Manganese(II) complexes of pyridyl-appended diazacyclo-alkanes: Effect of ligand backbone ring size on catalytic olefin oxidation

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• 作者：Natarajan Saravanan^a ; Mallayan Palaniandavar^a ; ^b ; ^{palanim51@yahoo.com} ; ^{palaniandavarm@gmail.com}
• 关键词：Manganese(II) complexes ; Diazacycloalkane ligands ; Epoxidation ; Electrochemistry ; Catalytic properties ; Iodosylbenzene
• 刊名：Inorganica Chimica Acta
• 出版年：2012
• 期刊代码：40_00201693
• 类别：ch
• 出版时间：30 April, 2012
• 卷：385
• 期：Complete
• 页码：100-111
• 文件大小：824 K

摘要

A series of Mn(II) complexes [Mn(L)Cl₂] dFont">1-dFont">5, where L is a tetradentate 4N ligand such as N,N-bis(2-pyridylmethyl)-1,2-diaminoethane (L1A), 1,4-bis(2-pyridylmethyl)piperazine (L2), N,N-bis(2-pyridyl-methyl)hexahydropyrimidine (L3), N,N-bis(2-pyridylmethyl)-1,4-diazepane (L4) and N,N-bis(2-pyridylmethyl)-1,5-diazocane (L5), has been isolated, characterized by using electronic and ESI-MS spectral techniques and screened for catalytic olefin oxidation with a representative set of olefins. Interestingly, when the ligand N,N-bis(2-pyridylmethyl)imidazolidine (L1) is treated with MnCl₂路6H₂O in methanol it undergoes imidazolidine ring hydrolysis to form the complex [Mn(L1A)Cl₂] possessing a distorted octahedral coordination geometry around Mn(II). The complex [Mn(L3)(OTf)₂(H₂O)] contains Mn(II) with a distorted pentagonal bipyramidal coordination geometry while [Mn(L4)Cl₂] contains Mn(II) with an octahedral coordination geometry. The complex [Mn(L5)Cl₂] adopts a rare trigonal prismatic coordination geometry, presumably because of steric interactions imposed by the ligand backbone. The catalytic ability of the solvent coordinated complex species [Mn(L)(ACN)₂]²⁺ show significant activity towards olefin epoxidation using iodosylbenzene (PhIO) as oxygen source and addition of N-methylimidazole to the reaction mixture increases the epoxide yield. The epoxidation of cis-cyclooctene catalyzed by the complexes proceeds with high conversion (22-65%) and selectivity (100%). The epoxide yield and product selectivity increase upon increasing the Lewis acidity of the Mn(II) center, as modified by the variation in the diazacycloalkane ligand backbone.