N-杂环螯合高柠檬酸钒（钼）及其同系物研究

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英文题名：N-heterocycle Chelated Homocitrato Oxovanadium(V/IV) and Oxomolybdenum(V/VI) Complexes and Their Homologues 作者：陈灿玉 论文级别：博士 学科专业名称：物理化学 中文关键词：铁钼辅基 ; 高柠檬酸 ; 柠檬酸 ; α-羟基羧酸 英文关键词：Iron-molybdenum cofactor ; Homocitrate ; Citrate ; α-Hydroxycarboxylate 学位年度：2008 导师：周朝晖 学科代码：070304 学位授予单位：厦门大学 论文提交日期：2008-06-01 答辩委员会主席：廖代伟

摘要

固氮酶是某些微生物在常温常压下固氮成氨的催化剂,其催化作用机理和化学模拟一直是国际上长期致力研究的对象。钼铁蛋白的单晶高分辨X光衍射分析表明,铁钼辅基的结构为MoFe_7S_9X(S-cys)(N-his)(homocit)(x=C,N,O)。其中,Mo原子处于一端的角落位置上,并和3个μ_3-硫配体、一个组氨酸和一个高柠檬酸配位,形成八面体的络合物。高柠檬酸以α-烷氧基氧和α-羧基氧直接同钼形成双齿配位。在钼元素缺乏的环境中,则形成了含钒的钒固氮酶。最新的研究成果表明,在固氮酶固氮铁铝辅基的生物合成过程中,其中心金属和高柠檬酸是在最后的步骤中才插入铁硫簇前驱体中。但中心金属和高柠檬酸是如何插入铁硫簇前驱体和以什么样的结构形式插入其中都还有待了解。另外,α-羧酸钼、钒主要以双核配合物存在,因而了解羧酸钼、钒的双核和单核之间的相互联系以及所形成配合物的成键规律,有利于进一步模拟铁钼辅基的结构,为固氮酶的固氮机理提供化学依据。
     本文以柠檬酸、苹果酸、柠苹酸和高柠檬酸为配体,钼、钒为金属源,再引入邻菲啰啉和联吡啶含氮配体,合成了一系列的配合物1-19:[Hneo]_4[V_2O_4(R-Hcit)(OH)][V_2O_4(S-Hcit)(OH)]·4H_2O(1),[Ni(phen)_3]_2[V_2O_4(R-Hcit)(OC_2H_5)][V_2O_4(S-Hcit)(OC_2H_5)]·4H_2O(2),[V_2O_3(phen)_3(Hcit)]·5H_2O(3),[V_2O_3(phen)_3(Hcit)_2(phen)_3O_3V_2]·12H_2O(4),Na_3(Hhomocit)·H_2O(5),[V_2O_3(phen)_3(R,S-H_2homocit)]Cl·6H_2O(6),[VO_2(phen)_2]_2[V_2O_4(R,S-H_2homocit)_2]·4H_2O·2C_2H_5OH(7),[V_2O_3(phen)_3(mal)H(mal)(phen)_3O_3V_2]Cl·14H_2O(8),[VO(bpy)(R,S-H_2cit)]·2H_2O(9),[VO(phen)(R,S-H_2cit)]·1.5H_2O(10),[VO(phen)(R,S-H_2cit)]·6.5H_2O(11),[VO(bpy)(R,S-Hmal)]·H_2O(12),[VO(phen)(R,S-Hmal)]·H_2O(13),[VO(bpy)(R-Hcitmal)]·2H_2O(14),[Co(phen)(R,S-H_2cit)(H_2O)]·3H_2O(15),[Ni(phen)(R,S-H_2cit)(H_2O)]·3H_2O(16),[(MoO_2)_2O(phen)(H_2cit)(H_2O)_2]·H_2O(17),[(MoO)_2O(bpy)_2(H_2cit)_2]·4H_2O(18),[MoO_2(bpy)(H_2cit)]·H_2O(19)。主要结果总结如下:
     一、1和2的配阴离子为不对称双核V_2O_2结构,钒与柠檬酸的比例为2:1,柠檬酸为三齿配体与两个钒配位,α-烷氧基氧作为桥氧与两个钒键联,α-羧基氧和β-羧基氧分别与钒配位,形成五元环或六元环。5是高柠檬酸的正盐,为链式结构。配合物3,4,6和8为不对称双核V_2O_3结构,钒与羧酸的比例为2:1,羧酸以α-烷氧基氧和α-羧基氧与钒配位,形成五元环。配合物7为对称双核V_2O_2结构,是配合物6的前驱体。配合物9-16为单核的金属配合物,羧酸α-烷氧基氧、α-羧基氧和β-羧基氧与金属配位,形成MN_2O_4(M=V,Co,Ni)结构。17为双核柠檬酸钼配合物,为Mo_2O_5L_1L_2结构,柠檬酸以α-烷氧基氧和α-羧基氧与钼配位,形成五元环。19为单核柠檬酸配合物,形成MoN_2O_4结构,柠檬酸也以α-烷氧基氧和α-羧基氧与钼配位,形成五元环。18是还原态钼(Ⅴ)的柠檬酸配合物,为对称的双核结构,两个钼通过一个桥氧键连,每个钼为MoN_2O_4结构,柠檬酸也以α-烷氧基氧和α-羧基氧与钼配位,形成五元环。在以上配合物中,羟基羧酸配体都可以用α-烷氧基氧和α-羧基氧与钼、钒进行双齿配位,形成五元环,这与固氮酶中高柠檬酸钼的配位形式相同,因此这种配位形式是羟基羧酸与钼、钒相互作用的一种典型模式,这一结论为通过羟基羧酸的金属钼和钒配合物研究固氮酶的配位提供了间接证据。
     二、通过配合物3和4中不同氢键模式的研究以及它们之间的转化反应,我们发现柠檬酸的β-羧基不仅可以与结晶水形成氢键,还可以在β-羧基之间形成分子内和分子间的氢键,且分子内和分子间氢键间存在相互转化。由此说明不仅高柠檬酸γ-羧基可以参与质子的传递,其β-羧基也有可能参与质子的传递。这为固氮酶中高柠檬酸搭桥的质子(电子)传递途径提供了佐证。
     三、不对称的双核钒和单核钒配合物的合成和表征显示,pH值对产物的形成和分离具有至关重要的作用。pH值在一定范围内,柠檬酸的烷氧基氧作为桥氧的化学键才容易断开,从而形成不对称的双核钒和单核钒结构。pH值不仅影响不对称双核V_2O_3钒配合物和单核钒配合物的形成,还关系到钒还原的速度。这两类配合物可以相互转化的,如:这为我们研究固氮酶反应体系中中心金属的价态变化提供了依据。
     四、双核柠檬酸钼(Ⅵ)和柠檬酸钼(Ⅴ)的成功合成分离,说明在双核柠檬酸钼配合物中,其三齿配位的β-羧基配位是可以被取代的。而首次双齿配位的柠檬酸单核钼(Ⅵ)的成功提取,也说明高柠檬酸和钼有可能在最后插入铁硫簇前驱体中时可以以单核的方式插入,且高柠檬酸与钼形成了双齿配位。
