利用理論計算探討Non-Innocent Ligand和Innocent Ligand應用在Ruthenium系統之水氧化反應的差異性

Abstract

由文獻中得知Non-Innocent Ligand（NIL）有多變的電子特性，而第一個應用NIL在催化劑的是Tanaka等人所合成出的雙核催化劑，此結構為[Ru2(OH)2(3,6-tBu2Q)2(btpyan)]2+ (tBu2Q, 3,6-di-tert-butyl-1,2-benzoquinone; btpyan, 1,8-bis(2,2′:6′,2′′-terpyridyl)anthracene)，發現turnover高達33,500並能夠產生出氧氣，而後Meyer等人利用單核催化劑[Ru(OH2)(Bpm)(tpy)]2+ (Bpm, 2,2'-bipyrimidine; tpy, 2,2':6',2"-terpyridine)證明只要單核催化劑就足夠將水氧化成氧氣，所以本篇利用電子結構、能量與吸收光譜圖分析NIL應用在Ruthenium單核催化劑上並與Innocent Ligand做比較，探討應用在Meyer所假設的水氧化反應機制之差異性。本篇研究結果發現NIL的特性能穩定中心金屬，分散金屬中心的電荷密度使之分子能量降低，因此Pourbaix Diagram相對於Innocent Ligand有多種結構存在於水溶液中，之後本篇藉由吸收光譜圖發現Meyer等人假設[RuV-O]3+的結構可能為[RuV(═O)(OH2)(tpy)(Bpm)]3+，另外Ru-NIL的催化劑從Ru(dπ)+Qn(π*)至Qn(π*)-Ru(dπ)的transition為MLCT且所需的能量相較於Innocent Ligand較低，還有藉由[RuV(═O)(OH2)]3+的吸收光譜圖觀察到H2O(pσ)上的電子激發至Ru(dπ*)+O(pπ*)+H2O(pσ*)的波長範圍在450-500 nm左右，Ru-NIL電子躍遷的能量較低且在可見光區內，或許在實驗中除了水溶液的酸鹼性和電壓外，可見光也能夠幫助此催化劑與H2O形成O-O鍵並完成催化循環，這個電子轉移的訊號或許可以透過以Transient Absorption Spectrum的方式，被利用來觀察O-O鍵形成的動力學光譜測量。

The non-innocent ligand (NIL) has been reported to play an important role on molecular water-oxidation catalysis. The first NIL catalyst was introduced by Tanaka and coworkers where the electrochemical property of quinone in [Ru2(OH)2(3,6-tBu2Q)2(btpyan)]2+ (tBu2Q, 3,6-di-tert-butyl-1,2-benzoquinone; btpyan, 1,8-bis(2,2′:6′,2′′-terpyridyl)anthracene) is believed to be responsible for its novel catalytic ability. The current study focuses on a systematic comparison between two Ruthenium complexes with Non-innocent ligand(NIL) and innocent ligand from the perspective of energy, redox potential and molecular orbital theory. The calculated absorption spectra, Pourbiax diagrams and energy curve of these complexes will be presented. More importantly, the impact to the acid-base type of oxygen-oxygen bond formation mechanism given rise to two ligands will be discussed. The current study demonstrates the capability of NIL for redistributing the charge density in order to stabilize the metal complex based upon the results of the various electronic state assignments in Pourbaix Diagram. We also reassign the [RuV(═O)(OH2)(tpy)(Bpm)]3+ state as the [RuV-O]3+ by Meyer et al. The energy of transferring electron from H2O(pσ) excited to Ru(dπ*)+O(pπ*)+H2O(pσ*) with Ru-NIL is lower than innocent ligand. That suggest the potential of forming O-O bond through absorbing a visible photon. The signal of this water-to-catalyst electron transfer process may be utilized for monitoring the kinetic of O-O bond formation in a typical Transient Absorption Spectrum setup.