Sequential deprotonation of meso-(p-hydroxyphenyl)porphyrins in DMF: From hyperporphyrins to sodium porphyrin complexes

Sequential deprotonations of meso-(p-hydroxyphenyl)porphyrins (p-OHTPPH2) in DMF + H₂O (V/V = 1:1) mixture have been verified to result in the appearance of hyperporphyrin spectra. However, when the deprotonations of these p-OHTPPH₂ are carried out in DMF, the spectral changes differ considerably from those in the mixture mentioned above. At low [OH^-], the optical spectra in the visible region are still considered to have characteristics of hyperporphyrin spectra. Further deprotonation at much higher basicity makes the optical spectra form three-banded spectra similar to those in the acidic solution. To clarify the molecular origins of these changes, UV - vis, resonance Raman (RR), proton nuclear magnetic resonance (¹H NMR) experiments are carried out. Our data give evidence that p-OHTPPH₂ in DMF can be further deprotonated of pyrrolic-H by higher concentrated NaOH, due to an aprotic medium like DMF effectively weakening the basicity of the porphyrin relative to that of the NaOH, and coordinates with two sodium ions (except the sodium ions that interact with the peripherial phenoxide anions) to form the sodium complexes of p-OHTPPH₂ (Na₂P, to lay a strong emphasis on the sodium ions that coordinate with the central nitrogen atom), which can be regarded as the porphyrin anions being perturbed by the sodium cations due to their highly ionic character. The negative centers generated by deprotonation of pyrrolic-H and phenolic-H are not thoroughly delocalized between the substituents and the porphyrin ring. Thus the negative centers generated by deprotonation of pyrrolic-H only act as electron-donating groups on the porphyrin π system, and the negative charges of the phenoxide anion are also mainly localized on the peripheral substituents. As a result, the porphyrin π orbitals cross over the phenoxide anion π orbital and turn into HOMOs, which turns hyperporphyrin spectra of deprotonated phenolic-H of p-OHTPPH₂ into three-banded spectra of regular metalloporphyrins.