β-Nitro-substituted free-base, iron(III) and manganese(III) tetraarylporphyrins: synthesis, electrochemistry and effect of the NO2 substituent on spectra and redox potentials in non-aqueous media

Two free-base and four metal derivatives of substituted tetraarylporphyrins containing a nitro-substituent on the beta-pyrrole position of the macrocycle were synthesized and characterized by UV-vis, FTIR, H-1 NMR and mass spectrometry as well as electrochemistry and spectroelectrochemistry in non-aqueous media. The porphyrins are represented as (NO2TmPP)M and (NO(2)TdmPP)M, where M = 2H, (FeCl)-Cl-III or (MnCl)-Cl-III, m is a CH3 group on the para-position of the four meso-phenyl rings of the tetraphenylporphyrin (TPP) and dm represents two OCH3 substituents on the meta-positions of each phenyl ring of the TPP macrocycle. UV-visible spectra of the nitro-substituted porphyrins exhibit absorption bands which are red-shifted by 4-11 nm as compared to bands of the same substituted tetraarylporphyrins lacking a nitro substituent. Three or four reductions are observed for each iron and manganese nitroporphyrin, the first of which is metal-centered, leading to formation of an Fe(II) or Mn(II) complex. Further reduction at the metal center occurs for the iron porphyrins but this reaction proceeds via an Fe(II) pi anion radical in the case of the two nitro-substituented derivatives. The beta-nitro-substituted porphyrins are easier to reduce and harder to oxidize than the corresponding compounds lacking a nitro group. The effect of NO2 substituent on reduction/oxidation potentials and the site of electron transfer was also discussed.