NO₂-induced synthesis of nitrato-iron(III) porphyrin with diverse coordination mode and the formation of isoporphyrin

Abstract

Two nitrato-iron(III) porphyrinates [Fe(4-Me-TPP)(NO₃)] 1 and [Fe(4-OMe-TPP)(NO₃)] 2 are reported. Interestingly, [Fe(4-Me-TPP)(NO₃)] 1 has nitrate ion coordinated as monodentate (by single oxygen atom), while [Fe(4-OMe-TPP)(NO₃)] 2 has nitrate coordination through bidentate mode. Compound 1 was found serendipitously in the reaction of [Fe(4-Me-TPP)Cl] with nitrous acid, which was performed for the synthesis of nitro-iron(III) porphyrin, [Fe(4-Me-TPP)NO₂]. The compound 2 was synthesized by passing NO₂ gas through a solution of [Fe(4-OMe-TPP)]₂O. Upon passing NO₂ gas through a solution of a μ-oxo-dimer, [Fe(4-Me-TPP)]₂O also produces 1. It is interesting that in more electron-rich porphyrin 2, binding of the nitrate in a symmetrical bidentate way while in less electron-rich porphyrin 1, binding of the anion is unidentate by a terminal oxygen atom. However, it is expected that the energy difference between the monodentate and bidentate coordination mode is very small and the interchange between these coordination is possible. Upon passing NO₂ gas through a solution of μ -oxo-dimeric iron(III) porphyrin, the nitrato-iron(III) porphyrin forms first, that later gets oxidized to π-cation radical to yield hydroxy-isoporphyrin in the presence of trace amount of water. These nitrato-iron(III) porphyrinates in moist air slowly converted back to their respective μ -oxo-dimeric iron(III) porphyrins.