New meso-substituted trans-A2B2 di(4-pyridyl)porphyrins as building blocks for metal-mediated self-assembling of 4 + 4 Re(i)–porphyrin metallacycles

The reaction between 5-(4-pyridyl)dipyrrylmethane and aromatic aldehydes affords meso-arylsubstituted trans-A2B2 di(4-pyridyl)porphyrins which are key building blocks in the of metal-mediated self-assembling of supramolecular structures. A careful optimization of the reaction conditions allowed to obtain 5,15-diphenyl-10,20-di(4-pyridyl)porphyrin (P1), and two analogues bearing on the meso-phenyl substituents two dipropyl- (P4) or diexyl- alkyl chains (P5), with yields ranging from 53 to 63%. Porphyrin P1 reacts with Re(CO5)Br to give the expected 4+4 Re(I)-porphyrin metallacycle which has been fully characterized by means of Infrared, NMR and UV-Vis (absorption and emission) spectroscopies and by guest inclusion studies. Unexpectedly the addition of alkyl chains to the porphyrin fragment, which increase the solubility of the porphyrin in organic solvents, has the opposite effect on the adduct with Re(I). Indeed, the reaction between Re(CO5)Br and porphyrins P4,5 gives very insoluble materials, hampering their complete characterization.