Oxoiron(IV) Tetramethylcyclam Complexes with Axial Carboxylate Ligands: Effect of Tethering the Carboxylate on Reactivity
Oxoiron(IV) Tetramethylcyclam Complexes with Axial Carboxylate Ligands: Effect of Tethering the Carboxylate on Reactivity
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DOI：
10.1021/acs.inorgchem.6b02659
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发表时间：
2017-03-20
影响因子：
4.6
通讯作者：
Que, Lawrence, Jr.
中科院分区：
文献类型：
--
作者：
Bigelow, Jennifer O.;England, Jason;Que, Lawrence, Jr.
含氧铁（IV）物种是非血红素单铁加氧酶中的活性中间体，通常作为氢原子从底物转移的试剂。组氨酸是迄今为止表征的大多数酶中最可能的配体反式到氧代单元，但在异青霉素N合酶的情况下被羧酸酯取代。反式羧酸盐的影响。配体对氧代铁（IV）单元性质的影响尚未系统研究，我们合成并表征了四种由四甲基环胺（TMC）大环支撑的氧代铁（IV）配合物，其具有羧酸根配体反式到氧代单元。两个配合物具有乙酸酯或丙酸酯轴向配体，而另外两个具有通过一个或两个亚甲基单元栓系到大环配体框架的羧酸酯官能团。有趣的是，这四种复合物表现出底物氧化速率相差超过100倍，尽管Fe=O单元还原的E-p（c）值仅跨越130 mV的范围。1，4-环己二烯氧化的Eyring参数表明，反应性的差异源于与束缚的羧酸盐和那些与untethered羧酸盐的络合物之间的活化焓的差异，与计算结果一致。如前所述，对于四个络合物的初始子集，Fe-IV（O）TMC（X）系列的11个络合物的氧原子转移速率的均方根值随观察到的E-Pi（c）值线性增加，反映了Fe=0单元的亲电性。与此相反，没有观察到相应的氢原子转移（HAT）反应速率与E-p，E-c值的相关性;相反，HAT速率随着计算的三重态五重态自旋态间隙变窄而增加，与Shaik的两态反应性模型一致。事实上，这两个配合物与未束缚的羧酸是最活跃的HAT代理在这一系列中，表明轴向配体可以发挥关键作用，在调整的HAT反应性在非血红素铁酶活性位点。
Oxoiron(IV) species are implicated as reactive intermediates in nonheme monoiron oxygenases, often acting as the agent for hydrogen-atom transfer from substrate. A histidine is the most likely ligand trans to the oxo unit in most enzymes characterized thus far but is replaced by a carboxylate in the case of isopenicillin N synthase. As the effect of a trans carboxylate. ligand on the properties of the oxoiron(IV) unit has not been systematically studied, we have synthesized and characterized four oxoiron(IV) complexes supported by the tetramethylcyclam (TMC) macrocycle and having a carboxylate ligand trans to the oxo unit. Two complexes have acetate or propionate axial ligands, while the other two have the carboxylate functionality tethered to the macro cyclic ligand framework by one or two methylene units. Interestingly, these four complexes exhibit substrate oxidation rates that differ by more than 100-fold, despite having E-p,(c) values for the reduction of the Fe=O unit that span a range of only 130 mV. Eyring parameters for 1,4-cyclohexadiene oxidation show that reactivity differences originate from differences in activation enthalpy between complexes with tethered carboxylates and those with untethered carboxylates, in agreement with computational results. As noted previously for the initial subset of four complexes, the logarithms of the oxygen atom transfer rates of 11 complexes of the Fe-IV(O)TMC(X) series increase linearly with the observed E-P,(c) values, reflecting the electrophilicity of the Fe=0 unit. In contrast, no correlation with E-p,E-c values is observed for the corresponding hydrogen atom transfer (HAT) reaction rates; instead, the HAT rates increase as the computed triplet quintet spin state gap narrows, consistent with Shaik's two-state-reactivity model. In fact, the two complexes with untethered carboxylates are among the most reactive HAT agents in this series, demonstrating that the axial ligand can play a key role in tuning the HAT reactivity in a nonheme iron enzyme active site.