Imidazoporphyrins as supramolecular tectons: synthesis and self-assembly of zinc 2-(4-pyridyl)-1H-imidazo[4, 5-b]porphyrinate. Issue 9 (12th February 2019) - Abdulaeva, Inna A.
Abstract : First evidence of self-assembling of imidazoporphyrins is revealed with the example of zinc(ii ) 5, 10, 15, 20-tetramesityl-2-(4-pyridyl)-1 H -imidazo[4, 5- b ]porphyrin in the solid state and in solution. Abstract : 5, 10, 15, 20-Tetramesityl-2-(4-pyridyl)-1 H -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 porphyrinPyPorZn 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 –NPy –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 ofTPPZn–Py were calculated by varying the tilt of the pyridine ligand. The change of the C para –NPy –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 ofPyPorZn in non-coordinating solvents was also investigated by NMR and UV-vis spectroscopy. In contrast to the nickel complexPyPorNi, 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.