具自我辨識能力多配向聯三吡啶配體與其對鋅鎘離子自組裝行為的探討

Abstract

藉由自然界天然物的啟發，如酵素或是一些具有特殊結構的蛋白質，化學家們盡全力開發一些具有催化效果或是接受外界刺激響應的人工組裝體。在合成這些組裝體的方法中，利用金屬配位鍵來建構這些組裝體十分受矚目，因為組裝體的形狀與功能可以藉由適當的配基設計與選用的金屬離子或是陰離子來達成。 在金屬超分子領域裡含有2,2':6',2'-聯三吡啶官能基的配體因為能與多種的過渡金屬形成鍵結強度不一的錯合物而被廣泛運用。然而如果選用的金屬產生強度太強的配位鍵會使得組裝過程中產生太多動力學副產物，另一方面配位鍵太弱則會產生目標產物與和其相似位能的熱力學副產物。除了以上藉由不同金屬產生不同配位鍵強度的優缺點外，多重位向聯三吡啶配體的自我辨識能力不佳，在形成巨大組裝體的同時常伴隨著誤接的副產物。因此在多重位向聯三吡啶配體裡設計與開發具有高度自我辨識能力的官能基是必須且日益重要的。 在這份工作中我們探討了弱配位金屬鋅與鎘的些微差異及他們與多重配向聯三吡啶配體的自組裝行為，並發展了一些方法來增加聯三吡啶官能基間的自我辨識度。第一個方法是利用官能基幾何配置來形成幾何互補式聯三吡啶官能基對。合成出來的雙位向聯三吡啶配體能夠與鋅只產生單一頭接尾的金屬三角形超分子。第二個方法則是放大弱配位鋅與鎘的些微差異。在一個三角稜狀的金屬“衣服”中，客體穿上這件金屬“衣服”的速度與選擇何種金屬離子來合成這件衣服有關。我們也運用了這種差異性來呈現具備選擇性的仿生物體主客化學。最後一個方法則引入了互補性聯三吡啶官能基對。這個官能基對是由一個在6,6'位置上修飾立障基團的聯三吡啶官能基、鎘金屬離子、與未修飾的聯三吡啶官能基所組成。藉由當量控制，雙配向的6,6'位置修飾聯三吡啶配體可以與未修飾的多配向聯三吡啶配體形成豐富的二維或是三維結構。此外我們也發現到有修飾的聯三吡啶配體能夠與鎘形成配體不易交換的二聚體，而這個二聚體是當量與動力學控制的產物。以上三種方法對於日後聯三吡啶金屬超分子開發具有莫大的助益。

Inspired by natural functional nanoobjects commonly occurring in biology such as enzymes or structure-specific proteins, chemists endeavor to make artificial assemblies that have similar catalytic activity or extend their applications as stimuli-responsive materials. Coordination-driven self-assembly to these manmade objects is especially appealing because size, shape or embedded functional groups could be designed/altered on demand by proper ligand design and choice of metal ion and counter anions. In the vast field of metallo-supramolecules, 2,2':6',2'-terpyridine (tpy)-based ligands have been widely applied because it could bind different transition metal ions and their coordination bond strengths could be modulated accordingly depending on choice of metal ions. If the bond strength is too strong, ligands are barely exchanged and kinetically trapped species dominate. On the other hand, if the coordination bond is too labile, a mixture of targets and byproducts with similar thermodynamic stability will be generated. In addition to the pros and cons of the metal ions selected, multitopic tpy ligands for building complicated structures often result in misconnected byproducts that detour the original assembly route due to poor self-recognition capability of tpy functional groups. Therefore, offering tpy segments on the multitopic tpy ligand to self-recognize themselves to the proper tpy partner is required for the sophiscated structures. Herein, we aim for synthesizing ZnII and CdII-based tpy assemblies that form discrete structures through proper ligand design and discussing the slight dynamic difference between the both traditionally-viewed labile transition metal ions. In the first method, we applied geometrical constraint to make a bistpy ligand possessing two mutually hereoleptic binding sites that self-recognize each other to form a single head-to-tail discrete metallo-triangle. In the second method, the coordination dynamic difference between ZnII and CdII was discussed and exemplified in a metallo-supramolecular pseudo-suit[3]ane. The wearing speed of the metallo-suit was heavily dependent on the choice of metal ions. We then demonstrated a biomimetic system that the guest molecule would prefer to wear one metallo-suit over a certain timeframe. In the last method, we extended application on a recently published heteroleptic dynamic ligand pair composing of CdII, non-substituted and 6,6'-substituted tpy ligands. Through stoichiometric control, the same metal/ligand components could generate either 2D or 3D structures. Furthermore, we also found that bistpy CdII complex of 6,6'-position substituted tpy was kinetically inert and the assembly process became kinetically controlled under certain stoichiometry. The finding and the toolkit developed in this work would be helpful for designing more and more novel and useful tpy-based materials.