EFFECT OF PORPHYRIN RING DISTORTION ON REDOX POTENTIALS OF BETA-BROMINATED-PYRROLE IRON(III) TETRAPHENYLPORPHYRINS

This work reports the first quantitative measure of how porphyrin ring distortion and inductive effects of substituted halogen groups on the pyrrole beta positions of a metallotetraphenylporphyrin will influence their redox potentials for oxidation and reduction in nonaqueous media, The investigated compounds are represented by (BrxTPP)FeCl where BrxTPP is the dianion of meso-tetraphenylporphyrin and x, which varies between 0 and 8, is the number of bromides on the pyrrole beta positions of the macrocycle. The half-wave potentials for the first reduction of each compound are linearly related to the number of Br groups and shift in a positive direction by 51 mV/Br upon going from x = 0 to x = 8. In marked contrast, the E(1/2) values for oxidation vary linearly in a positive direction with the number of Br group only from x = 0 to x = 2 and then reverse and shift in a negative direction from x = 3 to x = 8, such that each compound is more easily oxidized with stepwise increase in the number of Br groups. The difference in E(1/2) between reduction of(Br8TPP)FeCl and (TPP)FeCl amounts to 390 mV, with the former compound being easier to reduce due to the inductive effects of the eight Br groups. However, most unexpectedly, (Br8TPP)FeCl is also more easily oxidized than (TPP)FeCl despite having eight electron-withdrawing Br groups on the macrocycle, This results in a more facile oxidation of (Br8TPP)FeCl by 309 mV with respect to the potential which would be observed if only inductive effects were to be operative.