Determination of the activation energies for ND tautomerism and anion exchange in a porphyrin monocation

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© 2016 World Scientific Publishing Company. An unusual feature of free base tetraarylporphyrins (H2TArP) is the small amount of monocation (H3TArP+) observed during titrations with acids. This anomalous behavior has been known for over forty years and has been explained in terms of the saddle deformation of the porphyrin that occurs upon protonation. In this paper, 1H NMR spectroscopy is used to investigate the protonation of H2OETPP, an H2TArP that is pre-deformed into a highly nonplanar saddle structure by steric crowding of its peripheral substituents. The goal of our studies is to determine if the saddle structure of H2OETPP stabilizes the monocation H3OETPP+, and, if it does, to conduct detailed NMR studies of H3OETPP+. NMR spectra clearly show the formation of H3OETPP+ when one equivalent of acid is added to H2OETPP in organic solvents, and for picric acid in toluene-d8 the monocation is the dominant species (∼70%). Crystals of H3OETPP+ (picrate) suitable for X-ray crystallography could not be obtained, but the structure of H4OETPP2+ (picrate)2 was determined. Variable temperature NMR studies of H3OETPP+ (picrate) reveal the presence of two dynamic processes. The first is picrate anion exchange in CD2Cl2, where the activation energy (G‡) is calculated to be 53 kJ.mol-1. A second process, ND tautomerism (G‡ = 42 kJ.mol-1) is also detected for D3OETPP+ at very low temperatures in toluene-d8. The much lower activation energy for ND tautomerism in D3OETPP+ vs. D2OETPP (G‡ = 63 kJ.mol-1) may be due to destabilization of the ground state of the monocation due to the presence of cis interactions between the inner deuterium atoms.

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Journal of Porphyrins and Phthalocyanines

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