Structures, Electronic Properties, and Oxidation-Reduction Reactivity of Halogenated Iron Porphyrins

Article abstract of DOI:10.1021/ic00123a026

Molecular structures of both Fe(III)(TFPPBr8)Cl and Fe(II)(TFPPBr8)(py)2 porphyrins reveal saddle distortions: Fe(III)(TFPPBr8)Cl in triclinic space group P1-, a = 13.649(4) ?, b = 14.474(4) ?, c = 14.537 ?, α = 89.26(2)°, β = 67.13(1)°, γ = 71.82(2)°, V= 2494.3(13) ?**3, Z= 2; Fe(II)(TFPPBr8)(py)2 in triclinic space group P1-, a = 12.459(7) ?, b = 13.125(8) ?, c = 20.989(11) ?, α = 84.72(5)°, β = 72.87(4)°, γ = 69.04(5)°, V = 3063(3) ?**3, Z = 2. Similar to other chloro-iron(III) porphyrins, μeff at room temperature (5.96 μB) is that of a (6)A1 state; μeff at 2 K (4.2 μB) indicates a large zero-field splitting. The Soret bands of both Fe(III)(TFPPBr8)Cl and Fe(II)(TFPPBr8)(py)2 are red-shifted by approximately 23 nm relative to those of corresponding planar Fe(III) porphyrins. The metal and porphyrin reduction potentials of Fe(III)(TFPPBr8)Cl and Fe(II)(TFPPBr8)(py)2 are anodically shifted more than 400 mV from those of Fe(III)(TPP)Cl. Although[Fe(II)(TFPPBr8)Cl](1-) reacts very slowly with dioxygen, it is oxidized rapidly by tert-butyl hydroperoxide (TBHP); these Fe(II) reactivity properties taken together with the finding that the corresponding Fe(III) complex is reduced rapidly by TBHP provide strong support for the proposal that Fe(III)(TFPPBr8)Cl/O2-catalyzed alkane oxygenations occur by a radical chain mechanism in which alkyl hydroperoxide intermediates are decomposed efficiently by both Fe(III) and Fe(II) species.