Ligand versus metal protonation of an iron hydrogenase active site mimic
Ligand versus metal protonation of an iron hydrogenase active site mimic
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DOI：
10.1002/chem.200700019
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发表时间：
2007-01-01
影响因子：
4.3
通讯作者：
Lomoth, Reiner
中科院分区：
文献类型：
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作者：
Eilers, Gerriet;Schwartz, Lennart;Lomoth, Reiner
用光谱、电化学和计算方法研究了铁氢化酶活性中心模拟物[Fe（2）（mu-adt）（CO）（4）（PMe（3））（2）（1; adt= N-苄基氮杂二硫醇盐）]的质子化行为. adt桥和供电子膦配体的组合允许adt氮质子化以产生[Fe（2）（mu-adt）（CO）（4）（PMe（3））（2）]（+）（[1H]（+）），Fe-Fe键质子化以产生[Fe（2）-（mu-adt）（mu-H）（CO）（4）（PMe（3））（2）]（+）（[1][Hy]（+）），或两个位点同时进行，得到[Fe（2）（mu-Hadt）（mu-H）（CO）（4）（PMe（3））（2）]（2+）（[1HHy]（2+））。配合物1及其质子化产物在乙腈溶液中的结构经IR、（1）H和（31）P NMR表征。所有质子化状态的溶液结构的特点是膦配体的基底/基底取向，这与I在固态中的基底/顶端结构形成对比。对所有质子化态进行了密度泛函计算，计算光谱与实验光谱的比较证实了结构归属。配体质子化络合物[1H]（+）（pK（a）= 12）是初始的亚稳态质子化产物，而氢化物[1Hy]（+）（pK（a）= 15）是化学稳定的单质子化形式。阳离子[1H]（+）到[1Hy]（+）的互变异构化不会自发发生。而HCl（k=2.2M（-1）S（-1））可催化该反应，从而选择性地生成阳离子[1Hy]（+）。两个碱性位点的质子化对它们的碱性有很强的相互影响，使得双质子化阳离子配合物[1HHy]（2+）的氢化物（pK（a）=8）和铵质子（pK（a）-5）比单质子化类似物的酸性大得多。用高氯酸或三氟甲磺酸（k = 0.15 M（-1）S（-1））从阳离子[1H]（+）形成二价阳离子[1HHy]（2+）的速度非常慢，而用氢溴酸（k > 102 M（-1）S（-1））形成二价阳离子的速度则快得多。在电化学上，I在-2.2 V处相对于二茂铁发生不可逆还原，质子化后，阳离子配合物[1H]（+）、[1Hy]（+）和[1HHy]（2+）的电位分别变为-1.6、-1.1和-1.0 V。双质子化形式[1HHy]（2+）在比所有先前报道的氢化酶模型更小的负电位下被还原，尽管在此电位下催化质子还原的特征在于缓慢的周转。
The protonation behavior of the iron hydrogenase active-site mimic [Fe(2)(mu-adt)(CO)(4)(PMe(3))(2) (1; adt=N-benzyl-azadithiolate) has been investigated by spectroscopic, electrochemical, and computational methods. The combination of an adt bridge and electron-donating phosphine ligands allows protonation of either the adt nitrogen to give [Fe(2)(mu-Hadt)(CO)(4)(PMe(3))(2)](+) ([1H](+)), the Fe-Fe bond to give [Fe(2)-(mu-adt)(mu-H)(CO)(4)(PMe(3))(2)](+) ([1][Hy](+)), or both sites simultaneously to give [Fe(2)(mu-Hadt)(mu-H)(CO)(4)(PMe(3))(2)](2+) ([1HHy](2+)). Complex 1 and its protonation products have been characterized in acetonitrile solution by IR, (1)H, and (31)P NMR spectroscopy. The solution structures of all protonation states feature a basal/basal orientation of the phosphine ligands, which contrasts with the basal/apical structure of I in the solid state. Densitv functional calculations have been performed on all protonation states and a comparison between calculated and experimental spectra confirms the structural assignments. The ligand protonated complex [1H](+) (pK(a) = 12) is the initial, metastable protonation product while the hydride [1Hy](+) (pK(a) = 15) is the thermodynamically stable singly protonated form. Tautomerization of cation [1H](+) to [1Hy](+) does not occur spontaneously. However, it can be catalyzed by HCI (k=2.2M(-1)S(-1)), which results in the selective formation of cation [1Hy](+). The protonations of the two basic sites have strong mutual effects on their basicities such that the hydride (pK(a),=8) and the ammonium proton (pK(a)-5) of the doubly protonated cationic complex [1HHy](2+) are considerably more acidic than in the singly protonated analogues. The formation of dication [1HHy](2+) from cation [1H](+) is exceptionally slow with perchloric or trifluoromethanesulfonic acid (k = 0.15 M(-1)S(-1)), while the dication is formed substantially faster (k > 102 M(-1) S(-1)) with hydrobromic acid. Electrochemically, I undergoes irreversible reduction at -2.2 V versus ferrocene, and this potential shifts to -1.6, -1.1, and -1.0 V for the cationic complexes [1H](+), [1Hy](+), and [1HHy](2+) respectively, upon protonation. The doubly protonated form [1HHy](2+) is reduced at less negative potential than all previously reported hydrogenase models, although catalytic proton reduction at this potential is characterized by slow turnover.