Nitrogenase catalyzes the reduction of dinitrogen to ammonia in the process of biological nitrogen fixation. In the past few decades, its catalytic mechanism and chemical simulation have been widely studied. The high resolution (1.16 (?)) X-ray structural analysis of the MoFe protein of nitrogenase reveals the FeMo-co (iron molybdenum cofactor) as a cage structure, MoFe_7S_9X(S-cys)(N-his)(homocit). The molybdenum atom is coordinated by three sulfur atoms, a nitrogen atom from histidine and two oxygen atoms from homocitrate. The homocitrate entity employs itsα-alkoxy andα-carboxy oxygen atoms chelating to the molybdenum atom. In addition, some organisms have alternative nitrogenases containing metal atoms like Fe and V, or Fe only. These alternative nitrogenases are inferred to have much similarity with Mo-nitrogenase in structure, but appear to be less efficient than the latter and are generally only expressed when Mo-nitrogenase is unavailable in the organism.
     Recent reference shows that potentially molybdenum and homocitrate are transferred into the NifEN protein in the last step. How do the molybdenum and homocitrate add to the precursor cluster and what are the detail composition and structure of molybdenum-homocitrate system remain unclear, as well as the molybdenum-hydroxycarboxylate system and vanadium-hydroxycarboxylate system.
     In order to further mimic the coordinative environment of molybdenum in FeMo-co and study theα-hydroxycarboxylato molybdenum and vanadium species in solutions, we have studied vanadium and molybdenum complexes 1-19 with citric acid, malic acid, citric acid, citramalic acid, homocitric acid as ligands, as well as 1,10-phenanthroline and 2, 2'-bipydine: [Hneo]_4[V_2O_4(R-Hcit)(OH)][V_2O_4(S-Hcit)(OH)]·4H_2O (1), [Ni(phen)_3]_2[V_2O_4(R-Hcit)(OC_2H_5)] [V_2O_4(S-Hcit)(OC_2H_5)]·4H_2O (2), [V_2O_3(phen)_3(Hcit)]·5H_2O (3),[V_2O_3(phen)_3(Hcit)_2(phen)_3O_3V_2]·12H_2O (4), Na_3(Hhomocit)·H_2O (5),[V_2O_3(phen)_3(R,S-H_2homocit)]Cl·6H_2O (6),[VO_2(phen)_2]_2[V_2O_4(R,S-H_2homocit)_2]·4H_2O·2C_2H_5OH (7),[V_2O_3(phen)_3(mal)H(mal)(phen)_3O_3V_2]Cl·14H_2O (8), [VO(bpy)(R,S-H_2cit)]·2H_2O (9), [VO(phen)(R,S-H_2cit)]·1.5H_2O (10),[VO(phen)(R,S-H_2cit)]·6.5H_2O (11), [VO(bpy)(R,S-Hmal)]·H_2O (12), [VO(phen)(R,S-Hmal)]·H_2O (13), [VO(bpy)(R-Hcitmal)]·2H_2O (14), [Co(phen)(R,S-H_2cit)(H_2O)]·3H_2O (15), [Ni(phen)(R,S-H_2cit)(H_2O)]·3H_2O (16), [(MoO_2)_2O(phen)(H_2cit)(H_2O)_2]H_2O (17), [(MoO)_2O(bpy)_2(H_2cit)_2]·4H_2O (18), [MoO_2(bpy)(H_2cit)]·H_2O (19). The results are summarized as follows:
