LOW-POTENTIAL NICKEL(III,II) COMPLEXES - NEW SYSTEMS BASED ON TETRADENTATE AMIDATE THIOLATE LIGANDS AND THE INFLUENCE OF LIGAND STRUCTURE ON POTENTIALS IN RELATION TO THE NICKEL SITE IN [NIFE]-HYDROGENASES
LOW-POTENTIAL NICKEL(III,II) COMPLEXES - NEW SYSTEMS BASED ON TETRADENTATE AMIDATE THIOLATE LIGANDS AND THE INFLUENCE OF LIGAND STRUCTURE ON POTENTIALS IN RELATION TO THE NICKEL SITE IN [NIFE]-HYDROGENASES
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DOI：
10.1021/ic00004a025
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发表时间：
1991-02-20
影响因子：
4.6
通讯作者：
HOLM, RH
中科院分区：
文献类型：
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作者：
KRUGER, HJ;PENG, G;HOLM, RH
在一个持续的研究镍（II，III）配合物与低氧化还原电位可能相关的镍网站在[NiFe]-氢化酶。合成了N，N ′-亚乙基双（2-巯基乙酰胺）（H_4ema）、N，N ′-1，2-亚苯基双（2-巯基乙酰胺）（H_4phma）和N，N ′-亚乙基双（2-巯基异丁酰胺）（H_4emi）四元阴离子的镍配合物。这些配体被分离为它们的S-乙酰基衍生物;它们在碱性条件下与Ni（II）反应得到红色抗磁性配合物[Ni（ema）]2-（5）、[Ni（phma）]2-（6）和[Ni（emi）]2-（7），它们被分离为Et 4 N+盐。通过（Et 4 N）2[Ni（ema）]·2 H2O的X射线结构测定证明了配体的四齿性质，其在正交晶系空间群Pmma中结晶，其中a = 28.973（8）埃，B = 12.239（4）埃，c = 8.389（2）埃，并且Z = 4。阴离子是平面的，Ni-S = 2.179（1）埃和Ni-N = 1.857（3）埃。配合物进行可逆的单电子氧化（E1/2 vs SCE，循环伏安法）：[Ni（ema）]-，2-，-0.34 V; [Ni（phma）]-，2-，-0.24 V; [Ni（emi）]-，2-，-0.42 V。其中[Ni（emi）]-最稳定，但在纯态下不能分离。这些电位是已知的Ni（III，II）对中最低的。总结了在本实验室和其他实验室制备的配合物中，这对夫妇的低电位的因素。其中主要是阴离子可极化配体和2-净电荷的Ni（II）配合物。对低电位配合物[Ni（pdtc）2]-，2-（pdtc =吡啶-2，6-双（硫代羧酸酯）（2-））和[Ni（nbdt）2]-1-，2-（nbdt =正戊烷-2，3-二硫纶（2-））进行了电子结构计算;对于相应的Ni（III）物种，指出了基态sigma-*（d（z）2）和pi-*（d（xz）或d（yz））。当名义上比较时，[Ni（emi）]-，2-电位接近于在脱硫弧菌（Desulfovibrio gigas）氢化酶中还原Ni-A态的电位（-0.39 V）。然而，当考虑溶剂和pH对氧化还原电位的影响时，可以得出结论，在合成物种中，[Ni（nbdt）2]-，2-对的电位最接近酶位点的电位。半胱氨酸可能是稳定Ni（III）的最有效的天然配体，但可能尚未发现金属连接的生物模式。
In a continuing study of Ni(II,III) complexes with low redox potentials that are of possible relevance to the nickel site in [NiFe]-hydrogenases. Ni(II) complexes of the tetraanions of the amide-thiol ligands N,N'-ethylenebis(2-mercaptoacetamide) (H4ema), N,N'-1,2-phenylenebis(2-mercaptoacetamide) (H4phma), and N,N'-ethylenebis(2-mercaptoisobutyramide) (H4emi) have been prepared. The ligands were isolated as their S-acetyl derivatives; their reaction with Ni(II) under basic conditions afforded the red diamagnetic complexes [Ni(ema)]2- (5), [Ni(phma)]2- (6), and [Ni(emi)]2- (7), which were isolated as Et4N+ salts. The tetradentate nature of the ligands was demonstrated by the X-ray structure determination of (Et4N)2[Ni(ema)].2H2O, which crystallizes in orthorhombic space group Pmma with a = 28.973 (8) angstrom, b = 12.239 (4) angstrom, c = 8.389 (2) angstrom, and Z = 4. The anion is planar with Ni-S = 2.179 (1) angstrom and Ni-N = 1.857 (3) angstrom. The complexes undergo reversible one-electron oxidations (E1/2 vs SCE, cyclic voltammetry) in DMF solutions: [Ni(ema)]-,2-, -0.34 V; [Ni(phma)]-,2-, -0.24 V; [Ni(emi)]-,2-, -0.42 V. The EPR spectra of the products demonstrate metal-centered oxidation to form Ni(III) species; of these, [Ni(emi)]- is the most stable, but it could not be isolated in pure condition. These potentials are among the lowest known for the Ni(III,II) couple. The factors affording low potentials of this couple in complexes prepared in this and other laboratories are summarized. Chief among these are anionic polarizable ligands and 2- net charge of the Ni(II) complexes. Electronic structural calculations were made for the low-potential complexes [Ni(pdtc)2]-,2- (pdtc = pyridine-2, 6-bis(thiocarboxylate)(2-)) and [Ni(nbdt)2]-1-,2- (nbdt = norbornane-2, 3-dithiolate(2-)); for the respective Ni(III) species the ground states sigma-*(d(z)2) and pi-*(d(xz) or d(yz)) are indicated. When nominally compared, the [Ni(emi)]-,2- potential is close to that (-0.39 V) for reduction of the Ni-A state in Desulfovibrio gigas hydrogenase. However, when the effects of solvent and pH on redox potentials are considered, it is concluded that, among synthetic species, the potential of the [Ni(nbdt)2]-,2- couple is closest to that of the enzyme site. Cysteinate is likely the most effective native ligand in stabilizing Ni(III), but not biological modes of metal ligation may have been discovered.