Iron(III) complexes of tridentate 3N ligands as functional models for catechol dioxygenases: the role of ligand N-alkyl substitution and solvent on reaction rate and product selectivity.
Iron(III) complexes of tridentate 3N ligands as functional models for catechol dioxygenases: the role of ligand N-alkyl substitution and solvent on reaction rate and product selectivity.
复制标题
三齿 3N 配体的铁 (III) 配合物作为儿茶酚双加氧酶的功能模型：配体 N-烷基取代和溶剂对反应速率和产物选择性的作用。
作者：
Kusalendiran Visvaganesan;R. Mayilmurugan;E. Suresh;M. Palaniandavar
[Fe(L)Cl3]类型的一系列铁(III)络合物，其中L是不同N-烷基取代的双(吡啶-2-基甲基)胺配体，例如双(吡啶-2-基甲基)胺(L1)、N,N-双(吡啶-2-基甲基)甲胺(L2)、N,N-双(吡啶-2-基甲基)-正丙胺(L3)， N,N-双(吡啶-2-基甲基)-异丁胺(L4)、N,N-双(吡啶-2-基甲基)-异丙胺(L5)、N,N-双(吡啶-2-基甲基)环己胺(L6)和N,N-双(吡啶-2-基甲基)-叔丁胺(L7)已通过元素分析、光谱和电化学方法进行分离和表征。配合物 [Fe(L2)Cl3] 2、[Fe(L3)Cl3] 3 和配合物-底物加合物 [Fe(L5)(TCC)(NO3)] 5a（其中 TCC2- 是四氯儿茶酚酸双阴离子）的晶体结构已通过单晶 X 射线晶体学测定。配合物[Fe(L2)Cl3] 2 和[Fe(L3)Cl3] 3 具有扭曲的八面体几何形状，其中线性三齿3N配体与铁(III)中心顺面配位，三个氯离子占据剩余的配位位点。如3中那样，2中的N-甲基被N-正丙基取代，导致形成Fe-Npy键以及反式不同长度的Fe-Cl键。儿茶酚加合物5a还具有扭曲的八面体几何结构，其中配体与铁(III)中心顺面配位，TCC2-与配体的两个吡啶基部分不对称螯合反式，硝酸根离子的一个氧原子占据第六配位位点。所有现有的配合物都与简单的和取代的儿茶酚相互作用。原位生成了儿茶酚加合物[Fe(L)(DBC)Cl]和[Fe(L)(DBC)(Sol)]+，其中H2DBC是3,5-二叔丁基儿茶酚，Sol=H2O/CH3CN，并在二甲基甲酰胺和二氯甲烷溶液中研究了它们的光谱和氧化还原特性以及双加氧酶活性。所有复合物均使用分子氧催化 H2DBC 裂解，以提供二醇内和二醇外裂解产物。 3N 配体的顺面配位和铁 (III) 中心上用于双氧结合的空配位点的可用性促进了 extradiol 裂解产物的形成。值得注意的是，3N 配体中 N-烷基取代基的性质控制着裂解的区域选择性，其中正丙基、异丁基、异丙基和环己基提高了二氯甲烷中 Extradiol 产物的产率 (46-68%)。氧化速率取决于溶剂和铁(III)中心的路易斯酸度，其通过空间要求的N-烷基基团长度和取代度进行修饰。 log (kO2) 与低能 DBC2-to-iron(III) LMCT 带能量的关系图是线性的，证明了铁 (III) 中心的路易斯酸度在决定双加氧酶反应速率方面的重要性。
A series of iron(III) complexes of the type [Fe(L)Cl3], where L is the variously N-alkyl-substituted bis(pyrid-2-ylmethyl)amine ligand such as bis(pyrid-2-ylmethyl)amine (L1), N,N-bis(pyrid-2-ylmethyl)methylamine (L2), N,N-bis(pyrid-2-ylmethyl)-n-propylamine (L3), N,N-bis(pyrid-2-ylmethyl)-iso-butylamine (L4), N,N-bis(pyrid-2-ylmethyl)-iso-propylamine (L5), N,N-bis(pyrid-2-ylmethyl)cyclohexylamine (L6), and N,N-bis(pyrid-2-ylmethyl)-tert-butylamine (L7), have been isolated and characterized by elemental analysis and spectral and electrochemical methods. The crystal structures of the complexes [Fe(L2)Cl3] 2, [Fe(L3)Cl3] 3, and the complex-substrate adduct [Fe(L5)(TCC)(NO3)] 5a, where TCC2- is the tetrachlorocatecholate dianion, have been determined by single-crystal X-ray crystallography. The complexes [Fe(L2)Cl3] 2 and [Fe(L3)Cl3] 3 possess a distorted octahedral geometry, in which the linear tridentate 3N ligands are cis-facially coordinated to the iron(III) center, and three chloride ions occupy the remaining coordination sites. The replacement of the N-methyl group in 2 by N-n-propyl group as in 3 leads to the formation of the Fe-Npy bonds and also the Fe-Cl bonds located trans to them of different lengths. The catecholate adduct 5a also possesses a distorted octahedral geometry, in which the ligand is cis-facially coordinated to iron(III) center, TCC2- is asymmetrically chelated trans to the two pyridyl moieties of the ligand, and one of the oxygen atoms of the nitrate ion occupies the sixth coordination site. All of the present complexes have been interacted with simple and substituted catechols. The catecholate adducts [Fe(L)(DBC)Cl] and [Fe(L)(DBC)(Sol)]+, where H2DBC is 3,5-di-tert-butylcatechol and Sol=H2O/CH3CN, have been generated in situ, and their spectral and redox properties and dioxygenase activities have been studied in dimethylformamide and dichloromethane solutions. All of the complexes catalyze the cleavage of H2DBC using molecular oxygen to afford both intra- and extradiol cleavage products. The formation of extradiol cleavage products is facilitated by cis-facial coordination of the 3N ligands and availability of vacant coordination site on iron(III) center for dioxygen binding. It is remarkable that the nature of the N-alkyl substituent in 3N ligands controls the regioselectivity of cleavage, with the n-propyl, iso-butyl, iso-propyl, and cyclohexyl groups enhancing the yield of extradiol products (46-68%) in dichloromethane. The rate of oxygenation depends upon the solvent and the Lewis acidity of iron(III) center as modified by the sterically demanding N-alkyl groups-length and degree of substitution. The plot of log (kO2) versus energy of the low-energy DBC2--to-iron(III) LMCT band is linear, demonstrating the importance of the Lewis acidity of the iron(III) center in dictating the rate of the dioxygenase reaction.