Substituted porphyrins
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Abstract
A series of ortho isomers of meso tetrakis N-alkylpyridylporphyrins (alkyl being methyl, ethyl, n-propyl, n-butyl, n-hexyl, and n-octyl) and their Mn(III) complexes were synthesized and characterized by elemental analysis, uv/vis spectroscopy, electrospray ionization mass spectrometry and electrochemistry. An increase in the number of carbon atoms in the alkyl chains from 1 to 8 is accompanied by an increase in: (a) lipophilicity measured by the chromatographic retention factor, Rf; (b) metal-entered redox potential, E1/2 from +220 to +367 mV vs NHE, and (c) proton dissociation constant, pKa2 from 10.9 to 13.2. A linear correlation was found between E1/2 and Rf of the Mn(III) porphyrins and between the pKa2 and Rf of the metal-free compounds. As the porphyrins become increasingly more lipophilic, the decrease in hydration disfavors the separation of charges, while enhancing the electron-withdrawing effect of the positively charged pyridyl nitrogen atoms. Consequently, the E1/2 increases linearly with the increase in pKa2, a trend in porphyrin basicity opposite from the one we previously reported for other water-soluble Mn(III) porphyrins. All of these Mn(III) porphyrins are potent catalysts for superoxide dismutation (disproportionation). Despite the favorable increase of E1/2 with the increase in chain length, the catalytic rate constant decreases from methyl (log kcat=7.79) to n-butyl, and then increases such that the n-octyl is as potent an SOD mimic as are the methyl and ethyl compounds. The observed behavior originates from an interplay of hydration and steric effects that modulate electronic effects.
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- 1. A compound of formula
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