The nuclear magnetic resonance spectra of porphyrins. Part X. Carbon-13 nuclear magnetic resonance spectra of some meso-tetraarylporphyrins and their metal chelates
The 13C n.m.r. spectra of meso-tetraphenylporphyrin (TPP), the zinc(II), cadmium(II), bis [mercury (II) acetate], and various thallium(III) derivatives, and some related analogues are reported and assigned. In meso-tetra-(o-tolyl)-porphyrin and meso-tetra-(1-naphthyl)porphyrin only one species was observed. The introduction of zinc(II), cadmium(II), or bis[mercury(II) acetate] into the TPP nucleus does not significantly change the 13C shifts, but the thallium(III) derivative shows both extensive TI-13C couplings and non-equivalence of o-phenyl carbon atoms, the latter being due to the exoplanar position of the thallium atom. Specific TI-13C couplings to the o-phenyl carbon atoms are also observed. The phenomenon of slow protonation (TPP⇄TPPH22+) was identified and examined by using 13C and 1H n.m.r. spectroscopy, and the 13C shifts and assignments for the dication are reported. The large protonation shifts of TPP and high ΔG‡ value for this process (ca. 16·3 kcal mol-1) are consistent with the further buckling of the macrocycle on protonation, due to decrease in the angle between the macrocycle and the phenyl rings. In contrast, the protonation shifts of meso-tetra-(o-tolyl) porphyrin are 'normal' owing to the additional steric requirements of the o-methyl groups. A hitherto unrecorded effect is the shift of the tetramethylsilane reference in trifluoroacetic acid compared with deuteriochloroform of 1·5 p.p.m. upfield with respect to other reference compounds. Identification of the coalescence temperature for the NH tautomerism in both the 13C and 1H spectra allows the kinetic parameters for this process to be determined (ΔG‡303 ca. 12·3 kcal mol-1, ΔH‡ ca. 9·2 kcal mol-1, and ΔS‡ ca. -10 cal K-1 mol-1). These, and the isotope effect (kNH/kND) of ca. 12 (measured at one temperature to remove entropy of activation effects) are consistent with a two-step (rather than a concerted) process.
Publication Source (Journal or Book title)
Journal of the Chemical Society, Perkin Transactions 2
Abraham, R., Hawkes, G., Hudson, M., & Smith, K. (1975). The nuclear magnetic resonance spectra of porphyrins. Part X. Carbon-13 nuclear magnetic resonance spectra of some meso-tetraarylporphyrins and their metal chelates. Journal of the Chemical Society, Perkin Transactions 2 (3), 204-211. https://doi.org/10.1039/P29750000204