鈦錳鐵錯合物之鍵性分析與電子組態之研究

Abstract

本論文著重於鈦錳鐵錯化合物之電子組態與其相關性質的研究。過程中，利用X光單晶繞射從長程有序的晶體結構出發，來研究電子密度的分佈及其化學鍵在特殊物性與化性的影響, 並輔以短程有序之X光吸收光譜技術，來探究過渡金屬d軌域電子在物性與化性所伴演之角色。 在第一部分中，研究對象為三個鈦錳金屬錯化合物：(1) (NH4)n[Ti(μ2-C2O4)2]n·2nH2O 、 (2) [Mn(μ3-mal)(H2O)2]n 及(3) [Mn3(μ2-mal)2(mal)2(H2O)8]·2H2O。透過引入一非球形對稱多極模型，對其由高解析度X光單晶繞射實驗數據所得之電子密度，做適切的精算描述，進而以電子密度分析的觀點，做深入的研究。其中，由多極項係數投影所得之各金屬的3d電子組態與其由局部結構對稱所推導之軌域能量分裂均相符合。在以電子密度分佈為基礎的拓撲學分析中，M-O鍵(M = Ti, Mn)表現出與C-O及C-C等共價鍵結完全不同的性質: :ρBCP of 0.26 ~ 0.40 e/Å3,▽2ρ > 0, HBCP > 0 vs.ρBCP of 1.8 ~ 3.0 e/Å3,▽2ρ < 0, HBCP < 0。此外，與形式電荷相比，金屬帶有較小的AIM (atom in molecule) 價數，如1中的TiIII 為 2.08 價，2中的MnII 為 1.31 價，3中的Mn(1)III 與Mn(2)II分別為1.62 與1.33價。 第二部分選擇的研究系統為一系列鐵二價磁自旋交換(spin crossover)錯化合物：(4) [Fe(μ2-btr)2(NCS)2]·H2O 、 (5) [Fe0.7Co0.3(μ2-btr)2(NCS)2]·H2O 及 (6) [Fe(μ2-btr)3](ClO4)2。利用X光吸收光譜能反應出受測元素電子組態之特性，研究對象中，由溫度改變或經低溫照光(27 K，532 nm)所引起之磁自旋交換行為，能清楚而直接的被觀測。伴隨著磁自旋交換所發生的局部結構微調，也由延伸X光吸收精細結構 (EXAFS) 數據所分析，其結果亦與文獻中其他相關單晶結構研究相符合。 由X光粉末繞射實驗所成功解出的 [Fe(μ2-btr)2(NCS)2] 無水結構，證實了在 [Fe(μ2-btr)2(NCS)2]·H2O 中，由脫水所引起的可逆結構相變 (monoclinic C2/c to triclinic P-1)。脫水後，二維結構的重新排列，例如層與層間垂直距離的縮短 (5.56 Å to 4.63 Å)，及二維層間彼此平行的滑動 (2.49 Å) 均造成在無水晶格中更緊密地堆疊。此立體阻礙的增加，推測將限制伴隨磁自旋交換所引發的局部結構微調，進而阻止磁自旋交換行為在此無水結構中發生的可能。 本論文的研究，說明X光單晶繞射及X光吸收光譜技術在過渡金屬錯化合物研究的重要性。其中以X光粉末繞射來解出無水之鐵二價磁自旋交換錯化合物的結構，更是當今晶體繞射學上的一項重要成果。此結果意謂著未來在鑑定應用於分子材料中的磁自旋交換錯化合物，X光粉末繞射已足以提供類似傳統單晶結構解析所要的資訊。

Specific features of electronic configurations of 3d transition metals in complexes and the corresponding interesting behaviors such as bonding characteristics and magnetic properties are pursued. In the first part of this dissertation three transition metal complexes, 1, (NH4)n[Ti(μ2-C2O4)2]n·2nH2O, 2, [Mn(μ3-mal)(H2O)2]n and 3, [Mn3(μ2-mal)2(mal)2(H2O)8]·2H2O, are investigated, of which electron density distributions are extracted from the corresponding high-resolution single crystal X-ray diffraction data by applying the aspherical multipole model refinement. The five 3d orbital populations of the metal sites (Ti in 1 and Mn in 2 and 3) derived from multipole terms are found to be well suited with the local coordination spheres. The total numbers of 3d electrons are also comparable with the corresponding net atom in molecule (AIM) charges. Significant difference between the two Mn atoms in 3 (which are expected to be MnIII and MnII, respectively) is not found though slight difference is still observable. M-O bonds in all the three complexes (M = Ti in 1 and Mn in 2 and 3) exhibit a total different bonding characteristic in comparison with those of organic fragments e.g. C-C, C-O and O-H, among which those with positive total energetic densities are characterized as close-shell interactions while those with negative total energetic densities are characterized as transit closed-shell interactions. The correlations between topological properties and the accompanied energetic densities of metal-oxygen bonds are inspected; the result shows similar behaviors with those of hydrogen bonding interactions. In the second part of the thesis, three spin crossover (SCO) systems (4, [Fe(μ2-btr)2(NCS)2]·H2O, 5, [Fe0.7Co0.3(μ2-btr)2(NCS)2]·H2O and 6, [Fe(μ2-btr)3](ClO4)2) are studied, of which transitions between spin states of metal sites are observed as the consequence of external perturbations. X-ray absorption spectroscopy at both metal K-edge and L-edge is performed to probe directly the exact electronic configurations of target metal centers. Typical spectra of FeII at HS and LS states are reproduced adequately. Spectrum measured after irradiation with a 532 nm laser source at 27 K is comparable with that at RT, revealing that the light-induced excited spin state trapping (LIESST) state exhibit a HS configuration. Spin state conversions with increasing temperature are monitored as well by a series of absorption spectra (at K-edge for 4 and L-edge for 6) where unexpected smoothened evolutions originated from sample grinding are obtained. For 4, the loss of SCO phenomenon after dehydration is pursued as well by synchrotron powder X-ray diffraction. A reversible structure phase transition from monoclinic C 2/c to triclinic P is evidenced. The anhydrous species yields a contracted unit cell especially in the direction along the normal of the 2D layer. The distance between adjacent layers is significantly shortened from 5.56 Å to 4.63 Å together with a slight sliding parallel to the layer. Consequently, the compressed free space may restrict the necessary structure fine-tuning accompanied with the change in spin state and thus limit the occurrence of spin transition.