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)

A series of derivatives M₂P₂ (M₂ ≡ H₄, Co₂, Ni₂, Cu₂, Zn₂, Pd₂, Pt₂, Co/Ni, Ni/Cu, Ni/Zn) of the ligand meso,meso′-bis(octaethylporphyrinyl)butadiyne has been prepared and characterized by ¹H NMR, FT Raman and visible absorption spectroscopies as well as by cyclic and a.c. voltammetry in CH₂Cl₂ 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 [M₂P₂]^1- and [M₂P₂]^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 = 50000-200000 M^-1 cm^-1) at c. 4500 and 11 500 cm^-1 ([M₂P₂]^1-) and 9500 cm^-1 ([M₂P₂]^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.