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 [Et 8 N 4 ] 4 - . The two-electron oxidation of [Et 8 N 4 Mn{Na(thf) 2 } 2 ], 4, with Cp 2 FeBPh 4 led to the corresponding monocyclopropane derivative [Et 8 N 4 (Δ)Mn], 6, [Δ ≡ cyclopropane], while the one-electron oxidation with CuCl 2 or O 2 led to the Mn(III)−porphyrinogen [Et 8 N 4 Mn][Li(thf) 4 ], 5, which can be further oxidized by an excess of CuCl 2 to [Et 8 N 4 (Δ) 2 Mn−Cl] + [Cu 9 Cl 11 ] 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, [Et 8 N 4 Fe{Li(thf) 2 } 2 ], 9, the oxidation led in a preliminary stage to the iron(III) derivative [Et 8 N 4 Fe][Li(thf) 4 ], 10, then to the metalated form of the biscyclopropane−porphyrinogen [Et 8 N 4 (Δ) 2 Fe−Cl]{μ-Cu 4 Cl 5 }], 11 . The supposed stabilization of the biscyclopropane by the copper(I) cluster was ruled out by carrying the oxidation of [Cy 4 N 4 Fe{Li(thf) 2 } 2 ], 11, to [Cy 4 N 4 (Δ) 2 Fe−Cl][Cu 2 Cl 4 ], 14 . The stepwise oxidation of [Et 8 N 4 M(thf) 4 ] [M = Li, 1; M = Na, 2 ] with Cp 2 FeBPh 4 led to [Et 8 N 4 (Δ)Li 2 thf 2 ], 15, [Et 8 N 4 (Δ)Li]BPh 4, 16, and [Et 8 N 4 (Δ)Na]BPh 4, 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 [Et 8 N 4 (Δ 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 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 [Et 8 N 4 Ni{Li(thf) 2 } 2 ], 20, with 2 equiv of Cp 2 FeBPh 4 led to the expected [Et 8 N 4 (Δ)Ni], 21, while the reaction of 20 with 1 equiv of Cp 2 FeBPh 4 led to the dimer [(β−β)(Et 8 N 4 ) 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 β position of a pyrrole. Such a reaction shows that the methodology can accede to oligomeric forms of metallaporphyrinogens.