Labrum Nicholas S, Cabelof Alyssa C, Caulton Kenneth G
Department of Chemistry, Indiana University, 800 E. Kirkwood Ave., Bloomington, IN, 47405-7102, USA.
Chemistry. 2020 Nov 2;26(61):13915-13926. doi: 10.1002/chem.202001749. Epub 2020 Sep 30.
Reduction of the bis-pyrazolyl pyridine complex [CrL] with 4 KC , followed by addition of one azobenzene (overall mole ratio 1:4:1), PhNNPh, transfers reducing equivalents to three azobenzenes, to form [K Cr(PhNNPh) ]. This has three κ PhNNPh ligands and K bound to nitrogen atoms of azobenzene. When the stoichiometry is modified to 1:4:3, the product is changed to [K CrL(PhNNPh) ], which has C symmetry except for the intimate ion pairing of two K ions to reduced azobenzene nitrogen atoms, and to pyrazolate and phenyl rings. The origin of the observed delivery of reducing equivalents to several, not to a single N=N bond, is traced to the resistance of the one-electron-reduced substrate to receiving a second electron, and is thus a general phenomenon. [CrL] alone is shown to be a two-electron reductant towards benzo[c]cinnoline (BCC) resulting in a product of formula [Cr L (BCC)], in which the reducing equivalents originate purely from Cr . An analogous study of the reaction of [CrL] with azobenzene yields [Cr L (PhNNPh)(THF)], an adduct in which one THF has displaced one of four hydrazide nitrogen/Cr bonds. Together these illustrate different modes for the Cr L unit to bind and reduce the N=N bond. Collectively, these results show that two divalent Cr, without added K , have the ability to reduce the N=N bond. Further KC reduction of preformed Cr L (RNNR) inevitably gives products in which K stabilizes the charge in the increasingly electron-rich nitrogen atoms, in a phenomenon which mimics proton coupled electron transfer: K performs the role of H . A least-squares fit of the two singly reduced DFT structures shows that the only major change is a re-orientation of one of the two phenyl rings in order to avoid repulsion with potassium but to still allow interaction of that phenyl π system with K . This shows both the impact of K , being modest to nitrogen/chromium interactions, but nevertheless accommodating some π donation of phenyl to potassium. Finally, delivering increasing equivalents of KC leads to complete cleavage of the N=N bond, and both N bind to three Cr . The varied impacts of the K electrophile on NN multiple bond reduction is discussed.
用4 KC还原双吡唑基吡啶配合物[CrL]，随后加入一个偶氮苯（总摩尔比1:4:1），即PhNNPh，将还原当量转移至三个偶氮苯，形成[K Cr(PhNNPh) ]。该配合物有三个κ PhNNPh配体，且K与偶氮苯的氮原子相连。当化学计量比改为1:4:3时，产物变为[K CrL(PhNNPh) ]，除了两个K离子与还原的偶氮苯氮原子、吡唑酸根和苯环紧密离子配对外，它具有C对称性。观察到还原当量传递至多个而非单个N=N键的原因可追溯到单电子还原底物接受第二个电子的阻力，因此这是一种普遍现象。单独的[CrL]被证明是对苯并[c]噌啉（BCC）的双电子还原剂，生成式为[Cr L (BCC)]的产物，其中还原当量纯粹源自Cr 。对[CrL]与偶氮苯反应的类似研究得到[Cr L (PhNNPh)(THF)]，这是一种加合物，其中一个THF取代了四个酰肼氮/Cr键中的一个。这些共同说明了Cr L单元结合和还原N=N键的不同模式。总体而言，这些结果表明，两个二价Cr在不添加K的情况下具有还原N=N键的能力。对预先形成的Cr L (RNNR)进一步用KC还原不可避免地会得到产物，其中K在越来越富电子的氮原子中稳定电荷，这一现象类似于质子耦合电子转移：K起到了H的作用。对两个单还原DFT结构的最小二乘拟合表明，唯一的主要变化是两个苯环之一的重新取向，以避免与钾的排斥，但仍允许该苯π体系与K相互作用。这既显示了K的影响，它对氮/铬相互作用影响不大，但仍能接受苯向钾的一些π电子给予。最后输送越来越多当量 的KC会导致N=N键完全断裂，且两个N都与三个Cr结合。讨论了K亲电试剂对NN多重键还原的不同影响。
