One- and Two-Electron Oxidative Pathway 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(Delta)Mn], 6, [Delta = cyclopropane], while the one-electron oxidation with CuCl2 or O-2 led to the Mn(III)-porphyrinogen [Et8N4Mn][Li(thf)(4)], 5, which can be further oxidized by an excess of CuCl2 to [Et8N4(Delta)(2)Mn-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(Delta)(2)Fe-Cl]{mu-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(Delta)(2)Fe-Cl][Cu2Cl4], 14. The stepwise oxidation of [Et8N4M(thf)(4)] [M = Li, 1; M = Na, 2] with Cp2FeBPh4 led to [Et8N4(Delta)Li(2)thf(2)], 15, [Et8N4(Delta)Li]BPh4, 16, and [Et8N4(Delta)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(Delta(2))] 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 beta 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(Delta)Ni], 21, while the reaction of 20 with 1 equiv of Cp2FeBPh4 led to the dimer [(beta-beta)(Et8N4)(2)Ni-2], 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 beta position of a pyrrole. Such a reaction shows that the methodology can accede to oligomeric forms of metallaporphyrinogens.