Reusable manganese compounds containing pyrazole-based ligands for olefin epoxidation reactions

We describe the synthesis of new manganese(II) and manganese(III) complexes containing the bidentate ligands 2-(3-pyrazolyl)pyridine, pypz-H, and 3(5)-(2-hydroxyphenyl)pyrazole, HOphpz-H, with formula [MnX2(pypz-H)2] (X = Cl−, 1, CF3SO3−, 2, OAc−, 3 or NO3− (4)), [MnCl2(pypz-H)(H2O)2], 5, or [MnCl- (Ophpz-H)2], 6. All the complexes have been characterized through analytical, spectroscopic and electrochemical techniques. Single X-ray structure analysis revealed a six-coordinated Mn(II) ion in complexes 15, and a five-coordinated Mn(III) ion in complex 6. Compound 5 is the first co-crystal of Mn(II) containing Cl and H2O ligands together with bidentate nitrogen ligands. The catalytic activity of complexes 16 has been tested with regard to the epoxidation of styrene and, in the case of 1, 5 and 6, other alkenes have been epoxidized using peracetic acid as oxidant in different media, among which glycerol, a green solvent never used in epoxidation reactions using peracetic acid as oxidant. The catalysts show moderate to high conversions and selectivities towards the corresponding epoxides. For complexes 1, 5 and 6, a certain degree of cis → trans isomerization is observed in the case of cis-β-methylstyrene. These observations have been explained through computational calculations. The reutilization of catalysts 1 and 6 for the epoxidation of alkenes has been evaluated in [bmim] : acetonitrile mixture (bmim = 1-butyl- 3-methylimidazolium), allowing the effective recyclability of the catalytic system and keeping high conversion and selectivity values up to 12 successive runs, in all casesThis research has been financed by MINECO of Spain through projects, CTQ2010-21532-C02-01 and CTQ2014-54306-P. EM thanks UdG for a predoctoral grant. Serveis Tècnics de Recerca (STR) from UdG are also acknowledged for technical support. AP thanks the Spanish MINECO for project CTQ2014-59832- JIN and European Commission for a Career Integration Grant (CIG09-GA-2011-293900). MS thanks EU for a FEDER fund (UNGI10-4E-801) and the Generalitat de Catalunya for project 2014SGR931 and ICREA Academia 2009 priz