Synthesis and studies of n-base appended manganese corroles

Three novel electron-deficient appendage manganese corroles including acetamido-, pyridyl- and imidazolyl- appendage were prepared. The binding constants between manganese 10-(o-acetamidophenyl)-5,15-bis(pentafluorophenyl) corrolate with four different nitrogenous ligands (triethylamine, Ν-methylimidazole, pyridine and 3-bromopyridine) were determined by photometric titration. The binding constant value of acetamido- appended manganese corrole with triethylamine is 8.77 x 10-3 M-1 and with N-methylimidazole is 0.108 M-1. The binding strength for the two pyridine-type axial ligands to coordinate to the manganese (III) corrole center is much weaker. Besides, the estimated binding constant of imidazolyl- appended manganese corrole exhibited a 318 fold improvement when compared to the intermolecular imidazole binding constant as a result of entropy effect. Apart from the binding properties, the presence of nitrogenous ligands can exert positive effect on the rate, and thereby lowering the activation energy on the oxygen atom transfer process from oxo-manganese (V) corroles to styrene. Kinetic study showed that the use of imidazolyl- appended oxo-manganese corrole can provide the greatest enhancement on the rate of oxygen atom transfer process. Twenty times improvement on the oxygen atom transfer when compared with the acetamido- appendage oxo-manganese corrole. Comparing the rate and the activation energy between the pyridyl- appendage and imidazolyl- appendage oxo-manganese corroles, it appeared that stronger binding can give more enhancement on the oxidation rate. The catalytic epoxidation results with styrene and cis-β-methylstyrene reveals a non-linear dependence of turnover number on the axial ligand concentration. The presence of 50 equivalent N-methylimidazole leads to the greatest enhancement on the total oxidation products yield and provides a substantial improvement on the stereoselectivity for catalytic epoxidation of cis-β-methylstyrene when compared with the results by using triethylamine and pyridine as axial ligand. Higher products yield were obtained with the imidazolyl- appended manganese corrole than with acetyl- appended manganese corrole in the presence of N-methylimidazole, but there is no further improvement on the stereoselectivity for catalytic epoxidation of cis-β-methylstyrene