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

A series of Mn(II) complexes [Mn(L)Cl2] 1–5, 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 MnCl2·6H2O 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.

A series of Mn(II) complexes of linear diazacycloalkane 4N ligands as catalysts for alkene epoxidation using PhIO as oxidant. Lewis acidity of the metal center tuned upon enlarging the diazacycloalkane ring backbone. The epoxide yield and product selectivity increase upon increasing the Lewis acidity of the Mn(II) center.Figure optionsDownload as PowerPoint slideHighlights
► Mn(II) complexes of linear diazacycloalkane 4N ligands has been prepared.
► The complex 5 adopts rare trigonal prismatic coordination geometry.
► Complexes screened for selective epoxidation of suitable alkenes.
► Correlation of Lewis acidity of Mn(II) center and stability of the high valent species responsible for catalytic reaction.