The nuclear magnetic resonance spectra of porphyrins. Part VIII. The 13C nuclear magnetic resonance spectra of some porphyrins and metalloporphyrins

Abstract

The ¹³C n.m.r. spectra of the methyl esters of the four coproporphyrin type-isomers and deuteroporphyrin-IX, and of octaethylporphyrin, and the zinc(II), thallium(III), and some diprotonated derivatives are reported and assigned. An immediate differentiation between the coproporphyrin type-isomers is provided by the spectra; the shifts of the meso and ‘pyrrole’ ring carbons are influenced by the adjacent peripheral substituents. The pattern of these carbon chemical shifts, when taken with other evidence, supports the 18 atom π-electron delocalisation pathway in these molecules. This conclusion is contrary to that of other workers, and illustrates the dangers of relying solely on ¹³C shifts. A novel phenomenon in the spectra is line-broadening of the ‘α-pyrrole’ carbon resonances in the neutral molecules, but not in the diprotonated or metal derivatives. This is attributed to NH tautomerism, and support for this is obtained from the spectra of the N-deuteriated species. Protonation of coproporphyrins gives upfield shifts of the ‘α-pyrrole’ carbons and downfield shifts of the ‘β-pyrrole’ and meso-carbon resonances; analogous behaviour has been observed earlier in other nitrogen heterocycles. The thallium(III) derivatives show extensive Tl–¹³C couplings, the two-bond Tl–‘α-pyrrole’ carbon couplings (15–20 Hz) being substantially less than the three-bond Tl–‘β-pyrrole’ carbon (104–109 Hz) and Tl–meso-carbon (145–147 Hz) couplings; comparisons can again be drawn with other metal–carbon couplings. These couplings appear to be primarily due to the contact mechanism.