Steric and inductive effects on the basicity of porphyrins and on the site of protonation of porphyrin dianions: radiolytic reduction of porphyrins and metalloporphyrins to chlorins or phlorins

Abstract

A series of sulfonated, water-soluble, phenyl-substituted porphyrins has been prepared, containing halogen or alkyl groups in the ortho, di-ortho or para positions. While the para-substituted compounds exhibited monomer–dimer behaviour, all the ortho and di-ortho substituted porphyrins were monomeric in aqueous solution at 0.1 mol dm^–3 ionic strength. The proton basicities varied over a 10⁵ range along the series, from the strongly basic tetra(4-methoxyphenyl) species to the weakly basic and less deformable sulfonated tetrakis(2,6-dichlorophenyl) porphyrin. Certain of these porphyrins and related metalloporphyrins were reduced by radiolytic methods in aqueous solutions. Pulse radiolysis studies provided the spectra of the short-lived π-radical anions and γ-radiolysis led to formation of stable chlorins or phlorins, the products of two-electron reduction and protonation at the β-pyrrole or at the meso position, respectively. Whereas H₂TPPS₄[tetrakis(4-sulfonatophenyl)porphyrin] yields phlorin at all pH values, Zn^II-, Al^III-, ln^III- and Sn^IV-TPPS₄ form phlorins at high pH but mostly chlorins at lower pH. The ratio of phlorin to chlorin production is enhanced by increased pH and by increased metal electronegativity. 2,6-Disubstitution at the phenyl rings diminishes the likelihood of phlorin formation while N-methyl substitution at one of the central nitrogens of ZnTPPS₄ enhances phlorin formation. These and other results indicate that electron withdrawal from the porphyrin π-system enhances the ratio of phlorin/chlorin production. On the other hand, steric crowding around the meso position retards protonation at this site by preventing the geometric reorientation necessary for phlorin formation.