Synthesis of “Face to Face” Porphyrin Dimers Linked by 5,15-Substituents: Potential Binuclear Multielectron Redox Catalysts

As part of the development of binuclear transition-metal complexes capable of facilitating multielectron redox reactions of small molecules (such as O₂ and N₂), we have devised an efficient and synthetically flexible route to 5,15-disubstituted porphyrins which affords multigram quantities of isomerically pure porphyrins without recourse to chromatography. The 5,15-substituents are principally amine ((CH₂)_nNHR (n = 1, 2, 3; R = H, CH₃)) and acid chloride ((CH₂)_nC(O)C₁ (n =1, 2)) groups. Reactions of the two types of substituted porphyrins lead to a new class of “face to face” porphyrins with interporphyrin amide bridges of varying length (4-7 atoms). An amine-linked dimer has been produced by the reduction of an appropriate amide linkage. Purely hydrocarbon-linked dimers have also been prepared by the direct coupling of bis(dipyrrylmethanes) bridged by aliphatic (CH₂)_n (n = 4, 6) chains. The dimeric porphyrin compounds have been fully characterized by their visible and NMR spectra. The crystal and molecular structure of the bis(copper(II)) complex of one of the amide-linked dimers, the first such determination of a “face to face” porphyrin, is reported. Crystal data for Cu₂C₇₄N₁₀O₂H₈₆.2H₂O.C₇H₈: space group C_2H-Pije, a = 11.878 (6) Å, b = 13.304 (7) Å, c = 23.725 (13) Å, β = 114.60 (2)° at -140 °C; Z = 2. Å crystallographic center of symmetry is imposed on the dimer. The structure was refined on all data including F₀² < 0 (4663 observations) to values for R and Rv on F1 of 0.103 and 0.152. For the portion of data having F₀² > 3σ(F₀²) (3213 observations) the values for R and R_w on F are 0.063 and 0.073. The spectral properties of these new porphyrin dimers are discussed with reference to the structural analysis. A striking feature of the structure is a shear-like displacement of one porphyrin unit with respect to the other by an average distance of 4.95 Å. The Cu-Cu separation is 6.332 (4) A and the interplanar separation of the two porphyrin rings is 3.87 Å. In solution at ambient temperatures, conformational changes involving motions of the two porphyrin rings cause considerable line broadening in the NMR spectra of all 5,15-bridged dimers. However, at temperatures above 90 °C conformational equilibria are sufficiently rapid to produce well-resolved spectra.