title: 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.
authors.affiliation: Departament de Química, Institut de Química Computacional i Catàlisi and Serveis Tècnics de Recerca, Universitat de Girona, Campus de Montilivi, E-17071 Girona, Spain. marisa.romero@udg.edu montse.rodriguez@udg.edu.
authors.forename: Ester
authors.initials: E
authors.lastname: Manrique
authors.forename: Albert
authors.initials: A
authors.lastname: Poater
authors.forename: Xavier
authors.initials: X
authors.lastname: Fontrodona
authors.forename: Miquel
authors.initials: M
authors.lastname: Solà
authors.forename: Montserrat
authors.initials: M
authors.lastname: Rodríguez
authors.forename: Isabel
authors.initials: I
authors.lastname: Romero
mesh_terms: D000097:Acetonitriles / Q000737:chemistry; D000475:Alkenes / Q000737:chemistry; D002384:Catalysis; D004563:Electrochemistry; D004852:Epoxy Compounds / Q000737:chemistry*; D007536:Isomerism; D008024:Ligands; D008345:Manganese / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D011720:Pyrazoles / Q000737:chemistry*
chemical_list: D000097:Acetonitriles; D000475:Alkenes; D004852:Epoxy Compounds; D008024:Ligands; D009942:Organometallic Compounds; D011720:Pyrazoles; C031280:pyrazole; D008345:Manganese; C032159:acetonitrile