STRUCTURES, ELECTRONIC-PROPERTIES, AND OXIDATION-REDUCTION REACTIVITY OF HALOGENATED IRON PORPHYRINS

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 P ($) over bar 1, a = 13.649(4) Angstrom, b = 14.474(4) Angstrom, c = 14.537 Angstrom, alpha = 89.26(2)degrees, beta 67.13(1)degrees, gamma = 71.82(2)degrees, V = 2494.3(13) Angstrom(3), Z = 2; Fe-II(TFPPBr8)(py)(2) in triclinic space group P ($) over bar 1, a = 12.459(7) Angstrom, b = 13.125(8) Angstrom, c = 20.989(11) Angstrom, alpha = 84.72(5)degrees, beta = 72.87(4)degrees, gamma = 69.04(5)degrees, V = 3063(3) Angstrom(3), Z = 2. Similar to other chloro-iron((III)) porphyrins, mu(eff) at room temperature (5.96 mu(B)) is that of a (6)A(1) state; mu(eff) at 2 K (4.2 mu(B)) 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](-) 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/O-2-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.