Synthesis and Properties of Tetraphenylporphyrin Molecules Containing Heteroatoms Other than Nitrogen. 5. High Resolution Nuclear Magnetic Resonance Studies of Inner and Outer Aromaticity

When the two NH groups in tetraphenylporphyrin are replaced by the group 6A heteroatoms S, Se, and Te, bonding interactions within the porphyrin core are found, as is apparent from X-ray analysis results. Because of these interactions, changes in the inner and outer aromatic pathways occur, which are expressed in the chemical shifts of the hydrogen atoms at the periphery of the molecule. The bonding interactions within the core are disrupted by either protonation or complexation, and this finds its expression in the chemical shifts in a consistent way. If two different heteroatoms are introduced, or when the p-phenyl hydrogens are substituted in such a way that two phenyl groups carry substituents of different electronic properties, charge transfer occurs between the heteroatoms. This charge transfer changes the inner aromatic pathway, which influences the chemical shift. These shifts relate linearly to the Hammett constant of the substituents. Starting from these results, the concept is developed that the interaction between the heteroatoms acts as an electron drain on the π electrons and especially on the pyrrolenine-N nonbonding electrons. This causes an increased contribution of the 20-membered, extended outer aromaticity Kekule structures.