Manganese(II) complexes of 1,1′-bis[(pyridin-2-yl)methyl)]-2,2′-bipiperidine (PYBP): Synthesis, structure, catalytic properties in alkene epoxidation with hydrogen peroxide, and related mechanistic studies

Abstract

Several manganese(II) complexes with the stereoisomers of ligand PYBP (1,1′-bis[(pyridin-2-yl)methyl]-2,2′-bipiperidine) and different anions were prepared and characterized by X-ray diffractometry. Complex [MnII(rac-PYBP)]2+ (1) was found to be an efficient catalyst of alkene epoxidation by hydrogen peroxide in the presence of acetic acid in acetonitrile at room temperature. Cyclooctene was converted to its epoxide with up to 91 % yield, 99.6 % selectivity, and the turnover number of 180 within 5 min. Fast epoxidations of cyclohexene, 1-decene, styrene, and cis-stilbene were also achieved. Isomeric complex [MnII(meso-PYBP)]2+ (2) was catalytically inactive under the same experimental conditions. Stopped-flow spectrophotometry and freeze-quenched EPR spectra show that complex 2 is not oxidized by H2O2 in the presence of acetic acid (AcOH) but instead undergoes partial ligand protonation and liberation of the Mn2+ cations due to the relatively poor chelating ability of ligand meso-PYBP. The rac-PYBP isomer acts as a better ligand and retains the coordinated Mn center when complex 1 is treated with the H2O2/AcOH mixture in acetonitrile solution yielding a mixture of intensely colored intermediates likely involving MnIII, MnIV, and MnV complexes. Magnetic susceptibility measurements, UV–vis and EPR spectra suggest that dinuclear complexes [MnIII2(μ-O)(μ-OAc)(rac-PYBP)2]3+ and [MnIIIMnIV(μ-O)2(rac-PYBP)2]3+ gradually accumulate in the reaction mixture as inactivated states of the catalyst. Complex 1 also causes fast decomposition of hydrogen peroxide into O2 gas and H2O, which competes with the epoxidation of alkenes and requires gradual addition of H2O2 for its efficient use. The catalytic activity of complex 1 is strongly influenced by its counterions and decreases in order ClO4− ≈ SbF6− > NO3– > Cl− indicating that labile ligands in the coordination sphere of Mn are required for the activation of H2O2.