Structures of nonheme oxoiron(IV) complexes from x-ray crystallography, NMR spectroscopy, and DFT calculations
Structures of nonheme oxoiron(IV) complexes from x-ray crystallography, NMR spectroscopy, and DFT calculations
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DOI：
10.1002/anie.200500485
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发表时间：
2005-01-01
影响因子：
16.6
通讯作者：
Que, L
中科院分区：
文献类型：
--
作者：
Klinker, EJ;Kaizer, J;Que, L
在单核非血红素铁酶的氧活化机制中，氧化铁(IV)物种经常被引用作为活性中间体。[1]2003年，报道了O2与2-氧戊二酸依赖的酶TauD与2-氧戊二酸及其底物牛磺酸反应中第一个非血红素氧铁(IV)中间体的捕获。[2-5]同时，还描述了第一个具有良好特征的合成氧铁(IV)与非血红素配体环境的络合物的例子。7]四氮杂配体如大环四甲基环丙烯(TMC)和三脚架三(2-吡啶甲基)胺(TPA)可以充分稳定可能具有反应性的氧合铁(IV)单元，从而使它们能够在低温下进行光谱表征。在TMC络合物的情况下，其相当的稳定性使其能够结晶，其结构可以通过X射线结晶学确定。这些结果代表了任何氧铁(IV)络合物、亚铁血红素或非亚铁血红素的第一个高分辨率结构数据。随后发现，五齿五氮杂配体如N，N-二(2-吡啶甲基)-N-二(2-吡啶)甲胺(N4Py)和N-苄基-N，N‘，N’-三(2-吡啶甲基)-1，2-二氨基乙烷(Bn-TPEN)的前体铁(II)络合物也可以通过过氧酸或PHIO处理而生成。事实上，在室温下，[FeIV(O)(N4Py)]2+(1)和[FeIV(O)(Bn-TPEN)]2+(2)可以通过与过量的固体碘代苯在CH3CN中反应而得到具有显著热稳定性的溶液。[8]在这里，我们报道了通过X射线结晶学、核磁共振和密度泛函理论计算得到的1和2的结构及其相对热稳定性。在室温下，配合物1和2的半衰期分别约为60和6h。1具有更高的热稳定性，因此可以分离单晶进行X射线晶体结构分析(图1)，从而仅提供氧铁(IV)络合物的第二个高分辨率结构。X-射线晶体结构1的Fe±O键长为1.639(5)，与[Fe(O)(Tmc)(NCCH_3)](SO_3CF3)_2(3)中类似键的长度1.646(3)基本相同。[6]在1的氧代配体上的反式是将五齿配体结合在一起的胺氮原子，Fe±N1键长为2.033(8)，这是分子中最长的金属配位键。O_1、Fe和N_1原子几乎共线，O_1-Fe-N_1角为178.6(3)8。在赤道平面上配位的是四个吡啶氮原子，它们的环平行于Fe-O轴排列，铁原子位于这四个吡啶氮原子所覆盖的平面上方0.252，朝向氧基配体。有趣的是，赤道Fe±N键的平均长度为1.957(5)；这些键比3中相应的赤道键短0.1%，反映了吡啶部分相对于叔胺更强的成键能力。
Oxoiron (iv) species are frequently invoked as reactive intermediates in the oxygen-activation mechanisms of mononuclear nonheme iron enzymes.[1] In 2003, the trapping of the first nonheme oxoiron (iv) intermediate in the reaction of O2 with the 2-oxoglutarate-dependent enzyme TauD, which was complexed with 2-oxoglutarate and its substrate taurine, was reported.[2–5] Contemporaneously, the first examples of wellcharacterized synthetic oxoiron (iv) complexes with nonheme ligand environments were also described.[6, 7] The presumably reactive oxoiron (iv) units could be stabilized sufficiently by tetraaza ligands such as macrocyclic tetramethylcyclam (TMC) and tripodal tris (2-pyridylmethyl) amine (TPA) to allow their spectroscopic characterization at low temperature. In the case of the TMC complex, its considerable stability allowed it to be crystallized and its structure to be determined by X-ray crystallography. These results represented the first high-resolution structural data for any oxoiron (iv) complex, heme or nonheme. Subsequently it was found that the oxoiron (iv) unit could also be generated from precursor iron (ii) complexes of pentadentate pentaaza ligands such as N, N-bis (2-pyridylmethyl)-N-bis (2-pyridyl) methylamine (N4Py) and N-benzyl-N, N’, N’-tris (2-pyridylmethyl)-1, 2-diaminoethane (Bn-TPEN) by treatment with a peracid or PhIO. In fact,[FeIV (O)(N4Py)] 2+(1) and [FeIV (O)(Bn-TPEN)] 2+(2) could be produced in high yields at room temperature by reaction with excess solid iodosylbenzene in CH3CN to afford solutions with significant thermal stability.[8] Herein, we report insight into the structures of 1 and 2 obtained from a combination of X-ray crystallography, NMR spectroscopy, and DFT (density functional theory) calculations, and their relative thermal stabilities. Complexes 1 and 2 exhibit half-lives of approximately 60 and 6h, respectively, at room temperature. The greater thermal stability of 1 allowed the isolation of single crystals for X-ray crystal structural analysis (Figure 1) to provide only the second high-resolution structure of an oxoiron (iv) complex. The X-ray crystal structure of 1 shows an FeÀO bond length of 1.639 (5), a value that is essentially identical to the length of 1.646 (3) reported for the analogous bond in [Fe (O)(TMC)(NCCH3)](SO3CF3) 2 (3).[6] Trans to the oxo ligand in 1 is the amine nitrogen atom that holds the pentadentate ligand together, with an FeÀN1 bond length of 2.033 (8), the longest metal–ligand bond in the molecule. The O1, Fe, and N1 atoms are nearly colinear with an O1-Fe-N1 angle of 178.6 (3) 8. Coordinated in the equatorial plane are the four pyridine nitrogen atoms, whose rings are aligned parallel to the Fe–O axis, and the iron atom lies 0.252 above the plane subtended by these four pyridyl nitrogen atoms towards the oxo ligand. Interestingly, the equatorial FeÀN bond lengths are on average 1.957 (5); these bonds are shorter than the corresponding equatorial bonds in 3 by 0.1 and reflect the stronger bonding ability of a pyridine moiety relative to a tertiary amine.