We report here a two-electron oxidation of a porphyrinogen tetraanion and its conversion into a dianionic form containing a cyclopropane unit [abbreviated as (Delta)]. The dianionic porphyrinogen form was structurally identified in nickel(II) and copper(II) derivatives. The redox chemistry of meso-octaethylporphyrinogen copper(II) helped to clarify the stepwise process leading to the oxidized form of porphyrinogen. The Ni(II)-meso-octaethylporphyrinogen complex [Et(8)N(4)NiLi(2)(THF)(4)] (1) can be converted into the oxidized form [Et(8)N(4)(Delta)Ni] (3) by reaction with excess of p-benzoquinone. Complex 3 contains the oxidized form of porphyrinogen which can be reduced, by lithium metal, back to complex 1. The structural features of the oxidized form have been elucidated by X-ray crystal analysis of 2. In contrast to the case of Ni(II), the oxidation of [Et(8)N(4)CuLi(2)(THF)(4)] (2) is a stepwise process initially leading to the formation of a diamagnetic square planar copper(III) derivative, [Et(8)N(4)Cu][Li(THF)(4)] (5). Further oxidation of 5, with CuCl2, led to the monocyclopropane form of porphyrinogen in [Et(8)N(4)(Delta)Cu] (4). There is an interesting redox relationship between 2, 4, and 5. The copper(III) derivative (5), disproportionates into 2, containing copper(II), and 4 containing formally a copper(IV), the cyclopropane counting for a +2 oxidation state. This reaction occurs in benzene, and it can be reversed in coordinating solvents such as THF. Such a finding emphasizes the inter- and intramolecular electron-transfer processes which can occur in such systems. The best way to prepare the monocyclopropane form of porphyrinogen is, however, via the oxidation of 2 with p-benzoquinone. The X-ray analysis carried out on 2, 4, and 5 allows description of the major conformational changes of the porphyrinogen skeleton during the oxidation process. Crystallographic details: 2 is monoclinic, space group C2/c, a = 20.808(2) Angstrom, b = 10.946(1) Angstrom, c = 22.811(2) Angstrom, alpha = gamma = 90 degrees, beta 104.57(2)degrees, Z = 4, and R = 0.056. 3 is orthorhombic, space group P2(1)2(1)2(1), a 11.526(1) Angstrom, b = 15.132(1) Angstrom, c = 21.098(2) Angstrom, alpha = beta = gamma = 90 degrees, Z = 4, and R = 0.057. 4 is orthorhombic, space group P2(1)2(1)2(1), a = 14.880(2) Angstrom, b = 22.050(2) Angstrom, c = 11.436(2) Angstrom, alpha = beta = gamma = 90 degrees, Z = 4, and R = 0.043. 5 is hexagonal, space group P3(1)21, a = b = 11.329(1) Angstrom, c = 34.939(2) Angstrom, alpha = beta = 90 degrees, gamma = 120 degrees, Z = 3, and R = 0.062.
Oxidation of Metal-meso-Octaethylporphyrinogen Complexes Leading to Novel Oxidized Forms of Porphyrinogen Other than Porphyrins. 1. The Redox Chemistry of Nickel(II)- and Copper(II)-meso-Octaethylporphyrinogen Complexes Occurring with the Formation and Cleavage of a Cyclopropane Unit / De Angelis, S.; Solari, E.; Floriani, C.; Chiesi Villa, A.; Rizzoli, Corrado. - In: JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. - ISSN 0002-7863. - 116(1994), pp. 5691-5701. [10.1021/ja00092a021]
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