Mixed β-pyrrole substituted meso-tetraphenylporphyrins and their metal complexes: Synthesis, structures and electrochemical redox properties

Synthesis of two new classes of mixed β-pyrrole substituted tetraphenylporphyrins, H2TPP(Ph)4(R)4 (R = 2-thienyl and phenylethynyl, PE) and H2TPP(CH3)4(R)4 (R = 2-thienyl and PE) and their metal complexes have been reported. These porphyrins showed dramatic red-shift in electronic absorption bands relative to their corresponding MTPP(R)4 derivatives. Crystal structures of H2TPP(R)4(2-thienyl)4 (R = Ph and CH3) and MTPP(CH3)4(PE)4 (M = Zn(II) and Cu(II)) derivatives are highly nonplanar as evidenced from the mean displacement of β-pyrrole carbon (ΔCb) in the range ±(0.33-1.07) Å. Normal-coordinate structural decomposition analysis of the 24-atom core in these crystal structures revealed largely saddle (73-93%) combined with different degree of ruffled, domed and wave distortions. Electrochemical redox properties of these porphyrins exhibited interesting trend in redox potentials relative to their corresponding H2TPP(R)4s. The tunable trend in redox potentials and electronic absorption bands of mixed substituted porphyrins suggests the role of nonplanarity of macrocycle and electronic effects of the substituents.