Synthesis and electrochemistry of 2,3,7,8,12,13,17,18-octachloro-5,10,15,20-tetrakis (3,5-dichloro-2,6-dimethoxyphenyl)porphyrin (H2tdcdmpp), [CoII(tdcdmpp)] and [M(tdcdmpp)Cl](M = FeIII or MnIII)

Abstract

Four new highly chlorinated porphyrins, 2,3,7,8,12,13,17,18-octachloro-5,10,15,20-tetrakis (3,5-dichloro-2,6-dimethoxyphenyl)porphyrin (H₂tdcdmpp), [Co^II(tdcdmpp)] and [M(tdcdmpp)Cl](M = Fe^III or Mn^III) have been prepared. The free base was synthesized by chlorination of H₂tdmpp [5,10,15,20-tetrakis(2,6-dimethoxyphenyl)porphyrin] and metallated to give the complexes following published procedures. The same transition-metal derivatives of tdmpp were also prepared and all eight synthesized compounds were electrochemically investigated in benzonitrile or pyridine containing 0.1 mol dm^–3 NBuⁿ₄ ClO₄. As expected the half-wave potentials of the tdcdmpp complexes are shifted towards positive values while those for oxidation or reduction of the tdmpp species are shifted negatively as compared with E_½ for oxidation or reduction of tetraphenylporphyrin (tpp) complexes having the same central metal ions. The magnitude of the anodic shifts in E_½ for reduction with respect to tpp is among the largest reported for chlorinated porphyrins and ranges between 590 and 860 mV depending upon the specific redox reaction and solvent. A smaller ΔE_½ of 330–400 mV is seen for oxidation of the same compounds and these values can be compared to a negligible shift in E_½ upon going from planar M(tpp) to distorted M(obtpp)(obtpp = 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrinate) complexes of cobalt and iron where the effect of macrocycle ruffling is known to predominate over the electronic effects of added halide substituents. This result indicates that the core distortion cannot compensate totally for the electron-withdrawing effect of the Cl substituents. The macrocyclic distortion also induces a large red shift of the UV/VIS absorption bands and this shift in λ_max ranges up to 40 nm for the chlorinated compounds with respect to the tdmpp complexes.