A proton NMR study of the reactions with acid of meso-tetraphenylporphyrins with various numbers of 4-(dimethylamino) groups

Abstract

Proton NMR spectroscopy has been used to study the protonation of meso-tetraphenylporphyrins with zero, one, or four para dimethylamino substituents. Changes in the spectra are similar for corresponding protons through this series of compounds. We give the following interpretations. The internal pyrrolenine nitrogen atoms always add two protons before any peripheral dimethylamino groups react. There is no evidence for a monoprotonated intermediate between the free base and diprotonated porphyrin in the chloroform-trifluoroacetic acid solvent system used. The chemical shifts of the NMR signals of the internal N-H protons are strongly acid-dependent and suggest a loss of aromatic ring current at the porphyrin core a t intermediate acid concentrations. Changes in chemical shifts of protons on the periphery of these aromatic systems are much smaller and in the opposite sense. The signals of the pyrrole Hp protons move discontinuously upfield upon diprotonation and then drift continuously downfield as acid concentration is increased beyond that required for diprotonation. These changes are also in the directions to be expected if aromatic ring currents of the porphyrin decrease to a minimum a t the acid concentration a t which the internal nitrogen atoms are fully protonated. W e suggest that this is due to the known distortion from planarity of the diprotonated tetraarylporphyrin aromatic system. Signals of protons attached to the meso-substituted aromatic rings move unidirectionally downfield as acid concentration increases.The changes in optical spectra of porphyrins on addition of acid have been used extensively to follow the evolution of solute species as basic sites are protonated.' Careful analysis of the shapes of these spectrophotometric titration curves can establish the sequence of distinguishable states and in favorable cases the number of protons added if successive sets of isosbestic points can be identified. However, there can be problems in determining the concentration of free acid in the nonaqueous solvents generally used for these studies. Furthermore, if several competing basic sites are available, the spectrophotometric data can at best tell only indirectly which sites are protonated. Although proton NMR spectra of porphyrins have been routinely recorded as a step in characterization, they often have not been used to count the number of protons at specific basic sites in these molecules, by integration of appropriate NMR signals. Also, with some exceptions, the spectra have been run only in a nonpolar neutral solvent or in trifluoroacetic acid but not at intermediate acid concentrations.2 Theexpectation has been that porphyrins without basic groups on the periphery of the ring system will add two protons on the two pyrrolenine nitrogen atoms which lack them in the free base. This expectation was confirmed by early acidbase titrations3 and NMR studies4 which have been interpreted as demonstrations of diprotonation.In this paper we report proton NMR studies on mesotetraphenylporphyrin with from zero...

Synthesis of 1:2 molecular complexes between free base meso-tetraarylporphyrins and sulfur trioxide

“…The UV-vis, and 1 H NMR correspondence between the spectral data of the 1:2 (donor:acceptor) molecular complexes of σor π-acceptors with meso-tetraarylporphyrins [2][3][4][5][6][7][8][9] or diprotonated species [13] and [H 2 T(4-X)PP(SO 3 ) 2 ] leads to the proposition of a similar structure for the porphyrin core in all of the species, with non-coplanar pyrrole rings tilted alternatively up and down (Tables 1 and 2). This conformation causes, the lone pairs of two pyrrolenine nitrogens to act as the electron donors to two molecules of the SO 3 s, that are located above and below the mean plane of the porphyrin and there are two hydrogen bonds between two pyrroles (N-H) and the oxygens of the sulfur trioxides (Fig.…”

Calculations on the origin of hyperporphyrin spectra in sequentially protonated meso-(dimethylaminophenyl) porphyrins

“…Systematic titration and NMR studies showed that proton addition to these compounds initially occurs at the central pyrrole nitrogens [4,5]. With these sites protonated but with any amine nitrogens still unprotonated, spectra classified as hyperporphyrins are observed [4].…”

Sandwich Intermediate Sitting-atop Complexation between Free Base meso-tetraarylporphyrins and Tellurium(IV) chloride

“…The ( 1 H, 13 C) NMR and UV-vis spectral data of the sandwich i-SAT complexes were similar to the related diprotonated species, [19][20][21] but the molar conductivities of two species were completely different. This indicates that the i-SAT complexes have been formed and not the diprotonated porphyrin species.…”