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 [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·thf_n, 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.