利用理論計算比較釕金屬撮合霧中的Innocent Ligand 和 Non-innocent Ligand 之 Pourbaix Diagrams 在水氧化反應差異

Abstract

將太陽能做能量轉換是水氧化的一個關鍵的半反應,主要以PCET為主,需要牽涉多個質子以及多個電子的轉移(2H2O → O2 + 4H+ + 4e−),水氧化的mechanism和催化劑已經有許多人發表,而Ru單核催化劑對於水氧化催化劑是重要的一個類別,NIL有三種氧化狀態, NILOx.NIL˙.NILRed, 我們得知non-innocent ligand (NIL)能夠分散金屬中心的電荷密度使分子的能量降低,而Tanaka的雙核催化劑對於Ru-NIL是一個有趣的例子,我們利用密度泛涵理論(DFT)來計算Ru(OH2)(tpy)(tBu2Qn), Ru(OH2)(tpy)(Bpm) and Ru(OH2)(tpy)(Bpy) (tBu2Q = 3,5-di-tert-butyl-2,2-benzoquinone, tpy = 2,2’:6’,2”-terpyridine, Bpm = 2,2’-bipyrimidine, Bpy = 2,2'-Bipyridine) complexes的pKa和還原電位,然後繪製pourbaix diagram,為了比較innocent和non-innocent ligand之不同, 本篇論文我們主要著重在研究PCET的過程和尋找低能量的路徑形成O-O bond, 並且利用計算出的pKa以及還原電位(Eo)所繪製出的pourbaix diagram來進行比較,然後我們發現在SMD系統中[RuIV=O] / [RuIII=O] couples有較低的電位,並且在quinone系統中發現了新的中間物[RuIII(3αO•-)(tpy)(βSQ)]+,而這個中間物可以進行radical-radical coupling形成O-O bond ,另外,我們也利用不同的計算方法比較其中的差異,發現在SMD系統中確實有較好的表現。

Water oxidation is a key half-reaction (2H2O → O2 + 4H+ + 4e−) employed in solar-fuel-based energy conversion, and it is dominated by proton-coupled electron transfer (PCET), given its multi-electron, multi-proton character. Mononuclear Ru-based water oxidation catalysts (WOCs) are a valuable class of WOCs used for water splitting. Noninnocent ligands (NILs) have three oxidation states, NILOx, NIL•, and NILRed, that have an electron redox transformation in common. NILs can help disperse the electron density at the metal center and keep the metal center in low oxidation states. The Tanaka catalyst, an anthracene-bridged dinuclear Ru complex, is an interesting example of a Ru–NIL framework in catalysis. We used density functional theory to calculate pKa and the standard reduction potential of Ru(OH2)(tpy)(tBu2Qn), Ru(OH2)(tpy)(Bpm), and Ru(OH2)(tpy)(Bpy) (tBu2Q = 3,5-di-tert-butyl-2,2-benzoquinone, tpy = 2,2':6',2″-terpyridine, Bpm = 2,2'-bipyrimidine, and Bpy = 2,2'-bipyridine) complexes, and we then constructed the Pourbaix diagram to compare innocent ligands and NILs. We focused on pH-dependent onset catalytic potentials indicative of a PCET-driven low-energy pathway for the formation of products with an O−O bond and investigated the differences between these complexes by using the Pourbaix diagram. We found a lower [RuIV=O]/[RuV=O] couples potential in the solvation model density (SMD) system and a new intermediate complex [RuIII(3αO•-)(tpy)(βSQ)]+ that can promote radical-radical coupling to form an O−O bond. In addition, we used different computational methods to compare the differences and to achieve better performance in the SMD system.