Manganese(iii) meso-tetrakis(ortho-N-alkylpyridyl)porphyrins. Synthesis, characterization, and catalysis of O2˙− dismutation

Abstract

A series of ortho isomers of meso-tetrakis(N-alkylpyridyl)porphyrins (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, as measured by the chromatographic retention factor, R_f; (b) metal-centered redox potential, E_1/2 from +220 to +367 mV vs. NHE, and (c) proton dissociation constant, pK_a2 from 10.9 to 13.2. A linear correlation was found between E_1/2 and R_f of the Mn(III) porphyrins and between the pK_a2 and R_f of the metal-free compounds. As the porphyrins become increasingly more lipophilic, the decrease in hydration disfavors the creation of charge, while enhancing the electron-withdrawing effect of the positively charged pyridyl nitrogen atoms. Consequently, E_1/2 increases linearly with the increase in pK_a2, 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 E_1/2 with the increase in chain length, the catalytic rate constant decreases from methyl (log k_cat = 7.79) to n-butyl, and then increases such that the n-octyl is as potent a 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.