Institute of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem , 91904 Jerusalem, Israel.
J Am Chem Soc. 2015 Jan 21;137(2):722-33. doi: 10.1021/ja509465w. Epub 2015 Jan 9.
This article addresses the intriguing hydrogen-abstraction (H-abstraction) and oxygen-transfer (O-transfer) reactivity of a series of nonheme Fe(IV)(O)(TMC)(Lax) complexes, with a tetramethyl cyclam ligand and a variable axial ligand (Lax), toward three substrates: 1,4-cyclohexadiene, 9,10-dihydroanthracene, and triphenyl phosphine. Experimentally, O-transfer-reactivity follows the relative electrophilicity of the complexes, whereas the corresponding H-abstraction-reactivity generally increases as the axial ligand becomes a better electron donor, hence exhibiting an antielectrophilic trend. Our theoretical results show that the antielectrophilic trend in H-abstraction is affected by tunneling contributions. Room-temperature tunneling increases with increase of the electron donation power of the axial-ligand, and this reverses the natural electrophilic trend, as revealed through calculations without tunneling, and leads to the observed antielectrophilic trend. By contrast, O-transfer-reactivity, not being subject to tunneling, retains an electrophilic-dependent reactivity trend, as revealed experimentally and computationally. Tunneling-corrected kinetic-isotope effect (KIE) calculations matched the experimental KIE values only if all of the H-abstraction reactions proceeded on the quintet state (S = 2) surface. As such, the present results corroborate the initially predicted two-state reactivity (TSR) scenario for these reactions. The increase of tunneling with the electron-releasing power of the axial ligand, and the reversal of the "natural" reactivity pattern, support the "tunneling control" hypothesis (Schreiner et al., ref 19). Should these predictions be corroborated, the entire field of C-H bond activation in bioinorganic chemistry would lay open to reinvestigation.
本文探讨了一系列非血红素Fe(IV)(O)(TMC)(Lax)配合物的引人入胜的氢提取(H-abstraction)和氧转移(O-transfer)反应性，其中 TMC 代表四甲基环酰胺配体，Lax 代表可变轴向配体。这些配合物对三种底物（1,4-环己二烯、9,10-二氢蒽和三苯基膦）具有反应性。实验结果表明，O-transfer 反应性遵循配合物的相对亲电性，而相应的 H-abstraction 反应性通常随着轴向配体成为更好的电子供体而增加，因此表现出反电性趋势。我们的理论结果表明，H-abstraction 中的反电性趋势受隧道效应贡献的影响。室温下的隧道效应随着轴向配体电子供体能力的增加而增加，这会反转自然的亲电性趋势，如不考虑隧道效应的计算所揭示的那样，并导致观察到的反电性趋势。相比之下，不受隧道效应影响的 O-transfer 反应性保留了亲电性依赖性的反应性趋势，这在实验和计算中都得到了证实。只有当所有的 H-abstraction 反应都在 quintet 态（S = 2）表面上进行时，经过隧道校正的动力学同位素效应（KIE）计算才与实验 KIE 值匹配。因此，目前的结果证实了这些反应最初预测的两态反应性（TSR）情景。隧道效应随轴向配体供电子能力的增加而增加，以及“自然”反应性模式的反转，支持了“隧道控制”假说（Schreiner 等人，参考文献 19）。如果这些预测得到证实，整个生物无机化学中 C-H 键活化领域将重新受到研究。
