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

Abstract

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 1–5, 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 1–6 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 cases.