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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

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posted on 22.10.1998, 00:00 authored by Raffaella Crescenzi, Euro Solari, Carlo Floriani, Angiola Chiesi-Villa, Corrado Rizzoli
This paper reports the oxidative transformations of the unprecedented meso-octaalkyl mono(pyridine)−tris(pyrrole) macrocycle [Et8(C5H3N)(C4H2NH)3], 1, which was obtained by a homologation of the corresponding porphyrinogen, [Et8(C4H2NH)4], A. The metallation of 1 was obtained via its deprotonation with LiBu, followed by the reaction with MCl2·thfn, leading to the bimetallic complexes:  [Et8(C5H3N)(C4H2N)3M{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 [Et8(C5H3N)(C4H2N)3Co]-[{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 [Et8(C5H3N){(C4H2N)3(Δ)}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, [Et8(C5H3N)(C4H2N)3Ni]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 β position of the pyrrole. Deprotonation of 9 by LiBu led to a dianionic form 10 [Et8(C5H3N)(C4H2N)3Ni{Li(thf)2}]. The formation of the cyclopropane functionality was also achieved via the oxidation of [Et8(C5H3N)(C4H2N)3Li3(thf)2}], 2, forming [Et8(C5H3N){(C4H2N)3(Δ)}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.