自旋轉換錯合物trans-[Fe(abpt)2(NCS)2] polymorph D電子密度及理論計算之研究

Charge Density Study of a Spin Crossover Complex, trans-[Fe(abpt)2(NCS)2], a Mixed High Spin and Low Spin State

施哲修(Che-Hsiu Shih)

指導教授 ： 王瑜

國立臺灣大學/理學院/化學研究所/碩士(2006年)

https://doi.org/10.6342/NTU.2006.01546

摘要

多相晶體trans-[Fe(abpt)2(NCS)2] phase D之晶體結構中，存在兩個不對稱相關的單核分子；兩分子個別以鐵為對置中心，其一Fe原子會隨溫度降低而有自高自旋 (S=2)到低自旋狀態 (S=0)的自旋轉換現象(Tc=162 K)；另一Fe'原子則在量測的溫度範圍維持者高自旋的順磁狀態。兩個單核分子主要的差別在於配位基abpt(abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole)上的平面度；介於未配位的吡啶(pyridine)和疊氮基(triazole)之間的雙面角(dihedral angle)在低溫結構(90 K)分別為6.2度(Fe)及20.1度(Fe')。
由於兩個分子在低溫下的電子組態十分不同，我們利用X-ray單晶繞射技術及DFT理論計算來研究其電子密度的分布。經過電子密度拓樸分析研究，我們可以觀察到在兩個分子間，中心金屬d電子軌域分佈的不同；Fe由於處在低自旋狀態所以在二次電子微分(Laplacian)圖中呈現非球性對稱分佈，Fe'則處於高自旋狀態所以二次電子微分(Laplacian)圖呈現較為球形對稱分佈。理論計算的結果相當符合實驗所得到的結果。
為了瞭解兩分子在低溫下呈現不同磁性及結構的成因，我們利用DFT理論計算的方式觀察其在熱力學上的不同；更進一步的，我們可以粗估其自旋轉換發生的溫度(spin-flip temperature)。由於配位基abpt的平面度在兩分子中存在一定的差異，我們試圖以費米孔洞分析去了解兩分子中兩芳香環上π電子共振的不同。此外，我們可以觀察雙面角的改變對中心金屬和配位氮距離影響。最後，我們利用ADF程式模擬出低自旋-低自旋(LS-LS)的結構，試圖以能量及結構上的觀點去了解磁性轉換的關係。

關鍵字

自旋轉換錯合物 ； 電子密度 ； 理論計算

並列摘要

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.

並列關鍵字

Charge Density ； Spin Crossover

參考文獻

2. P. Coppens, N. K. Hansen, Acta Cryst, 1978, A34, 909.

3. Roothan, C. C. J. Rev. Mod. Phys. 1951, 23, 69.

5. McWeeny, R. and Dierksen, G. J. Chem. Phys. 1968, 49, 4852.

7. Parr R, G, Yang W Density functional theory of atoms and molecules, 1st edn. Oxford University Press, New York, 1989

8. Slater J. C. Phys Rev, 1951, 81, 385.