Ion pairing in tetraphenylporphyrin oxidation: A semiquantitative study

In 1,2-difluorobenzene (DFB), electrolyte conductivity measurements and cyclic voltammetric titration on the traditional benchmark tetraphenylporphyrin, H₂tpp, permit the first estimate of ion pair association constants for singly- and doubly-oxidized free-base porphyrins. From ion titration cyclic voltammetry and digital simulation, measured association constants for H ₂tpp⁺X^- were 65, 120, 210, 520 and 730 M ^-1, for X^- = PF₆^-, ClO ₄^-, NTf₂^-, BF₄ ^- and OTf^-, respectively, relative to the association constant for the H₂tpp⁺TFPB^- complex. By similar methods it was found that the association constants for the corresponding dication, H₂tpp²⁺, were at least 3.0 × 10⁴ M^-1 (PF₆^-), 2.5 × 10 ⁶ M^-1 (ClO₄^-), 5.2 × 10 ⁵ M^-1 (NTf₂^-), 1.9 × 10 ⁶ M^-1 (BF₄^-) and 2.7 × 10 ⁶ M^-1 (OTf^-). We demonstrate that differences in association constants allow the formal potential of the second oxidation of H₂tpp to be shifted by more than 800 mV simply by varying the solvent and electrolyte. In addition, calculated electrostatic potential energy maps for porphyrin dications suggest that exposure of the core N - H groups is responsible for the change in ordering of anion affinities that occurs upon oxidation of H₂tpp⁺.