Hydrogen Bond Control of Active Oxidizing Species in Manganese Porphyrin Hydroxylation Catalysts

Some meso-tetra aryl porphyrinato manganese (III) acetate or chloride complexes including meso-tetraphenyl porphyrinato manganese (III) chloride (TPPMnCl), meso-tetrakis(2,3-dimethoxyphenyl)porphyrinato manganese(III) acetate, (T(2,3-OMeP)PMnOAc) and meso-tetrakis(pentaflourophenyl)porphyrinato manganese (III) acetate (TPFPPMnOAc) were synthesized. These porphyrins were used as catalyst in the oxidation of various alkanes in the presence of pyridine and imidazole as axial ligands. It was revealed that the catalytic activity depends on the existence of hydrogen bonding between the axial base and the substituents on the ortho position of the phenyl ring, in addition to usual electronic and steric effects. Therefore, T(2,3-OMeP)PMnOAc and TPFPPMnOAc exhibited higher catalytic activity than TPPMnCl owing to the presence of such hydrogen bonding between substitutions on the periphery of the porphyrin ring and coordinated axial ligand. Also, the selectivity of these two Manganese porphyrins significantly varies in the presence of pyridine and imidazole for the alkane hydroxylation and is reversed for the alkene epoxidation, suggesting different active oxidizing agent produced by pyridine and imidazole.