The Intra- and Intermolecular Oxidative Coupling of Ni(II)-meso-Octaethyl Mono(pyridine)-Tris(pyrrole) Complex Leading to C-C Bonds: Pathways to Oligomeric Porphyrinogens

This paper reports the oxidative transformations of the unprecedented meso-octaalkyl mono(pyridine)-tris(pyrrole) macrocycle [Et-8(C5H3N)(C4H2NH)(3)], 1, which was obtained by a homologation of the corresponding porphyrinogen, [Et-8(C4H2NH)(4)], A. The metallation of 1 was obtained via its deprotonation with LiBu, followed by the reaction with MCl2. thf(n), leading to the bimetallic complexes: [Et-8(C5H3N)(C4H2N)(3)M{Li(thf)(2)}] [M = Fe, 3; M = Co, 4; M = Ni, 6; M = Cu, 7]. The cobalt derivative occurs also in the separated ion-pair form [Et-8(C5H3N)(C4H2N)(3)Co](-)-[{Li(thf)(4)}](+), 5. The reaction of 4 and 6 with an excess of Cp2FeBPh4 led to the oxidation of the macrocycle with the formation of a cyclopropane functionality [Et-8(C5H3N){(C4H2N)(3)(Delta)}Ni]+BPh4-, 8. The cyclopropane unit underwent reductive cleavage back to the initial macrocycle using lithium metal. The use of a Ni/Cp2Fe+ 1:1 ratio allowed the identification of a dimer derived formally from the monoelectronic oxidation of the macrocycle 1, which formed a radical coupling to 9, [Et-8(C5H3N)(C4H2N)(3)Ni](2). The same compound formed from a disproportionation redox reaction occurring between 6 and 8. The C-C bond across the two metallamacrocycles involved the beta position of the pyrrole. Deprotonation of 9 by LiBu led to a dianionic form 10 [Et-8(C5H3N)(C4H2N)(3)Ni{Li(thf)(2)}]. The formation of the cyclopropane functionality was also achieved via the oxidation of [Et-8(C5H3N)(C4H2N)(3)Li-3(thf)(2)}], 2, forming [Et-8(C5H3N){(C4H2N)(3)(Delta)}Li(thf)], 11, which was ready to be used in metallation reaction. The proposed structures were supported by X-ray analyses on 4, 7, 8, 9, and 11.