Roles of phenol groups and auxiliary ligand of copper(II) complexes with tetradentate ligands in the aerobic oxidation of benzyl alcohol
Roles of phenol groups and auxiliary ligand of copper(II) complexes with tetradentate ligands in the aerobic oxidation of benzyl alcohol
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四齿配体铜(II)配合物中酚基团和辅助配体在苯甲醇有氧氧化中的作用
DOI：
10.1039/c7dt01716b
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发表时间：
2017
影响因子：
4
通讯作者：
Liu Xiaoming
中科院分区：
文献类型：
--
作者：
Zhan Guangli;Zhong Wei;Wei Zhenhong;Liu Zhenzhen;Liu Xiaoming
本文中，六种具有多齿配体的铜（II）配合物，[Cu（HL 1）（OAc）（HOAc）]（1）、[Cu（HL 2）（OAc）]（2）、[Cu（HL 3）（OAc）]（3）、[CuL 4（OAc）]（4）、[Cu（HL 2）Cl]（5）和[Cu（HL 3）Cl]（6）{H2 L1 = [双（3-叔丁基-2-羟基苄基）]（2-吡啶基甲基）胺，H2 L2 = [（3-叔丁基-2-羟基苄基）（3-三氟甲基-2-羟基苄基）H2 L3 = [双（3-三氟甲基-2-羟基苄基）]（2-吡啶基甲基）胺和HL 4 = [双（2-吡啶基甲基）]（3-叔丁基-2-羟基苄基）胺}。通过紫外-可见光谱、高分辨质谱、X射线单晶衍射和电化学等手段对配合物进行了表征。这些铜（II）配合物已被研究作为催化剂的苯甲醇有氧氧化为苯甲醛介导的克里思（2，2，6，6-四甲基哌啶基-1-氧基）自由基在水中在室温下。机理研究表明，酚盐/苯酚参与了分子内质子转移与结合底物的催化作用。因此，在苯酚环上的三氟甲基基团的存在下显着影响催化过程，因为取代基影响苯酚的酸度，随后，从结合底物的分子内质子转移。在催化过程中，辅助配体（Cl−或OAc−）的解离发生在SN 1途径中，并且需要与底物结合。为了完成催化循环，裂解的辅助配体重新结合到金属中心以再生催化剂。
Herein, six copper(II) complexes with multidentate ligands, [Cu(HL1)(OAc)(HOAc)] (1), [Cu(HL2)(OAc)] (2), [Cu(HL3)(OAc)] (3), [CuL4(OAc)] (4), [Cu(HL2)Cl] (5), and [Cu(HL3)Cl] (6) {H2L1 = [bis(3-tert-butyl-2-hydroxybenzyl)](2-pyridylmethyl)amine, H2L2 = [(3-tert-butyl-2-hydoxybenzyl)(3-trifluoromethyl-2-hydroxybenzyl) (2-pyridylmethyl)]amine, H2L3 = [bis(3-trifluoromethyl-2-hydroxybenzyl)] (2-pyridylmethyl)amine, and HL4 = [bis(2-pyridylmethyl)] (3-tert-butyl-2-hydroxybenzyl)amine}, are reported. The complexes were characterized by UV-vis spectroscopy, high-resolution mass spectrometry, X-ray single-crystal diffraction analysis and electrochemistry. These copper(II) complexes have been investigated as catalysts for the aerobic oxidation of benzyl alcohol to benzaldehyde mediated by TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) radical in water at ambient temperature. Mechanistic investigations have revealed that the phenolate/phenol is involved in the intramolecular proton transfer with a bound substrate in catalysis. Hence, the presence of the trifluoromethyl group on the phenol ring significantly affects the catalysis process since the substituent affects the acidity of phenol, and subsequently, the intramolecular proton transfer from the bound substrate. During catalysis, the dissociation of the auxiliary ligand (Cl− or OAc−) occurred in the SN1 pathway, and it is necessary for the substrate to bind. To complete the catalytic cycle, the cleaved auxiliary ligand rebinds to the metal center to regenerate the catalyst.