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.