Bittner Michael M, Lindeman Sergey V, Popescu Codrina V, Fiedler Adam T
Department of Chemistry, Marquette University , Milwaukee, Wisconsin 53201, United States.
Inorg Chem. 2014 Apr 21;53(8):4047-61. doi: 10.1021/ic403126p. Epub 2014 Apr 3.
This study describes the O2 reactivity of a series of high-spin mononuclear Fe(II) complexes each containing the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine ((Ph2)TIP) ligand and one of the following bidentate, redox-active ligands: 4-tert-butylcatecholate ((tBu)CatH(-)), 4,6-di-tert-butyl-2-aminophenolate ((tBu2)APH(-)), or 4-tert-butyl-1,2-phenylenediamine ((tBu)PDA). The preparation and X-ray structural characterization of [Fe(2+)((Ph2)TIP)((tBu)CatH)]OTf, [3]OTf and Fe(2+)((Ph2)TIP)((tBu)PDA)2, 42 are described here, whereas [Fe(2+)((Ph2)TIP)((tBu2)APH)]OTf, [2]OTf was reported in our previous paper [Bittner et al., Chem.-Eur. J. 2013, 19, 9686-9698]. These complexes mimic the substrate-bound active sites of nonheme iron dioxygenases, which catalyze the oxidative ring-cleavage of aromatic substrates like catechols and aminophenols. Each complex is oxidized in the presence of O2, and the geometric and electronic structures of the resulting complexes were examined with spectroscopic (absorption, EPR, Mössbauer, resonance Raman) and density functional theory (DFT) methods. Complex [3]OTf reacts rapidly with O2 to yield the ferric-catecholate species Fe(3+)((Ph2)TIP)((tBu)Cat) (3(ox)), which undergoes further oxidation to generate an extradiol cleavage product. In contrast, complex 4 experiences a two-electron (2e(-)), ligand-based oxidation to give Fe(2+)((Ph2)TIP)((tBu)DIBQ) (4(ox)), where DIBQ is o-diiminobenzoquinone. The reaction of 2 with O2 is also a 2e(-) process, yet in this case both the Fe center and (tBu2)AP ligand are oxidized; the resulting complex (2(ox)) is best described as Fe(3+)((Ph2)TIP)((tBu2)ISQ), where ISQ is o-iminobenzosemiquinone. Thus, the oxidized complexes display a remarkable continuum of electronic structures ranging from Fe(3+)(L(2-)) (3(ox)) to Fe(3+)(L(•-)) (2(ox)) to Fe(2+)(L(0)) (4(ox)). Notably, the O2 reaction rates vary by a factor of 10(5) across the series, following the order 3 > 2 > 4, even though the complexes have similar structures and Fe(3+/2+) redox potentials. To account for the kinetic data, we examined the relative abilities of the title complexes to bind O2 and participate in H-atom transfer reactions. We conclude that the trend in O2 reactivity can be rationalized by accounting for the role of proton transfer(s) in the overall reaction.
本研究描述了一系列高自旋单核铁（II）配合物的氧反应性，每个配合物都含有面配位的三（4,5-二苯基-1-甲基咪唑-2-基）膦（（Ph2）TIP）配体以及以下双齿、氧化还原活性配体之一：4-叔丁基邻苯二酚（（tBu）CatH（-））、4,6-二叔丁基-2-氨基酚（（tBu2）APH（-））或4-叔丁基-1,2-苯二胺（（tBu）PDA）。本文描述了[Fe（2+）（（Ph2）TIP）（（tBu）CatH）]OTf（[3]OTf）和Fe（2+）（（Ph2）TIP）（（tBu）PDA）2（42）的制备及X射线结构表征，而[Fe（2+）（（Ph2）TIP）（（tBu2）APH）]OTf（[2]OTf）已在我们之前的论文[Bittner等人，《化学 - 欧洲杂志》，2013年，19卷，9686 - 9698页]中报道。这些配合物模拟了非血红素铁双加氧酶的底物结合活性位点，该酶催化诸如邻苯二酚和氨基酚等芳香族底物的氧化环裂解反应。每种配合物在氧气存在下被氧化，并用光谱（吸收、电子顺磁共振、穆斯堡尔、共振拉曼）和密度泛函理论（DFT）方法研究了所得配合物的几何和电子结构。配合物[3]OTf与氧气迅速反应生成铁 - 邻苯二酚盐物种Fe（3+）（（Ph2）TIP）（（tBu）Cat）（3（ox）），其进一步氧化生成二醇裂解产物。相比之下，配合物4经历两电子（2e（-））、基于配体的氧化反应，生成Fe（2+）（（Ph2）TIP）（（tBu）DIBQ）（4（ox）），其中DIBQ是邻二亚氨基苯醌。2与氧气的反应也是一个2e（-）过程，但在这种情况下，铁中心和（tBu2）AP配体都被氧化；所得配合物（2（ox））最好描述为Fe（3+）（（Ph2）TIP）（（tBu2）ISQ），其中ISQ是邻亚氨基苯半醌。因此，氧化后的配合物展现出从Fe（3+）（L（2-））（3（ox））到Fe（3+）（L（•-））（2（ox））再到Fe（2+）（L（0））（4（ox））的显著连续电子结构。值得注意的是，尽管这些配合物具有相似的结构和Fe（3+/2+）氧化还原电位，但整个系列中氧气反应速率相差10^5倍，顺序为3 > 2 > 4。为了解释动力学数据，我们研究了标题配合物结合氧气和参与氢原子转移反应的相对能力。我们得出结论，通过考虑质子转移在整个反应中的作用，可以合理解释氧气反应性的趋势。
