Novel core-modified expanded porphyrins with meso-aryl substituents: synthesis, spectral and structural characterization

Abstract

The synthesis, spectral and structural characterization of meso-aryl sapphyrins and rubyrins containing heteroatoms such as S, O, Se in addition to pyrrole nitrogens are reported. The synthesis of the desired expanded porphyrins has been achieved using a single precursor, the modified tripyrranes containing heteroatoms, through an unprecedented oxidative coupling reaction in moderately good yields. The product distribution and the isolated yields were found to be dependent on the nature of the acid catalyst and its concentration. Use of 0.1 equiv of acid exclusively gave 26π rubyrins while a higher concentration of acid gave a mixture of 18π porphyrin, 22π sapphyrin, and 26π rubyrin. Two additional products, 22π oxasmaragdyrin and 18π oxacorrole, were isolated in the reaction of oxatripyrrane. All of the sapphyrins and rubyrins exhibit well-defined intense Soret and Q-bands in the visible region, and the intensity and the position of the absorption maxima were dependent on the number and the nature of the heteroatoms present in the cavity. The solid-state structures of sapphyrins 8 and 9 show small deviations from planarity with formation of supramolecular ladders stabilized by weak C-H···S, C-H···Se, and C-H···N hydrogen bonds. ¹H NMR studies reveal retainment of supramolecular arrays in solution. The TFA adduct of 8 shows unusual binding in which both the hydroxyl oxygen and the carbonyl oxygen participate, which is reminiscent of metal carboxylate binding and in total contrast to that observed for Beta-substituted sapphyrins. ¹H NMR studies on rubyrins indicate rapid rotation of heterocyclic rings at room temperature, and protonation leads to a decrease in rate of rotation at room temperature. ¹H NMR spectra of 10 and 17 in its free base form recorded at -50 °C reveal that the heterocyclic rings are inverted and protonation leads to dramatic ring flipping. However, 11 shows normal structure in the solution. The single-crystal X-ray structures of 10, 11, and 17 show that the heterocyclic rings, thiophene in 10, selenophene in 11, and furan and thiophene in 17, are inverted in the solid state.