The nmr spectra of porphyrins—15 : Self-aggregation in zinc(II) protoporphyrin-IX dimethyl ester

Abstract

Complex formation in Zn(II) protoporphyrin-IX dimethyl ester has been studied by proton and 13C NMR spectroscopy. The large concentration dependence of the spectra has been studied by the technique of porphyrin/axial ligand titration, which together with selective decoupling and regiospecific deuterium labelling allows the assignment of all the peripheral proton and 13C nuclei in both the monomeric and aggregated species. Titration of the metalloporphyrin with various basic ligands (pyrrolidine, pyridine, lutidine) showed that dissociation of the aggregate was complete for a 1:1 porphyrin/added base ratio. The concentration dependence of the spectra was then analyzed to give the monomer and monomer-dimer shifts for all the assigned nuclei. Analysis of the monomer-dimer shifts in terms of the ring current model gives good agreement with a dimer geometry in which the inter-ring separation is ca. 4.5 Å and there is a smaller lateral displacement of the porphyrin rings. The dimer geometry is such that rings A and B of one porphyrin molecule are situated over rings C and D of the other. These results confirm our earlier suggestions of intermolecular metal-to-porphyrin binding in these metalloporphyrins, and further suggest that charge-transfer interactions may also be present in appropriate cases. The discrepancy between the absolute values of the observed and calculated monomer-dimer shifts, which was formerly attributed to multiple aggregation, is now suggested to be due to ensemble-averaging in the dimer structure.