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

The synthesis, spectral and structural characterization of meso-aryl sapphy rins and rubyrins containing heteroatoms such as S, O, Se in addition tb py rrole nitrogens are reported. The synthesis of the desired expanded porphyr ins has been achieved using a single precursor, the modified tripyrranes co ntaining 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 conc entration. Use of 0.1 equiv of acid exclusively gave 26 pi rubyrins while a higher concentration of acid gave a mixture of 18 pi porphyrin, 22 pi sapp hyrin, and 26 pi rubyrin. Two additional products, 22 pi oxasmaragdyrin and 18 pi oxacorrole, were isolated in the reaction of oxatripyrrane. All of t he sapphyrins and rubyrins exhibit well-defined intense Soret and Q-bands i n the visible region, and the intensity and the position of the absorption maxima were dependent on the number and the nature of the heteroatoms prese nt in the cavity. The solid-state structures of sapphyrins 8 and 9 show sma ll deviations from planarity with formation of supramolecular ladders stabi lized by weak C-H ... S, C-H ... Se, and C-H ... N hydrogen bonds. H-1 NMR studies reveal retainment of supramolecular arrays in solution. The TFA add uct of 8 shows unusual binding in which both the hydroxyl oxygen and the ca rbonyl oxygen participate, which is reminiscent of metal carboxylate bindin g and in total contrast to that observed for beta-substituted sapphyrins. H -1 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. H-1 NMR spectra of 10 and 17 in its free base form rec orded at -50 degrees C reveal that the heterocyclic rings are inverted and protonation leads to dramatic ring flipping. However, 11 shows normal struc ture 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.