A bench stable formal Cu(iii) N-heterocyclic carbene accessible from simple copper(ii) acetate
A bench stable formal Cu(iii) N-heterocyclic carbene accessible from simple copper(ii) acetate
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DOI：
10.1039/c8sc01834k
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发表时间：
2018-11-21
期刊：
影响因子：
8.4
通讯作者：
Kuehn, Fritz E.
中科院分区：
文献类型：
--
作者：
Ghavami, Zohreh S.;Anneser, Markus R.;Kuehn, Fritz E.
多年来，Cu(iii)NHCs被认为是Cu(i)NHC催化反应中的活性中间体，通过还原消除产生所需的产物，但直到今天，还没有人报道过这种化合物的表征。在研究仿生过渡金属（NHC）配合物的合成及其在均相催化中的应用时，我们最近发现了一种特殊环四元（NHC）配体存在时醋酸铜（ii）的高度不寻常的反应性。其中，通过二甲亚砜（DMSO）中Cu（ii）乙酸酯的简单歧化，观察到第一个稳定的CuNHC化合物的形成，其形式氧化态为+III。在高温下，NHC配体发生选择性单氧化，即使在厌氧条件下也是如此。通过氧-18标记实验，确定了乙酸盐是氧原子的来源。通过循环伏安法进一步证明了该化合物具有很高的稳定性。对其反应性的深入研究揭示了另外四种化合物的参与。其中3个化合物可以通过H-1/ c -13核磁共振、单晶XRD、质谱分析和元素分析进行分离和表征。第四种是Cu(NHC)（3+）化合物通过形式还原消除形成的Cu(i)NHC中间体，通过H-1/C-13-NMR和计算方法进行了原位表征。
For years, Cu(iii)NHCs have been proposed as active intermediates in Cu(i)NHC catalyzed reactions, yielding the desired products by reductive elimination, but until today, no one has ever reported the characterisation of such a compound. When working on the synthesis of biomimetic transition metal (NHC) complexes and their application in homogeneous catalysis, we recently found a highly unusual reactivity for Cu(ii) acetate in the presence of a particular cyclic tetra(NHC) ligand. Therein, the formation of the first stable CuNHC compound, displaying Cu in the formal oxidation state +III, by simple disproportionation of Cu(ii) acetate in dimethyl sulfoxide (DMSO) was observed. At elevated temperatures selective mono-oxidation of the NHC ligand occurs, even under anaerobic conditions. Acetate was identified as the origin of the oxygen atom by O-18-labelling experiments. The remarkably high stability of the title compound was furthermore proven electrochemically by cyclic voltammetry. An in-depth investigation of its reactivity revealed the involvement of four additional compounds. Three of them could be isolated and characterised by H-1/C-13-NMR, single crystal XRD, mass spectrometry and elemental analysis. The fourth, a Cu(i)NHC intermediate, formed by formal reductive elimination from the Cu(NHC)(3+) compound, was characterised in situ by H-1/C-13-NMR and computational methods.