Porphyrin Dimers Linked by Conjugated Butadiyne Bridges: Preparations, Spectra, Voltammetry and Reductive Spectroelectrochemistry of {[M(OEP)](μ-C₄)[M(OEP)]}(M₂≡H₄, Co₂, Ni₂, Cu₂, Zn₂, Pd₂, Pt₂, Co/Ni, Ni/Cu, Ni/Zn)

Abstract

A series of derivatives M 2 P 2 ( M 2 ≡ H 4 , Co 2 , Ni 2 , Cu 2 , Zn 2 , Pd 2 , Pt 2 , Co / Ni , Ni / Cu , Ni / Zn ) of the ligand meso,meso′-bis(octaethylporphyrinyl)butadiyne has been prepared and characterized by 1 H NMR, FT Raman and visible absorption spectroscopies as well as by cyclic and a.c. voltammetry in CH 2 Cl 2 solution at 20 and −40 °C. The electronic spectra exhibit multiple Soret bands and the voltammetry reveals successive one-electron reductions indicating the accessibility of ‘mixed valence’ π-radical anions and π-dianions. Using in situ thin layer spectroelectrochemistry, the UV to near-IR spectra of [ M 2 P 2 ] 1− and [ M 2 P 2 ] 2− ( M as above) were recorded at ≤ −40 °C. Apart from the Co complexes (reduced at the metal ion), the bis(porphyrin) anions have spectra which include sharp, intense near-IR bands (ε = 50 000–200 000 M −1 cm −1 ) at c. 4500 and 11 500 cm −1 ([ M 2 P 2 ] 1− ) and 9500 cm −1 ([ M 2 P 2 ] 2− ). An empirically constructed semiquantitative frontier orbital model explains the observed electronic absorption bands. Inter-porphyrin conjugation, mediated by the butadiyne bridge, is responsible for the Soret band multiplicity. The near-IR bands of the anions are assigned to long-axis polarized π → π* transitions within the newly occupied upper manifold of the two-porphyrin eight-orbital framework. The small comproportionation constants found for the diporphyrin monoanions contradict the usual assumption that electronically coupled dimers should have a large voltammetric separation between the first and second redox steps.