The proton magnetic resonance spectra of porphyrins-VII. Synthesis and NMR studies of thallium(III) coproporphyrin chelates
The four coproporphyrin "type-isomers" have been synthesized as their tetramethyl esters (1) through modifications of existing procedures. When treated with thallium(III) trifluoroacetate, these porphyrins furnish the corresponding aquo porphinatothallium(III) hydroxides (2) after ligand exchange induced by chromatography on deactivated alumina. The proton NMR spectra of the chloroform solutions of thallium(III) coproporphyrins show a pronounced concentration dependence, all resonances moving to low field upon dilution; the spectra of the type-III and -IV isomers show additional fine structure in solution. Both the meso- and β-Me protons show thallium-proton spin couplings. These results are interpreted in terms of a monomer-dimer equilibrium. In the dimers of the type-I and -II chelates, the rings lie directly one above another, whereas with the type-III and -IV complexes, steric repulsions of the propionate side-chains cause lateral displacement of one molecule in the dimer relative to the other, resulting in the observed fine structure in the spectra. The inter-porphyrin distances and lateral displacements are calculated on this basis and are compared with the corresponding dimers of the parent coproporphyrins, with which there are considerable similarities. © 1973.
Publication Source (Journal or Book title)
Abraham, R., Barnett, G., Bretschneider, E., & Smith, K. (1973). The proton magnetic resonance spectra of porphyrins-VII. Synthesis and NMR studies of thallium(III) coproporphyrin chelates. Tetrahedron, 29 (3), 553-560. https://doi.org/10.1016/0040-4020(73)80008-0