【作者】 陈林；

【导师】 王梅；

【作者基本信息】 大连理工大学 ， 应用化学， 2013， 博士

【摘要】 铁铁氢化酶是人类迄今为止发现的最高效的水还原催化剂,其催化速率高达6000-9000s-1。自从其活性中心的单晶结构被测定后,人们围绕它的结构模拟、功能模拟、催化机理做了大量研究工作。通过对一些结构特殊的模型配合物的研究,人们在实验室条件下重现了自然界氢化酶的某些特征。尤其是获得的单电子氧化态中间体与氢化酶活性中心氧化态的构型颇为相像,并表现出相似的电子分布和顺磁特征。相似的构型却未能复制出氢化酶的高催化效率,铁铁模型配合物的功能模拟至今还没有大的突破。相比之下,以钴、镍为中心的金属配合物却往往表现出较好的光和电催化质子还原能力,成为目前氢化酶功能模拟的主要研究方向。本论文合成并表征了四核和六核铁硫簇合物2、2’和3。这种迷你型的树枝状铁硫配合物表现出与以往报道的多核铁簇合物不同的氧化还原性质。配合物2和3可以在-1.33V～-1.81V(vs Fc+/0)电势范围内分别发生基于金属中心的连续两次和三次的双电子还原反应,在很窄的电势范围内分别获得四个和六个电子。SEC-IR谱图证明配合物2和3的[Fe2S2(CO)6]基团在还原状态下有桥羰基形成。配合物2和3在氧化还原过程中表现出的良好的可逆性,说明苯桥的共轭及刚性对于稳定其还原态物种起到重要的作用。论文设计合成了一种特殊桥连的氢化酶活性中心模型配合物6,利用“帽”式桥引入不对称的配位环境。来自桥上的斥力使其铁中心相比类似配合物6b更易被氧化。配合物6被氧化后,桥连结构上的酮羰基与溶剂中的水发生加成反应,酮羰基的氧配到铁中心氧化后配体旋转留下的空位上,得到了相对稳定的氧化态产物7。7具有与氢化酶活性中心的一种氧化态(Hoxar)相似的构型、相同的价态,为氢化酶活性中心结构模拟提供了一种新的参考模型。论文还发现了钴配合物9无论在中性缓冲液中还是在海水中都是一个非常好的电催化还原水的催化剂,其催化水还原过电位低(80mV),活性高,寿命长。在pH7的磷酸缓冲液中,-1.0V电压下,经过20小时的电解,表现出1470mol H2(mol cat)-1h-1(cm2Hg)-’的催化速率,而且没有明显失活。配合物9的另一个优点是能够用廉价的无机盐通过自组装反应得到,这让人联想到自然界的许多金属酶都是通过自组装生成的。这些研究结果表明通过合理设计分子催化剂的结构可以大幅提高电解水制氢的效率,有可能以地球上大量存在的元素作为原料,合成能够代替贵金属铂的高效的分子催化剂,用于水还原产氢。更多还原

【Abstract】 [FeFe]-hydrogenase ([FeFe]-H2ase) is so far the most efficient catalyst for water reduction, which has a catalytic rate of6000～9000mol H2per second. Chemists have extensively studied its structural and functional mimics and catalytic mechanism since the crystal structure of [FeFe]-H2ase was determined. Chemists have got some features of the hydrogenases existing in nature by studying specially designed model complexes. The intermediates of one electron oxidation state of [FeFe]-H2ases models obtained in lab show almost the same configuration as the Hox state of hydrogenases. These models have similar electron distribution and magnetic property as the Hox. Unfortunately, although the models possess similar structure of [FeFe]-H2ases, they have catalytic properties different from hydrogenases. Some Co and Ni complexes show good photochemical and electrochemical catalytic H2-evolving properties, giving examples for mimicking the function of hydrogenases by small molecules, though no breakthrough is found in functional mimic of [FeFe]-H2ases.Tetra-and hexanuclear iron-sulfur complexes2,2’and3were prepared and structurally characterized. These mini dendritic iron complexes display redox feature quite different from that of most reported polynucleariron complexes. Complexes2and3display, respectively, two and three consecutive two-electron, metal-center-based reductionevents in the range of-1.33to-1.81V, with relatively narrow potential spans for four-and six-electron transformations. SEC-IR spectra give experimental evidence for the formation of a μ-CO bridge in the two-electron-reduced [Fe2S2(CO)6] unit of2and3. The good reversibility of the reduction processes implies the stabilization of the reduced species of2and3by the conjugated and rigid bridge.A specially degnined bridge was introduced into the model complex6of [FeFe]-hydrogenase, The "cap-like" bridge resulted in an asymmetric coordination environment of6. The repulsive force from the bridge made the iron center oxidized more easily than its analogue6b. When complex6was oxidized, the ketone carbonyl of the bridge reacted with the water in solvent, and formed a gem-diol sepcies. The oxygen of the ketone coordinates to the open site on the iron center resulting from rotation, thereby forming a covalent bond with the iron atom to afford a relative stable oxidized product7. This complex shows a similar structure with the oxidized state (Hoxair) of the [FeFe]-H2ase active site, and provides a new Fe"Fe" model for the mimic of [FeFe]-H2ases.A cobalt complex9proves to be an excellent catalyst for electrochemical production of hydrogen both in neutral water and sea water, which displays an extremely low overpotential (80mV) while maintaining high activity and good durability. It displays an activity of1470mol H2(mol cat)-1h-1(cm2Hg)-1over20h CPE (controlled potential electrolysis) experiment at an applied potential of-1.0V in phosphate buffer at pH7, without apparent deactivation. The other merit of this catalyst is its self-assembling property from simple salts of earth-abundant elements, which is reminiscent of self-formation of many metalloenzymes in nature. The discovery of this unusual molecular cobalt catalyst shows promise in using earth-abundant metal-based catalysts as surrogates of precious metal-based ones such as platinum to create highly efficient and economical catalyst systems for water reduction to a sustainable fuel-hydrogen.更多还原