Electrochemical and catalytic properties of novel manganese(III) complexes with substituted 2-(2′-hydroxyphenyl)oxazoline ligands -: X-ray structures of tris[5-methyl-2-(2′-oxazolinyl)phenolato]manganese(III) and tris[5-chloro-2-(2′-oxazolinyl)phenolato] manganese(III)

A series of manganese complexes of the type MnL3 were synthesised and characterised, with HL being (substituted) 2(2'-hydroxyphenyl)oxazoline ligands. The molecular structures of [Mn(5'-Mephox)(3)] and [Mn(5'-Clphox)(3)] reveal their meridional configuration with a significant Jahn-Teller distortion, The ligand substituents exert a significant influence on the redox and catalytic properties of the manganese complexes. Cyclic voltammetry experiments show that the electrochemically quasi-reversible Mn-II/Mn-III and reversible Mn-III/Mn-IV redox couples shift towards higher potentials with more electron-withdrawing substituents. The same trend applies for the oxidation of the manganese(IV) complexes. Electrolysis experiments monitored with UV/Vis and EPR spectroscopy revealed that the reduction of the manganese(III) complexes to manganese(II), and their oxidation to manganese(IV) are both chemically reversible, while subsequent oxidation of the manganese(IV) species was found to be irreversible. All complexes are active oxidation catalysts with dihydrogen peroxide as the oxidant. In the oxidation of styrene, up to 220 turnover numbers were obtained towards styrene oxide, The turnover numbers in the oxidation reactions as well as the selectivity in the oxidation of styrene and cis-stilbene are affected significantly by the substituents, but no clear correlation with the electrochemical properties was observed. The most striking substituent effect is the higher stability of [Mn(5'-NO(2)phox)(3)] compared to the other complexes during styrene oxidation with dihydrogen peroxide in methanol.