Synthesis and characterization of manganese(III) and high-valent manganese-oxo complexes and their roles in conversion of alkenes to cyclic carbonates

A novel manganese(III) complex (2) of an amido-amine ligand (1) was successfully synthesized and characterized using various analytical techniques such as H-1 NMR and single-crystal X-ray diffraction. Xray crystallography data of 2 has demonstrated that the complex is a six-coordinated species. Electrospray ionization mass spectrometry (ESI-MS) also showed the characteristic peak (m/z 401.6, negative ion mode) of 2 indicating the formation of the complex. 2 was successfully employed to convert a variety of olefins to cyclic carbonate in the presence of carbon dioxide (CO2), an oxidant (e.g. tert-butylhydroperoxide, TBHP) and a co-catalyst (tetrabutylammonium bromide, TBAB) in a single-pot under mild reaction conditions. Upon reaction with an oxidant, 2 generated a non-heme high-valent manganese-oxo species (3), which was responsible for conversion of alkene to epoxide, and further cycloaddition of CO2 occurs in the presence of 2 and TBAB. The effect of reaction conditions (temperature and pressure), oxidants, and alkenes were systematically investigated for cyclic carbonate synthesis. High turnover numbers (50-240) and yield were obtained when various alkenes were employed. Additionally, 3 was characterized using various spectroscopic and mass spectrometry techniques. Fourier transform infrared spectrum (Fr-IR) of 3 indicated a peak at 839 cm(-1) corresponding to manganese-oxo stretching. Isotope labeling studies of the oxo group of 3 revealed a peak shift to 804 cm(-1) upon exchange of the O-16 with O-18. 3 was also characterized by ESI-MS. 180 isotope studies were performed to understand the role of 3 in the catalytic cycle and to further establish the mechanism of the reaction. (C) 2014 Elsevier Ltd. All rights reserved.