仿生性雙硫醇架橋-雙核釕複合物及其衍生物模擬雙硫鐵產氫酶活化中心之研究

Abstract

氫能源是一種乾淨無污染並且可作為替代石化燃料的再生性能源。然而目前的氫氣製備，主要是以蒸氣重組 (steam reforming) 或電裂解水(water splitting)的方式來產氫，不僅效能不佳且成本昂貴。在自然界中，產氫酶與氫氣的代謝息息相關，其主要是催化氫分子和氫質子之間的互相轉換 (2H++2e- ⇆ H2)。在適合的反應環境條件下此酵素能快速地進行催化反應而產生氫氣。近年來利用結晶學的技術，科學家已成功地解出Desulfovibrio desulfuricans 以及Clostridium pasteurianum中鐵-鐵產氫酶 ([FeFe]-Hydrogenase) 的X-ray晶體結構。而這類酵素活化中心的氫簇分子 (H-cluster) 就是使其具有高效能產生氫氣的催化中心。因此我們的目標就在於模擬並合成出鐵-鐵產氫酶活化中心-氫簇分子(H-cluster)的化學結構-[(μ-DT)Fe2(CO)6] (DT: dithiolate)，並以置換其中心金屬、或在其雙硫架橋上取代成不同的官能基以及修飾上不同的磷衍生物來探討這些仿生酵素活化中心催化氫氣產生的效率以及機制。我們合成了一系列雙硫醇架橋之雙核釕複合物 [Ru2(S2C3H6)(μ-X) (CO)6] (X=H, COOH, N-Boc) 去模擬氫簇分子 (H-cluster) 的結構，並藉由核磁共振光譜(NMR)、電噴灑質譜儀(ESI-MS)以及傅立葉轉換紅外線光譜 (FT-IR) 來分析及鑑定其結構。其後我們利用有機相和水相兩種不同的反應系統來探討這些有機雙核釕金屬複合物的產氫活性。在有機相系統中主要是以甲酸 (Formic acid, HCOOH) 來當作氫氣來源，並外加三種具有不同的推拉電子特性之含磷化合物 (三苯基膦 (triphenylphosphine (P(Phe)3)), 三吡咯膦(tripyrrolephosphine (P(Pyr)3))及三吡咯啶膦(tripyrrolidinephosphine (P(Pyrldn)3)))來探討其照光催化之產氫活性分析。另一方面，在水相光催化產氫系統中，我們是以抗壞血酸 (ascorbic acid) 當作電子提供者並輔以一個釕金屬複合物光敏化劑 (Ru(bpy)32+)，使得仿生有機雙核釕金屬複合物在照光條件下可以催化裂解水，並測得其在水相系統下之光催化產氫效率。在產氫活性的比較上，有機相反應的效率較水相來的好；而在有機相當中，又以三吡咯啶膦(P(Pyrldn)3)的添加更能夠去增加其活性。在未來的應用上，我們希望透過此仿生性雙核釕複合物的建構，對於鐵-鐵產氫酶 ([FeFe]-Hydrogenase) 其活化氫簇分子中心的催化產氫機制有更進一步的探討，並使其能應用於未來氫能源工業上。

Hydrogen is regarded as a clean fuel and a promising alternative energy to fossil fuels, because current hydrogen production via water splitting is still inefficient and expensive. In contrast, hydrogenases (H2ases), which are enzymes for hydrogen (H2) evolution, catalyze the reversible inter-conversion of proton and hydrogen (2H++2e- ⇆ H2) with convenient and high efficiency under mild conditions. The crystal structures of one kind of hydrogenases, the [FeFe] hydrogenases from Desulfovibrio desulfuricans and Clostridium pasteurianum, have been elucidated by X-ray crystallography. The organometallic H-cluster unit of its active site is involved in hydrogen production and provided a very high rate of hydrogen generation. Our aim is to mimic the structure of the H-cluster, [(μ-DT)Fe2(CO)6] (DT: dithiolate) in [FeFe] hydrogenases, with the substitution of different metal center, replacement of bridging dithiolate ligands and the modification of various phosphine ligands to study their catalytic activity for hydrogen production. In this study, we first synthesize a series of ruthenium (Ru)-substituted H-cluster mimics (dithiolate diruthenium complexes [Ru2(S2C3H6)(μ-X) (CO)5L] (X=H, COOH, N-Boc). All of the artificial ruthenium (Ru)-substituted H-cluster mimics were characterized by IR, NMR and MS spectra. Further, the efficiency of photo-induced hydrogen production by using these compounds as catalysts were investigated in different phase systems. One is the organic phase which utilizes formic acid as hydrogen donor. The hydrogen evolution activity of synthesized ruthenium (Ru)-substituted H-cluster was investigated. Moreover, three kinds of phosphine ligands (triphenylphosphine (P(Phe)3), tripyrrolephosphine (P(Pyr)3) and tripyrrolidinephosphine (P(Pyrldn)3)) with different chemical-electronic properties were added inside to analyze their effect on hydrogen production efficiency. The other system is aqueous phase which needs a photosensitizer existence to process the light-driven water splitting reaction. In the presence of the (Ru)-substituted H-cluster functioning as a catalyst, Ru(bpy)32+ as a photosensitizer and ascorbic acid as a sacrificial electron donor, the reduction of water to H2 were studied. The results showed that the hydrogen evolution in organic phase is more efficient than that of aqueous phase. Moreover, in organic phase, P(Pyrldn)3 is the ligand with the best hydrogen generation efficiency. In the future, these artificial ruthenium-substituted H-cluster mimics will be one of promising catalysts in photoinduced hydrogen generation industry and the detailed mechanism of these artificial biomimetic hydrogenases will be further discussed.