单铁氢化酶五配位模型化合物的合成、表征及催化反应性

氢化酶仿生化学是当前有机金属化学领域研究的前沿课题,其主要内容为针对氢化酶的活性中心结构和功能进行化学模拟研究.自然界中已经发现的氢化酶有三种,其中[NiFe]氢化酶、[FeFe]氢化酶研究较多.单铁氢化酶发现于1990年,是产甲烷杆菌在厌氧和镍缺乏的条件下合成的.区别于其他两种氢化酶,其活性中心不含Fe-S簇,且仅含有一个Fe原子,并且仅能在底物存在的情况下,催化异裂氢分子并选择性还原特定底物,为产甲烷杆菌代谢提供能量.研究单铁氢化酶的结构和功能,模拟其活化氢、利用氢的过程,对于探索清洁能源的利用和开发新的非贵金属催化剂具有重要意义.本文以单铁氢化酶(Hmd)结构和功能模拟为导向,针对单铁氢化酶一级配位结构,设计合成了两个新模型化合物.通过IR, NMR, X射线单晶衍射等手段表征分析了模型化合物的性质并确认其结构.探索了其质子化反应特性、电催化还原质子制氢的特性.为了进一步模拟Hmd催化裂解氢气、完成氢转移的功能,以所合成模型物为催化剂实现了在常温常压下,以乙醇作为质子源的催化转移氢化过程.新单铁模型配合物Fe(CO)2PR3(NN)(R = Cy (3), Ph (4), NN,邻苯二胺二价阴离子配体)由NN二齿配体与前体化合物Fe(CO)3I2PR3进行配体取代反应合成.模型化合物活性中心为一个二价铁原子,拥有两个处于cis-位置的羰基配体,一个邻苯二胺双齿配体(两个氮原子进行配位)以及一个有机膦配体.通过红外光谱表征所合成的具有不饱和五配位结构化合物的光谱性质,可以得到配合物Fe(CO)2PCy3(NN)的羰基红外特征谱峰为1974,1919 cm–1,配合物Fe(CO)2PPh3(NN)的红外特征谱峰在1985和1929 cm–1处.通过单晶X射线衍射表征确认了两个化合物结构,并获取晶体学数据.经研究发现, Fe(CO)2PR3(NN)能够发生酸碱调控下可逆的质子化/脱质子化过程.基于红外光谱和密度泛函理论计算推断邻苯二胺阴离子配体可以作为内部碱基.在酸性条件下, Fe(CO)2PR3(NN)分子内部碱基氮原子通过质子化反应结合一个质子,生成Fe(CO)2PR3(NN)·H+.加入碱之后,重新生成起始化合物Fe(CO)2PR3(NN).表明N原子作为内部碱基,具有结合和转移质子的能力.该性质与Hmd中半胱氨酸硫配体具有一致性.通过循环伏安曲线研究了配合物Fe(CO)2PCy3(NN)和Fe(CO)2PPh3(NN)的电化学性质.其中配合物Fe(CO)2PCy3(NN)和Fe(CO)2PPh3(NN)均具有两个不可逆的还原峰和氧化峰.在电化学制氢研究中,配合物Fe(CO)2PPh3(NN)的还原峰电流随着乙酸的加入增幅较大,展现出较强的催化质子还原的性质.通过与其他单铁模型配合物对比,可以推断第一个还原峰归属为配合物由FeI转化为FeI,第二个可逆还原峰归属为配合物由FeI转化为Fe0.同时,配合物Fe(CO)2PPh3(NN)第一个还原峰向高电位移动,该现象与双铁模型化合物的电化学性质较为一致.进一步研究发现,模型化合物具有催化转移氢化的活性.在常温下,乙醇溶剂中, Fe(CO)2PCy3(NN)能够催化对苯醌还原转化为对苯二酚,其中对苯醌的转化率达到89%,对苯二酚的产率达到40%.结合实验数据以及文献资料分析,认为乙醇在催化氢化中可以作为质子源,并且提出了催化转移氢化反应过程的机理.认为催化氢化过程中形成了-Fe-H-C-O-H-N-六元环,通过分子间相互作用完成了氢原子转移过程.该研究结论对单铁氢化酶活性中心模型化合物在催化氢化反应中的应用具有一定的参考价值.     

Synthesis and Reactivity of Mononuclear Iron Models of [Fe]‐Hydrogenase that Contain an Acylmethylpyridinol Ligand

[Fe]‐hydrogenase has a single iron‐containing active site that features an acylmethylpyridinol ligand. This unique ligand environment had yet to be reproduced in synthetic models; however the synthesis and reactivity of a new class of small molecule mimics of [Fe]‐hydrogenase in which a mono‐iron center is ligated by an acylmethylpyridinol ligand has now been achieved. Key to the preparation of these model compounds is the successful C?O cleavage of an alkyl ether moiety to form the desired pyridinol ligand. Reaction of solvated complex [(2‐CH₂CO‐6‐HOC₅H₃N)Fe(CO)₂(CH₃CN)₂]⁺(BF₄)^? with thiols or thiophenols in the presence of NEt₃ yielded 5‐coordinate iron thiolate complexes. Further derivation produced complexes [(2‐CH₂CO‐6‐HOC₅H₃N)Fe(CO)₂(SCH₂CH₂OH)] and [(2‐CH₂CO‐6‐HOC₅H₃N)Fe(CO)₂(CH₃COO)], which can be regarded as models of FeGP cofactors of [Fe]‐hydrogenase extracted by 2‐mercaptoethanol and acetic acid, respectively. When the derivative complexes were treated with HBF₄?Et₂O, the solvated complex was regenerated by protonation of the thiolate ligands. The reactivity of several models with CO, isocyanide, cyanide, and H₂ was also investigated.     

