Interaction of meso-tetraarylporphyrins with formic acid: A variable temperature 1H NMR study

The dications of meso-tetraphenylporphyrin, H2TPP, meso-tetra(4-Cl)phenylporphyrin, H2T(4-Cl)PP and meso-tetra(4-OCH3phenyl)porphyrin, H2T(4-OCH3)PP with formic acid as a weak carboxylic acid have been studied by UV–vis and variable temperature 1H NMR spectroscopies. As would be expected in the presence of a strongly basic counter anion, the formation of very strong N–H–O hydrogen bonds between the diprotonated species and the formate anion leads to considerable broadening of the NH resonance of H2T(4-OCH3)PP(HCOOH)2, H2T(4-Cl)PP(HCOOH)2 and H2TPP(HCOOH)2, so that the signal cannot be observed at or close to room temperatures, but appears at 0 °C for H2T(4-OCH3)PP(HCOOH)2 and at − 30 °C for the latter dications. While the signal of the β protons of H2TPP(HCOOH)2 moves continuously upfield from 20 to − 60 °C, the corresponding signal of H2T(4-OCH3)PP(HCOOH)2 and H2T(4-Cl)PP(HCOOH)2 moves upfield at higher temperatures and downfield at lower temperatures, corresponding to a decrease followed by an increase in the porphyrin ring current.