Oxidation of Metal-meso-Octaethylporphyrinogen Complexes Leading to Novel Oxidized Forms of Porphyrinogen Other than Porphyrins. 2. The Redox Chemistry of Iron(II)- and Cobalt(II)-meso-Octaethylporphyrinogen Complexes Occurring with the Formation and Cleavage of Two Cyclopropane Units

In this paper we report a four-electron oxidation of a porphyrinogen tetraanion leading to a neutral form containing two cyclopropane units complexes to iron(II), iron(III), and cobalt(II). The redox chemistry of the meso-octaethylporphyrinogen-cobalt(II) complex clarified the stepwise process leading to the oxidized forms of porphyrinogen. The reaction of the iron(II)-meso-octaethylporphyrinogen complex [Et(8)N(4)FeLi(2)(THF)(4)] (1) with CuCl2 led to a four-electron oxidation of the porphyrinogen skeleton and the formation of two cyclopropane units. Such an oxidation converts the meso-octaethylporphyrinogen tetraanion into a neutral ligand which is bonded via the nitrogen atoms to the [FeCl]+ cation and with a C=C double bond of each pyrrole to the four copper(I) atoms of the [Cu4Cl5](-) cluster in [Et(8)N(4)(Delta)(2)FeCl...Cu4Cl5] (2). The stabilization of the four-electron oxidized form is independent of interaction with the [Cu4Cl5](-) cluster, as proven by the isolation of a copper free form. The oxidation of [Et(8)N(4)](-)[Li(THF)(4)](+) (3) with CuCl2 under different conditions led to the porphyrinogen-bis(cyclopropane) form binding exclusively iron(III) in [Et(8)N(4)(Delta)(2)FeCl](2+)[FeCl4](2-) (4) [(Delta) = abbreviation for cyclopropane]. The structure of 4 has been established by an X-ray analysis. A comparison between the structures of 1 and 4 singles out the conformational changes occurring on the porphyrinogen skeleton upon a four-electron oxidation and formation of two cyclopropane units. The stepwise nature of such an oxidation has been clarified by studying the oxidation of [Et(8)N(4)CoLi(2)(THF)(4)] (5), which can be converted by reaction with 1 mol of CuCl2 to the corresponding cobalt(III) derivative {[Et(8)N(4)Co](-)[Li(THF)(4)](+)} (6) and undergoes a further oxidation by CuCl2 to [Et(8)N(N)4(Delta)Co] (7), containing the oxidized form of porphyrinogen with a cyclopropane unit. An intermolecular redox process, dependent on the nature of the solvent, occurs between 7 and 5, leading to the formation of 6. In the presence of coordinating solvents, 6 is the stable form, while in hydrocarbons, it disproportionates into 5 and 7. The best synthesis of 7 is achieved by the oxidation of 5 with an excess of p-benzoquinone. The cyclopropane unit can be reduced by two electrons by reacting 7 with excess lithium metal or by reductive demetalation with H2S. The reaction of 7 with CuCl2 leads to a further oxidation of porphyrinogen by two electrons and the formation of a second cyclopropane unit. The fully oxidized form of porphyrinogen is a neutral ligand. Complex 8, [{Et(8)N(4)(Delta)(2)- CoCl}...{Cu4Cl5}], has been structurally characterized and like 2 contains the oxidized form bonded to [CoCl](+) via nitrogen atoms and to the [Cu4Cl5](-) cluster via the pyrrolic C=C bonds. Crystallographic details: 1 is monoclinic, space group C2/c, a = 20.749(2) Angstrom, b = 10.930(1) Angstrom, c = 22.797(2) Angstrom, alpha = gamma = 90 degrees, beta = 104.69(2)degrees, Z = 4, and R = 0.044. 4 is orthorhombic, space group Pbca, a = 23.144(4) Angstrom, b = 30.264(7) Angstrom, c 13.362(3) Angstrom, alpha = beta = gamma = 90 degrees, Z = 8, and R = 0.053. 7 is monoclinic, space group P2(1), a = 15,804(2) Angstrom, b = 12.237(1) Angstrom, c 17.026(3) Angstrom, alpha = gamma = 90 degrees, beta = 91.15(2)degrees, Z = 4, and R = 0.069. 8 is triclinic, space group P (1) over bar, a 15.361(3) Angstrom, b = 15.548(4) Angstrom, c = 12.060(5) Angstrom, alpha = 107.66(2)degrees, beta = 99.62(2)degrees, gamma = 86.30(2)degrees, Z = 2, and R = 0.080.