The nuclear magnetic resonance spectra of porphyrins—XI : Self-aggregation of zinc(II) meso-trifluoroacetoxyoctaethylporphyrin

Abstract

Complex formation in zinc(II) meso-trifluoroacetoxyoctacthylporphyrin (1) and the corresponding acetate (3) has been studied by 1H and 13C NMR spectroscopy. The large concentration dependence of the 1H chemical shifts of 1 has been analysed in terms of a monomer-dimer equilibrium to give K ∼ 3.01 mole−1 and monomer-dimer shifts of up to 4.2 ppm (for the γ-meso proton). The dimers are immediately dissociated upon addition of methanol. In complete contrast, 3 shows no concentration dependence nor any change upon addition of methanol.The conformations of the model compounds phenyl acetate (4) and phenyl trifluoroacetate (5) were studied by CNDO and solvation calculations and, for 4, by a LIS experiment. Compound 4 exists as the endo conformer in non-polar media, but the exo conformer is preferentially solvated and is also formed when 4 complexes with Eu(fod)3. In compound 5, the exo-endo energy difference is smaller and is not so affected by solvation; 5 shows no LIS.The large complex shifts found for compound 1 are best explained on the basis of a dimer structure in which the O·CO·CF3 groups play no part in the association, which is presumably due to a novel metal to porphyrin interaction.