Characterization and catalytic activity of iron(III) mono(4-N-methyl pyridyl)-tris(halophenyl) porphyrins in homogeneous and heterogeneous systems

The synthesis, characterization and catalytic activity of the cationic iron porphyrins Fe[M(4-N-MePy)TDCPP]Cl₂ and Fe[M(4-N-MePy)TFPP]Cl₂ in the epoxidation of (Z)-cyclooctene by PhIO in homogeneous solution and supported on silica gel (SG), imidazole propyl gel (IPG) or SG modified with 2-(4-sulfonatophenyl)ethyl groups (SiSO₃) have been accomplished. When supported on IPG, both cationic FeP bind to the support via Fe–imidazole coordination. Fe[M(4-N-MePy)TDCPP]IPG contains a mixture of low-spin bis-coordinated Fe^IIIP and high-spin mono-coordinated Fe^IIIP species, whereas Fe[M(4-N-MePy)TFPP]IPG only contains high-spin mono-coordinated Fe^IIIP. These FePIPG catalysts also contain Fe^IIP species, whose presence was confirmed by EPR spectroscopy using NO as a paramagnetic probe. Both cationic FePs coordinate to SG through Fe–O ligation and they are present as high-spin Fe^IIIP species. The cationic FePs supported on SiSO₃⁻ are also high-spin Fe^IIIP species and they bind to the support via electrostatic interaction between the 4-N-methylpyridyl groups and the SO₃⁻ groups present on the matrix. In homogeneous solution, both Fe[M(4-N-MePy)TDCPP]Cl₂ and Fe[M(4-N-MePy)TFPP]Cl₂ have similar catalytic activity to Fe(TDCPP)Cl and Fe(TFPP)Cl, leading to cis-epoxycyclooctane yields of 92%. When supported on inorganic matrices, both FePs lead to epoxide yields comparable to their homogeneous analogues and their anchoring enables catalyst recovery and re-use. Recycling of Fe[M(4-N-MePy)TDCPP]SiSO₃⁻ shows that this FeP maintains its activity in a second reaction.