Electrochemistry of nonplanar copper(ii) tetrabutano- and tetrabenzotetraarylporphyrins in nonaqueous media

Abstract

Two series of copper tetraarylporphyrins containing β,β′-fused tetrabutano or tetrabenzo groups were synthesized and characterized as to their electrochemistry and spectroelectrochemistry in nonaqueous media. The examined compounds are represented as butano-(TpYPP)Cu^II and benzo-(TpYPP)Cu^II, where TpYPP is the porphyrin dianion and Y is a CH₃, H or Cl substituent on the para-position of the four meso-phenyl rings of the compound. Each neutral porphyrin in the two series is ESR active and shows a typical d⁹ Cu(II) signal in frozen CH₂Cl₂ solution. Each Cu(II) porphyrin also undergoes two reversible one-electron reductions and two reversible one-electron oxidations in DMF or CH₂Cl₂ containing 0.1 M tetra-n-butylammonium perchlorate to give a π-anion radical and dianion upon reduction and a π-cation radical and dication upon oxidation. A third one-electron oxidation is also observed for butano-(TpYPP)Cu (Y = CH₃ and H) and benzo-(TPP)Cu in PhCN and this process is assigned to the Cu^II/Cu^III transition. The reversible half-wave potential for the first oxidation of each compound in both series is shifted negatively by about 500 mV as compared to E_1/2 values for oxidation of the related copper tetraarylporphyrin without the four fused benzo or butano rings while smaller positive shifts of 60 and 300 mV are seen for reduction of the tetrabenzotetraarylporphyrins and tetrabutaotetraarylporphyrins, respectively, as compared to the same redox reactions of the related tetraarylporphyrins. The electrochemically measured HOMO–LUMO gap averages 1.76 ± 0.05 V for benzo-(TpYPP)Cu^II, 2.04 ± 0.06 V for butano-(TpYPP)Cu^II and 2.33 ± 0.03 for (TpYPP)Cu in CH₂Cl₂.