軸配基的鍵結性質對金屬卟啉電子組態的影響

Abstract

論文摘要 鐵卟啉是生物無機化學中最常用來模擬血紅素蛋白(hemoprotein)相關生物活性的模型化合物。近年來一系列的研究顯示，當卟啉環上的取代基過於擁擠時，可能會造成大環的變形；主要分為馬鞍型變形及皺摺型變形：八乙基四苯基卟啉(OETPP)是文獻中最早被注意到的馬鞍型變形卟啉，卟啉環的變形會使鹼性明顯增加，金屬dx2-y2軌域能階提升，相對地dz2能階下降，金屬dxy軌域會和卟啉大環的a1u軌域對稱相符合，在晶場強度改變下，將有利於三價鐵中自旋態的形成。四異丙基卟啉(TiPP)是文獻中最常被使用的皺摺型變形模擬化合物，皺摺變形會使大環孔徑變小，再加上扭曲的因素，金屬dxy軌域會和卟啉大環的a2u軌域作用，形成不尋常的三價鐵低自旋(dxz,dyz)4(dxy)1電子組態。 除了卟啉上取代基的影響，在本文中我們將重點放在軸配基，透過不同軸配基配位於金屬卟啉探討其鍵結特性。一般認為強場的配位基屬於好的σ授予者，亦是好的π接受者，透過金屬卟啉和軸配基的鍵結分析，發現CN-確實是很好的σ授予者，但受到負電荷的影響，π接受的能力並不如中性的HNC明顯。中性的HNC也因此特性造成dπ軌域能量下降因此發生Jahn-Teller distortion，使平面的[Fe(TPP)(HNC)2]+以(dxz,dyz)4(dxy)1電子組態存在；而在[Fe(OETPP)(HNC)2]+的系統中，更因為大環成馬鞍型變形較易被氧化，而呈現[FeⅡ(OETPP) +• (HNC)2]+不尋常的電子組態。 先前已有許多證據顯示[Fe(TPP)(Im)2]+確實以(dxy)2 (dxz,dyz)3電子組態存在，而[Fe(TPP)(Py)2]+的情形較為複雜。經由核磁共振光譜之分析及平衡常數的測定，顯示Py雖然不像HNC擁有好的π接受能力，但在配位時受到立體障礙的影響會互相垂直於meso位置，引發微量的皺摺變形，導致電子組態有(dxz,dyz)4(dxy)1的貢獻。 最後我們本來希望利用Ramsey equations ，經由測定59Co化學位移，找到在弱場的配位基中，有利於低自旋兩價鈷卟啉發生大環氧化，形成卟啉π陽離子自由基的直接證據，卻無意間得到測定晶場能量大小的方法。

Abstract Recent researches demonstrate that porphyrin distortion is mainly divided into two kinds: saddle and ruffle. The saddle distortion in OETPP increases basicity. The phenomenon promotes dx2-y2 orbital energy and decreases dz2 orbital energy relatively in metalloporphyrin. Porphyrin's a1u orbital has the same symmetry as dxy orbital in saddle distortions. Iron (III) porphyrin is advantageous to intermediate-spin under crystal field intensity change. The ruffle distortion in TiPP decreases porphyrin core and creates dxy orbital bonding interaction with porphyrin's a2u orbital. Iron (III) porphyrin becomes the unusual (dxz, dyz)4(dxy)1 electronic state. Beside this substituents effect, we are focusing on axial ligand in this report. It is generally believed that a strong field ligand is a good σ- donor and is also a good π-acceptor. Through the bonding analysis, it has been found that CN- is a good σ- donor but the π-acceptor of neutral HNC is better than CN- which make large difference in their electronic properties in metalloporphyrin . The electronic state of planar [Fe(TPP)(HNC)2]+ is (dxz, dyz)4(dxy)1 and exhibits Jahn-Teller distortion. In case of [Fe(OETPP)(HNC)2]+ the unpair electron is unusually present on the porphyrin's a2u orbital and make [FeII(OETPP)+•(HNC)2]+. The electronic state of [Fe(TPP)(Im)2]+ is (dxy)2(dxz, dyz)3 in many evidential demonstrations where Im is mainly good σ- donor. But [Fe(TPP)(Py)2]+ is more complex where Py is also good σ- donor as well as π-acceptor. However, the π-acceptor properties of Py is not better than HNC but it still has a little ruffle distortion because two pys are peripendicular to meso porphyrin which lead the electronic state to (dxz, dyz)4(dxy)1. At last, we tried to explain, by using of Ramsey equations and 59Co chemical shift, why the weak field axial ligand low-spin Co(II) porphyrin oxidizes on the porphyrin ring which become π cation radical. But we found another way to determine the crystal field energy by accident.