Imidazoporphyrins as supramolecular tectons: synthesis and self-assembly of zinc 2-(4-pyridyl)-1H-imidazo[4,5-b]porphyrinate

Abstract

5,10,15,20-Tetramesityl-2-(4-pyridyl)-1H-imidazo[4,5-b]porphyrin (PyPor) bearing a pyridine donor site connected to the tetrapyrrolic macrocycle by a rigid imidazole linker was prepared in high yield by the condensation of the corresponding 2,3-dioxochlorin and 4-formylpyridine in the presence of ammonium acetate. After the insertion of a metal ion capable of metal–ligand axial coordination, namely Zn(II), this compound afforded self-complementary porphyrin PyPorZn which was suitable for self-assembly. In the crystals, this complex exists as a zigzag coordination polymer formed through the axial coordination of the pyridine nitrogen atom to the zinc ion of the neighbouring porphyrin molecule. The Zn atoms adopt a distorted tetragonal pyramidal environment and the pyridine rings significantly deviate from the orthogonal orientation to the mean N₄ plane of the adjacent macrocycle (the C_para–N_Py–Zn angle is equal to 148°). This distortion was analysed by using DFT calculations of a zinc 5,10,15,20-tetraphenylporphyrinate–pyridine complex (TPPZn–Py). The energy and the geometry of TPPZn–Py were calculated by varying the tilt of the pyridine ligand. The change of the C_para–N_Py–Zn angle from 180° to 150° induces a small increase (within 1.5 kcal mol⁻¹) of the total energy of the complex. Such a small increase can be easily compensated by the intermolecular CH⋯H and CH⋯π interactions in the crystals. The self-assembly of PyPorZn in non-coordinating solvents was also investigated by NMR and UV-vis spectroscopy. In contrast to the nickel complex PyPorNi, the zinc complex exists in 10⁻⁴ M solution in toluene and chloroform as a mixture of short linear oligomers. These oligomers dissociate affording monomer species upon heating, dilution or addition of a substituting ligand.