     1, The anions of complexes 1 and 2 have asymmetric V_2O_2 core containing hydroxy or ethoxy bridges, respectively. The molar ratio of vanadium and citrate is 2: 1, the citrate acts as tridentate ligand via itsα-alkoxy,α-carboxy andβ-carboxy groups. 5 is trisodium homocitrate. 3,4,6 and 8 have asymmetric V_2O_3 core containing oxygen atom bridges. The molar ratio of vanadium and citrate is also 2: 1, theα-hydroxycarboxylates act as bidentate ligands via theirα-alkoxy andα-carboxy groups. The complex 7 has symmetric V_2O_ core, which is a precursor of complex 6. The a-hydroxycarboxylates act as tridentate ligand via theirα-hydroxy,α-carboxy andβ-carboxy groups in complexes 9-16, and each metal formed MN_2O_4 configuration (M=V,Co,Ni). Complex 17 is asymmetric dinuclear citrato molybdate, the citrate chelates bidentately to molybdenum via its a-alkoxy and a-carboxy groups. Complex 19 is mononuclear citrato molybdate, formed MoN_2O_4 configuration, the citrate also chelates bidentately to molybdenum via itsα-alkoxy andα-carboxy groups. Symmetric dinuclear structure was found in molybdenum (V) complex 18. Each molybdenum atom is in MoN_2O_4 configuration, and the citrate also acts as bidentate ligand via its a-alkoxy andα-carboxy groups. The bidentate coordination modes of molybdenum or vanadium in these hydrocarboxylato complexes are similar to that of homocitrato molybdate in FeMo-co. Itseems that the complexes could be served as model complexes for exploring the coordination environment of molybdenum in nitrogenase.
     2, By studying the different hydrogen bond models and their relationship in complex 3 and 4, it is found that theβ-carboxy groups of citrate not only hydrogen bond to lattice water, but also form intramolecular and intermolecular hydrogen bonds each other. The species with intramolecular hydrogen bond can transformed into the species with intermolecular hydrogen bond. Thus, it is proposed that the homocitrate may participate in proton's transformation byγ-carboxy groups in the nitrogen fixation, but also withβ-carboxy groups.
     3, The synthesis and characterization of asymmetric dinuclear and mononuclear vanadium complexes show that the pH value is very important for the products' formation and isolation. In some pH range, theα-alkoxy bridge was easy to break and form asymmetric dinuclear and mononuclear vanadium structure. The pH value also affects the speed of reduction of vanadium. The asymmetric dinuclear and mononuclear vanadium complexes can be transformed each other, e.g.:
     4, Dinuclear citrato molybdenum (Ⅴ,Ⅵ) complexes 17 and 18 were successfully synthesized and isolated, show that theβ-carboxy group of citrate can be replaced in dinuclear citrato molybdenum complexes. Meanwhile, the first mononuclear citrato molybdenum (Ⅵ) complex 19 was isolated and the citrate acted as bidentate ligand. It is proposed that the homocitrate and molybdenum were transferred into precursor cluster by mononuclear mode,while the homocitrate acts as bidentate ligand.

引文

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