One- and Two-Electron Oxidative Pathways Leading to Cyclopropane-Containing Oxidized Porphyrinogens and C−C-Coupled Porphyrinogens from Alkali Cation− and Transition Metal−meso-Octaethylporphyrinogen Complexes

This report deals with the different transition metal- and alkali cation-assisted oxidation pathways of the meso-octaethylporphyrinogen tetraanion [Et8N4]4-. The two-electron oxidation of [Et8N4Mn{Na(thf)2}2], 4, with Cp2FeBPh4 led to the corresponding monocyclopropane derivative [Et8N4(Δ)Mn], 6, [Δ ≡ cyclopropane], while the one-electron oxidation with CuCl2 or O2 led to the Mn(III)−porphyrinogen [Et8N4Mn][Li(thf)4], 5, which can be further oxidized by an excess of CuCl2 to [Et8N4(Δ)2Mn−Cl]+[Cu9Cl11]0.5, 7. The formation of 7 does not follow the expected sequence Mn(II) → Mn(III) → Mn(II)−monocyclopropane → Mn(II)−biscyclopropane−porphyrinogen. In the case of iron(II)−porphyrinogen, [Et8N4Fe{Li(thf)2}2], 9, the oxidation led in a preliminary stage to the iron(III) derivative [Et8N4Fe][Li(thf)4], 10, then to the metalated form of the biscyclopropane−porphyrinogen [Et8N4(Δ)2Fe−Cl]{μ-Cu4Cl5}], 11. The supposed stabilization of the biscyclopropane by the copper(I) cluster was ruled out by carrying the oxidation of [Cy4N4Fe{Li(thf)2}2], 11, to [Cy4N4(Δ)2Fe−Cl][Cu2Cl4], 14. The stepwise oxidation of [Et8N4M(thf)4] [M = Li, 1; M = Na, 2] with Cp2FeBPh4 led to [Et8N4(Δ)Li2thf2], 15, [Et8N4(Δ)Li]BPh4, 16, and [Et8N4(Δ)Na]BPh4, 17. The reaction of 1 with 16 leading to 15 showed how the C−C moiety in cyclopropane can be engaged in an intermolecular electron transfer. The reaction of 17 with 18-crown-6 allowed the release of biscyclopropane−porphyrinogen [Et8N4(Δ2)]. Particularly interesting is the thermal rearrangement of 15 to 19 occurring via intra- and intermolecular electron transfers with the transposition of the C−C bond of the cyclopropane to a C−C bridge across the β position of two adjacent pyrroles. In the case of metals, such as Ni(II), which do not undergo oxidation state changes, the primary oxidation product of a metalla-meso-octaalkylporphyrinogen is the monocyclopropane derivative, which reacting with the starting material masks an overall one-electron oxidation. In fact, the reaction of [Et8N4Ni{Li(thf)2}2], 20, with 2 equiv of Cp2FeBPh4 led to the expected [Et8N4(Δ)Ni], 21, while the reaction of 20 with 1 equiv of Cp2FeBPh4 led to the dimer [(β−β)(Et8N4)2Ni2], 22, which forms equally well from the reaction of 20 and 21. Complex 22 is a quite unique metallaporphyrinogen dimer, where the two monomeric units are joined via a C−C bond in the β position of a pyrrole. Such a reaction shows that the methodology can accede to oligomeric forms of metallapor