph D of the complex trans-[Fe(abpt)2(NCS)2], (abpt = 4-amino-3,5 bis(pyridine-2-yl)-1,2,4-triazole)was synthesized and structurally characterized. There are two unique mononuclear molecules in the crystal lattice, one (Fe) undergoes a gradual thermal spin transition from high spin (HS) to low spin (LS) state around 162 K, while the other (Fe'') remains in HS at the temperature range of 300-2K. The main difference in these two molecules is the planarity of the abpt ligand; the dihedral angle between the triazole ring and the uncoordinated pyridine ring is 6.2 and 20.1º for Fe and Fe'' complex respectively.
In order to understand the differences in electronic density distribution between these HS and LS states, trans-[Fe(abpt)2(NCS)2] polymorph D is chosen to be the ideal case to investigate the charge density using single crystal X-ray diffraction at 90 K. The electron density can also be calculated by DFT calculation. Through the topological analysis on charge density derived from both experimental measurement and quantum calculation, we are able to demonstrate the apparent differences in the environment of metal ion in two states. Furthermore, the d-orbital populations at HS (Fe) and LS (Fe'') states can also be derived. The agreement between the experiment and theory is reasonably good. The relationship between the planarity of the ligand, abpt, and the π electron conjugation is depicted in terms of Fermi-hole distribution.
The difference of ~0.2Ǻ in Fe–N distance is often observed between HS and LS states. The significant difference in the intra-ligand dihedral angle in this case is also highly correlated with the magnetic behaviour. A few models based on such consideration are calculated to rationalize the HS-LS transition. The energies of HS-HS; HS-LS and LS-LS pairs are computed accordingly; the absence of LS-LS case in this compound can be realized. The spin-flip temperature can be predicted.
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