Four-electron oxidative formation of aryl diazenes using a tantalum redox-active ligand complex.
Four-electron oxidative formation of aryl diazenes using a tantalum redox-active ligand complex.
复制标题
使用钽氧化还原活性配体络合物四电子氧化形成芳基二氮烯。
作者：
Ryan A. Zarkesh;J. Ziller;A. F. Heyduk
能够介导多电子转换的过渡金属配合物是各种小分子转换的关键组分。例如，C-H键的氧化和质子还原为H2都是两电子转换。水氧化成O2是一个四电子过程，氮还原成氨是一个六电子过程。金属配合物的设计，以促进或催化这些多电子反应通常依赖于一个或多个过渡金属离子能够两个电子的变化，在一个正式的氧化态。另一种策略是将氧化还原活性配体并入金属配位球中，以在多电子转化期间提供还原或氧化当量。在此，我们报告了使用三齿氧化还原活性配体，N，N-双（3，5-二叔丁基-2-苯氧基）酰胺（[ONO]），配位到钽，影响四电子氧化形成芳基二氮烯。在其还原形式中，[ONO]是一种平面的三齿配体，其以双对称几何构型与过渡金属配位。有机金属合成子TaMe 3Cl 2 [7]已用于制备[ONO] TaMe 2（1），然后将其转化为桥接亚氨基二聚体{[ONO]Ta[m-N（对甲苯基）]L}2（2a，L= NH 2（对甲苯基）; 2b，L=吡啶（py）;方案1）。2b的氧化导致（对甲苯基）N=N（对甲苯基）的定量消除。据我们所知，这是从钽（V）桥接亚氨基二聚体中形成N=N双键和消除有机二氮烯的第一个例子。相关配合物[ONO] TaCl 2（4）与PhICl 2的氧化研究表明，氧化还原活性配体在钽配位圈内收集氧化当量方面发挥着关键作用。本文提出的工作突出了一个新的策略，能够多电子氧化反应的金属配合物的设计。通过二甲基络合物1制备桥接亚氨基络合物2a和2b（方案1）。用nBuLi（2当量）使H3[ONO ]双去质子化，然后用TaCl 2 Me 3处理，在从戊烷中重结晶后以49%产率得到1。化合物1的1H和13 C NMR谱显示了[ONO]配体的特征共振峰。1的甲基配体在1H和13 C NMR谱中分别在d = 0.77和59.7ppm处共振。1的甲基配体易于被苯胺质子化，这导致与两个桥连亚氨基配体形成双金属络合物。如方案1所示，1的苯溶液与2当量的NH 2（对甲苯基）一起加热至回流，导致形成
Transition-metal complexes capable of mediating multielectron transformations are critical components for a variety of small-molecule transformations. For example, the oxidation of C H bonds and the reduction of protons to H2 are both two-electron transformations. The oxidation of water to O2 is a four-electron process and the reduction of nitrogen to ammonia is an overall six-electron process. The design of metal complexes to promote or catalyze these multielectron reactions usually relies on one or more transition-metal ions capable of two-electron changes in a formal oxidation state. An alternative strategy is to incorporate redox-active ligands into the metal coordination sphere to supply reducing or oxidizing equivalents during a multielectron transformation. Herein, we report the use of a tridentate redox-active ligand, N,N-bis(3,5-di-tert-butyl-2-phenoxide)amide ([ONO] ), coordinated to tantalum, to effect the fourelectron oxidative formation of aryl diazenes. In its reduced form, [ONO] is a planar, tridentate ligand that coordinates to transition metals in a meridional geometry. The organometallic synthon TaMe3Cl2 [7] has been used to prepare [ONO]TaMe2 (1), which was then converted into the bridging imido dimer {[ONO]Ta[m-N(p-tolyl)]L}2 (2a, L= NH2(p-tolyl); 2b, L= pyridine (py); Scheme 1). Oxidation of 2b resulted in the quantitative elimination of (p-tolyl)N=N(ptolyl). To the best of our knowledge, this is the first example of N=N double bond formation and organic diazene elimination from a tantalum(V) bridging imido dimer. Oxidation studies of the related complex [ONO]TaCl2 (4) with PhICl2 suggest that the redox-active ligand plays the pivotal role of collecting oxidizing equivalents within the tantalum coordination sphere. The work presented herein highlights a new strategy for the design of metal complexes capable of multielectron oxidation reactions. The bridging imido complexes 2a and 2b were prepared via dimethyl complex 1 (Scheme 1). Double deprotonation of H3[ONO ] with nBuLi (2 equiv) followed by treatment with TaCl2Me3 afforded 1 in 49% yield following recrystallization from pentane. The H and C NMR spectra of 1 showed diagnostic resonances for the [ONO] ligand. The methyl ligands of 1 resonated at d = 0.77 and 59.7 ppm in the H and C NMR spectra, respectively. The methyl ligands of 1 are susceptible to protonolysis by anilines, which results in the formation of bimetallic complexes with two bridging imido ligands. As shown in Scheme 1, benzene solutions of 1 heated to reflux with two equivalents of NH2(p-tolyl) resulted in the formation of