Synthesis and characterization of a series of model complexes of the active site of [Fe]-hydrogenase (Hmd)

A series of Fe complexes were synthesized and characterized as small molecule mimics for the active site of [Fe]-hydrogenase (Hmd). The collection includes both structurally new compounds and analogues of previously reported models. These complexes contain the essential ligands of the enzyme, namely, acyl, CO, pyridone, and sulfur ligands. They serve as IR and Mo?ssbauer spectroscopic models for the Fe center in [Fe]-hydrogenase. The field-dependent Mo?ssbauer study of representative model complexes shows that the sign and absolute value of the quadrupole splitting are sensitive to the change in the ligand environment of the Fe center.     

Multiple-timescale photoreactivity of a model compound related to the active site of [FeFe]-hydrogenase

Ultraviolet (UV) photolysis of (mu-S(CH 2) 3S)Fe 2(CO) 6 ( 1), a model compound of the Fe-hydrogenase enzyme system, has been carried out. When ultrafast UV-pump infrared (IR)-probe spectroscopy, steady-state Fourier transform IR spectroscopic methods, and density functional theory simulations are employed, it has been determined that irradiation of 1 in an alkane solution at 350 nm leads to the formation of two isomers of the 16-electron complex (mu-S(CH 2) 3S)Fe 2(CO) 5 within 50 ps with evidence of a weakly associated solvent adduct complex. 1 is subsequently recovered on timescales covering several minutes. These studies constitute the first attempt to study the photochemistry and reactivity of these enzyme active site models in solution following carbonyl ligand photolysis.     

Iron acyl thiolato carbonyls: structural models for the active site of the [Fe]-hydrogenase (Hmd)

Phosphine-modified thioester derivatives are shown to serve as efficient precursors to phosphine-stabilized ferrous acyl thiolato carbonyls, which replicate key structural features of the active site of the hydrogenase Hmd. The reaction of Ph(2)PC(6)H(4)C(O)SPh and sources of Fe(0) generates both Fe(SPh)(Ph(2)PC(6)H(4)CO)(CO)(3) (1) and the diferrous diacyl Fe(2)(SPh)(2)(CO)(3)(Ph(2)PC(6)H(4)CO)(2), which carbonylates to give 1. For the extremely bulky arylthioester Ph(2)PC(6)H(4)C(O)SC(6)H(3)-2,6-(2,4,6-trimethylphenyl)(2), oxidative addition is arrested and the Fe(0) adduct of the phosphine is obtained. Complex 1 reacts with cyanide to give Et(4)N[Fe(SPh)(Ph(2)PC(6)H(4)CO)(CN)(CO)(2)] (Et(4)N[2]). (13)C and (31)P NMR spectra indicate that substitution is stereospecific and cis to P. The IR spectrum of [2](-) in ν(CN) and ν(CO) regions very closely matches that for Hmd(CN). XANES and EXAFS measurements also indicate close structural and electronic similarity of Et(4)N[2] to the active site of wild-type Hmd. Complex 1 also stereospecifically forms a derivative with TsCH(2)NC, but the adduct is more labile than Et(4)N[2]. Tricarbonyl 1 was found to reversibly protonate to give a thermally labile derivative, IR measurements of which indicate that the acyl and thiolate ligands are probably not protonated in Hmd.     

A [RuRu] Analogue of an [FeFe]‐Hydrogenase Traps the Key Hydride Intermediate of the Catalytic Cycle

The active site of the [FeFe]‐hydrogenases features a binuclear [2Fe]_H sub‐cluster that contains a unique bridging amine moiety close to an exposed iron center. Heterolytic splitting of H₂ results in the formation of a transient terminal hydride at this iron site, which, however is difficult to stabilize. We show that the hydride intermediate forms immediately when [2Fe]_H is replaced with [2Ru]_H analogues through artificial maturation. Outside the protein, the [2Ru]_H analogues form bridging hydrides, which rearrange to terminal hydrides after insertion into the apo‐protein. H/D exchange of the hydride only occurs for [2Ru]_H analogues containing the bridging amine moiety.     

Evidence for acyl-iron ligation in the active site of [Fe]-hydrogenase provided by mass spectrometry and infrared spectroscopy

[Fe]-hydrogenase catalyzes the reversible heterolytic cleavage of H(2) and stereo-specific hydride transfer to the substrate methenyltetrahydromethanopterin in methanogenic archaea. This enzyme contains a unique iron guanylylpyridinol (FeGP) cofactor as a prosthetic group. It has recently been proposed-on the basis of crystal structural analyses of the [Fe]-hydrogenase holoenzyme-that the FeGP cofactor contains an acyl-iron ligation, the first one reported in a biological system. We report here that the cofactor can be reversibly extracted with acids; its exact mass has been determined by electrospray ionization Fourier transform ion cyclotron resonance mass-spectrometry. The measured mass of the intact cofactor and its gas-phase fragments are consistent with the proposed structure. The mass of the light decomposition products of the cofactor support the presence of acyl-iron ligation. Attenuated total reflection infrared spectroscopy of the FeGP cofactor revealed a band near wave number 1700 cm(-1), which was assigned to the C=O (double bond) stretching mode of the acyl-iron ligand.