利用理論計算探討釕金屬之Innocent和Non-innocent ligand在電催化還原二氧碳反應機制上的差異

Abstract

Koji Tanaka 為少數利用有機金屬錯合物直接將CO2進行多電子還原的科學家之一，在1993年利用RuII(bpy)(trpy)(CO), bpy = 2,2'-Bipyridine, tpy = 2,2':6',2”-terpyridine,作為催化劑，並可在通入-1.7V的電壓環境下使CO2還原成常見的CO、HCOOH，甚至可得到CH3OH、HC(O)H、以及增加碳-碳鍵的產物H(O)CCOOH及HOCH2COOH，並且在1994年推測出一個完整的催化循環。但此催化反應各中間物的詳細訊息在實驗數據上並不明瞭。因此希望藉由理論計算方法分析各步驟的還原電位、pKa值及自由能去探討其反應的可行性，藉此更加瞭解Tanaka所推測的反應機制。此外我們也推測RuII(tpy)(OBQ)(CO), trpy = 2,2':6',2”-terpyridine, OBQ = ortho-benzoquinone還原CO2的反應機制，並將RuII(trpy)(OBQ)(CO)視為non-innocent ligand，而RuII(bpy)(trpy)(CO)視為innocent ligand做比較，探討電子在錯合物上非定域化的程度是否大幅影響催化劑還原CO2的能力以及催化途徑的推測。結果很成功地利用兩個催化劑推測完整的催化機制，並可發現鍵結上non-innocent ligand的催化劑，可使反應更利於脫除甲醇，但相對地不好和CO2進行鍵結。

Koji Tanaka is one of the few scientists who use organometallic complexes as a catalyst for multi-electron reduction reactions. In 1993, Tanaka reported that RuII(bpy)(trpy)(CO) (bpy = 2,2 '-bipyridine, trpy = 2,2': 6',2"-terpyridine) acts as a catalyst, at −1.7V (vs. NHE), for the reduction of CO2 to generate products of economic value like CO, HCOOH, CH3OH, HC(O)H, and products involving C–C bond formation like H(O)CCOOH and HOCH2COOH. In 1994, Tanaka speculated a complete catalytic cycle; however, the detailed information was not available on each intermediate of the reaction. Therefore, we relied on theoretical calculations to analyze each step of the reaction in terms of reduction potential, pKa values, and free energy, to explore the feasibility of the reaction, thereby understanding more of the reaction mechanism than what Tanaka hypothesized. In addition to RuII(bpy)(trpy)(CO), we speculate that RuII (trpy) (OBQ) (CO) (OBQ = ortho-benzoquinone) can act as a catalyst for the CO2 reduction reaction. RuII(trpy)(OBQ)(CO) as a non-innocent ligand and RuII(bpy)(trpy)(CO) as an innocent ligand were compared to determine whether the degree of electron delocalization between the metal and the ligands significantly affects the catalytic ability for CO2 reduction and to determine the catalytic pathways. We successfully predicted the mechanism of CO2 reduction with two different types of ligands (innocent and non-innocent). A non-innocent ligand has the advantage of reducing CO to form methanol easily on the complex, but the CO2 activation after removal of methanol on ethanol is weaker than innocent ligand system (bpy).