Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur-741246, India.
Department of Chemistry, Indian Institute of Technology Jammu, Jammu 181221, India.
Dalton Trans. 2020 May 26;49(20):6816-6831. doi: 10.1039/d0dt00466a.
We have synthesized a series of new redox non-innocent azo aromatic pincer-like ligands: 2-(phenylazo)-6-(arylaminomethyl)pyridine (HLa-c: HLa = 2-(phenylazo)-6-(2,6-diisopropylphenylaminomethyl)pyridine, HLb = 2-(phenylazo)-6-(2,6-dimethylphenylaminomethyl)pyridine, and HLc = 2-(phenylazo)-6-(phenylaminomethyl)pyridine), in which one side arm is an arylaminomethyl moiety and the other arm is a 2-phenylazo moiety. Nickel(ii) complexes, 1-3, of these ligands HLa-c were synthesized in good yield (approximately 70%) by the reaction of ligands : (NiCl2·6H2O) in a 1 : 1 molar ratio in methanol. The amine donor in each of the ligands HLa-c binds to the Ni(ii) centre without deprotonation. In the solid state, complex 3 is a dimer; in solution it exists as monomer 3a. The reduction of acetonitrile solutions of each of the complexes 1, 2 and 3a, separately, with cobaltocene (1 equivalent), followed by exposure of the solution to air, resulted in the formation of new complexes 7, 8 and 9, respectively. Novel free ligands Lx and Ly have also been isolated, in addition to complexes 7 and 8, from the reaction of complexes 1 and 2, respectively. Complexes 7-9 and free ligands Lx and Ly have been formed via a dehydrogenation reaction of the arylaminomethyl side arm. The mechanism of the reaction was thoroughly investigated using a series of studies, including cyclic voltammetry, EPR, and UV-Vis spectral studies and density functional theory (DFT) calculations. The results of these studies suggest a mechanism initiated by ligand reduction followed by dioxygen activation. A Cl-/I- scrambling experiment revealed that the dissociation of the chloride ligand(s) was associated with the one-electron reduction of the ligand (azo moiety) in each of the complexes 1, 2 and 3a. The dissociated chloride ligand(s) were reassociated with the metal following the dehydrogenation reaction to yield the final products. All of the newly synthesized compounds were fully characterized using a variety of physicochemical techniques. Single-crystal X-ray structures of the representative compounds were determined to confirm the identities of the synthesized molecules.
我们合成了一系列新的氧化还原非自由基偶氮芳基 Pincer 类配体：2-(苯基偶氮)-6-(芳胺甲基)吡啶（HL a-c：HL a = 2-(苯基偶氮)-6-(2,6-二异丙基苯氨基甲基)吡啶，HL b = 2-(苯基偶氮)-6-(2,6-二甲基苯氨基甲基)吡啶，HL c = 2-(苯基偶氮)-6-(苯氨基甲基)吡啶），其中一个侧臂为芳胺甲基部分，另一个臂为 2-苯基偶氮部分。这些配体 HL a-c 的镍（ii）配合物 1-3 通过配体的反应以良好的收率（约 70%）合成：（NiCl2·6H2O）在甲醇中以 1:1 的摩尔比反应。每个配体 HL a-c 中的胺供体与 Ni(ii)中心结合而不脱质子化。在固态下，配合物 3 是二聚体；在溶液中，它以单体 3a 的形式存在。分别将每个配合物 1、2 和 3a 的乙腈溶液用环戊二烯基二羰基合钴（1 当量）还原，然后暴露于空气中，得到新的配合物 7、8 和 9。除了配合物 7 和 8 之外，还从配合物 1 和 2 的反应中分离出了新型游离配体 Lx 和 Ly。配合物 7-9 和游离配体 Lx 和 Ly 通过芳胺甲基侧臂的脱氢反应形成。通过一系列研究，包括循环伏安法、EPR、UV-Vis 光谱研究和密度泛函理论（DFT）计算，彻底研究了反应的机理。这些研究的结果表明，该反应的机理是由配体还原引发的，随后是氧气的活化。Cl-/I- scrambling 实验表明，在配合物 1、2 和 3a 中，每个配合物中氯配体（s）的解离与配体（偶氮部分）的单电子还原有关。脱氢反应后，解离的氯配体（s）与金属重新结合，生成最终产物。所有新合成的化合物均采用多种物理化学技术进行了充分表征。通过单晶 X 射线结构确定了代表性化合物的结构，以确认合成分子的身份。
